add noreturn and unreachable

2026-07-22 00:27:13 +02:00
parent 402871ef7a
commit 17508ff751
21 changed files with 286673 additions and 279245 deletions
@@ -25,7 +25,7 @@ roadmap and milestone history.

 ### scalar, aggregate, and pointer types

- exact-width integers, concrete pointer-sized `isize` / `usize`, `f32`, `f64`, `bool`, `void`, and `anyopaque`; contextual `int` accepts the whole integer family, while `uint` accepts only unsigned native and target-classified C integers
+- exact-width integers, concrete pointer-sized `isize` / `usize`, `f32`, `f64`, `bool`, `void`, `noreturn`, and `anyopaque`; `noreturn` is a bottom type valid as a native function result and coerces to any expected value type; contextual `int` accepts the whole integer family, while `uint` accepts only unsigned native and target-classified C integers
 - target-dependent C scalar primitives from `c_char` through `c_longdouble`, kept semantically distinct from native scalars
 - contextual integer/float/character literals, backward type-demand inference through names and arithmetic/bitwise expressions, and typed compile-time evaluation for numeric constant expressions
 - strict numeric conversion by default, widening where valid, C scalar coercions at C boundaries, and explicit scalar keyword casts such as `i32(x)` / `c_float(x)`
@@ -98,6 +98,7 @@ fields. `_` is not a keyword member name.
 - `for` loops over ranges, arrays, slices, and pointers-to-arrays with copy captures, pointer captures `|@item|`, and optional `usize` index captures; `expand for` specializes a comptime aggregate into one checked body per element
 - `break`, `continue`, labeled `break :label`, labeled `continue :label`, and labeled plain blocks; `break :label` can cross nested scopes to exit a labeled block
 - bare block scopes, `defer`, and fallible-function `errdefer` with optional error capture; cleanup is block-scoped and LIFO
+- always-trapping `unreachable`, a `noreturn` expression that diagnoses use during comptime evaluation and terminates the current runtime path
 - bare void `return`, same-line `return value`, value blocks, value `if` with implicit single-expression branches, value loops, value `match`, and strictly value-producing `yield value` / `yield :label value`
 - `match` statements/expressions over enums, tagged unions, and scalars, including exhaustiveness checks, payload captures, pointer payload captures, multi-pattern arms, and scalar range patterns
 - a final `expand |value|:` enum arm or `expand |payload[, tag]|:` tagged-union arm generates one specialized arm for each variant not covered earlier; enum values and optional tags are comptime-known, while union payloads keep their concrete variant type
@@ -949,6 +949,12 @@

 47. rename optional `none` to `null` (implemented)

+48. add `noreturn` and `unreachable` (implemented)
+  - `noreturn` is the native bottom type, permitted as a function result but rejected for storage,
+    parameters, record fields, and the C ABI
+  - `noreturn` expressions coerce to any expected value type and terminate path analysis
+  - `unreachable` always traps at runtime and reports an error during comptime evaluation
+
 ## A word on unchecked casts

 For casts that bypass safety checks, Honey provides builtin functions:
@@ -79,6 +79,7 @@ Expr_Kind :: enum u8 {
 	Bool,
 	Array,
 	Null,
+	Unreachable,
 	Undefined,
 	Inference_Hole,
 	Type,
@@ -394,7 +394,7 @@ block_reads_name :: proc(checker: ^Checker, statements: []ast.Stmt_Id, name: sym
 		case .Add, .Sub, .Mul, .Div, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left, .Shift_Right,
 		     .Shift_Left_Saturating, .Index, .Orelse, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
 			append(&expr_stack, expr.left, expr.right)
-		case .Invalid, .Integer, .Float, .String, .Bool, .Null, .Undefined, .Inference_Hole,
+		case .Invalid, .Integer, .Float, .String, .Bool, .Null, .Unreachable, .Undefined, .Inference_Hole,
 		     .Type, .Name, .Function_Literal, .Anonymous_Struct_Type:
 		}
 	}
@@ -518,7 +518,7 @@ write_type_label :: proc(checker: ^Checker, builder: ^strings.Builder, value: ty
 		strings.write_string(builder, "enum")
 	case .Alias, .Distinct, .Named:
 		write_type_label(checker, builder, item.child)
-	case .Invalid, .Void, .Anyopaque, .Int_Constraint, .Uint_Constraint, .Float_Constraint, .Range_Constraint, .Scalar:
+	case .Invalid, .Void, .Noreturn, .Anyopaque, .Int_Constraint, .Uint_Constraint, .Float_Constraint, .Range_Constraint, .Scalar:
 		strings.write_string(builder, types.name(value))
 	}
 }
@@ -2219,7 +2219,8 @@ call_mapping_semantically_valid :: proc(
 	}
 	if is_runtime_type(checker, expected) {
 		result := function_channel_type(checker, function)
-		if !is_runtime_type(checker, result) || !can_implicitly_convert_type(checker, result, expected) {
+		if !types.is_noreturn(result) && !is_runtime_type(checker, result) ||
+		   !can_implicitly_convert_type(checker, result, expected) {
 			return false, fmt.aprintf(
 				"result type %s cannot convert to expected type %s",
 				type_label(checker, result), type_label(checker, expected),
@@ -3365,7 +3366,7 @@ function_value_signature :: proc(
 		return nil, types.INVALID, false
 	}
 	result = function_channel_type(checker, function)
-	if !types.is_void(result) && !is_runtime_type(checker, result) {
+	if !types.is_void(result) && !types.is_noreturn(result) && !is_runtime_type(checker, result) {
 		return nil, types.INVALID, false
 	}
 	params = make([]types.Type, len(function.params), checker.allocator)
@@ -3402,6 +3403,9 @@ function_type_for_template :: proc(
 		}
 	} else {
 		spec = find_spec(checker, template, params)
+		if spec == INVALID_SPEC {
+			spec = ensure_spec(checker, template, params)
+		}
 		mark_spec_demanded(checker, spec, demanded)
 	}
 	return function_type, spec, spec != INVALID_SPEC || !demand_spec
@@ -3506,7 +3510,7 @@ mark_expr_imports_used :: proc(checker: ^Checker, expr_id: ast.Expr_Id, file: as
 		case .Add, .Sub, .Mul, .Div, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left, .Shift_Right,
 		     .Shift_Left_Saturating, .Index, .Orelse, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
 			append(&stack, expr.left, expr.right)
-		case .Invalid, .Integer, .Float, .String, .Bool, .Null, .Undefined, .Inference_Hole, .Type, .Name, .Anonymous_Struct_Type:
+		case .Invalid, .Integer, .Float, .String, .Bool, .Null, .Unreachable, .Undefined, .Inference_Hole, .Type, .Name, .Anonymous_Struct_Type:
 		}
 	}
 }
@@ -3695,6 +3699,13 @@ validate_declarations :: proc(checker: ^Checker) {
 					"void is only valid as a function result type",
 				)
 			}
+			if !param.comptime_value && types.is_noreturn(param_type) {
+				checker.template_diagnostics[function_id] = source.add(
+					checker.diagnostics,
+					param.span,
+					"noreturn is only valid as a native function result type",
+				)
+			}
 			if param.name != checker.sink_symbol && contains_name(locals[:], param.name) {
 				source.addf(
 					checker.diagnostics,
@@ -3720,6 +3731,13 @@ validate_declarations :: proc(checker: ^Checker) {
 		}
 		if !has_comptime && !signature_poisoned {
 			result_type := type_from_syntax(checker, function.result, function.pkg, function.file)
+			if function.c_abi && types.is_noreturn(result_type) {
+				checker.template_diagnostics[function_id] = source.add(
+					checker.diagnostics,
+					function.span,
+					"noreturn is not supported across the C ABI",
+				)
+			}
 			if diagnostic := add_unsupported_type_diagnostic(checker, function.span, result_type);
 			   diagnostic != source.INVALID_DIAGNOSTIC {
 				checker.template_diagnostics[function_id] = diagnostic
@@ -3788,6 +3806,7 @@ validate_declarations :: proc(checker: ^Checker) {
 			}
 			result := type_from_syntax(checker, function.result, function.pkg, function.file)
 			if add_unsupported_type_diagnostic(checker, function.span, result) == source.INVALID_DIAGNOSTIC &&
+			   !types.is_noreturn(result) &&
 			   !types.contains_c_struct_by_value(result, &checker.module.types) &&
 			   !types.is_c_signature_type(result, &checker.module.types, true) {
 				checker.template_diagnostics[function_id] = source.addf(
@@ -4116,7 +4135,7 @@ validate_meta_schema :: proc(checker: ^Checker) {
 	type_item, type_ok := types.node(&checker.module.types, type_info)
 	type_fields := types.fields_for(&checker.module.types, type_info)
 	expected_tags := []string{
-		"invalid", "void", "anyopaque", "bool", "integer", "float", "array", "pointer", "slice",
+		"invalid", "void", "noreturn", "anyopaque", "bool", "integer", "float", "array", "pointer", "slice",
 		"range", "optional", "function", "enum", "record", "union", "fallible", "distinct",
 	}
 	valid = valid && type_ok && type_item.kind == .Union &&
@@ -4308,8 +4327,8 @@ validate_type_nodes :: proc(checker: ^Checker) {
 						source.add(checker.diagnostics, source.Span{}, "native function pointer parameters must be concrete runtime types")
 					}
 				}
-				if !types.is_void(item.child) && !is_runtime_type(checker, item.child) {
-					source.add(checker.diagnostics, source.Span{}, "native function pointer results must be concrete runtime types or void")
+				if !types.is_void(item.child) && !types.is_noreturn(item.child) && !is_runtime_type(checker, item.child) {
+					source.add(checker.diagnostics, source.Span{}, "native function pointer results must be concrete runtime types, void, or noreturn")
 				}
 			}
 		}
@@ -4631,6 +4650,8 @@ infer_compound_expr :: proc(
 		return types.array(store, element, u64(len(expr.args)), false)
 	case .Null:
 		return expected if types.is_optional(expected, store) else types.INVALID
+	case .Unreachable:
+		return types.NORETURN
 	case .Undefined:
 		return types.INVALID
 	case .Enum_Literal:
@@ -4928,7 +4949,7 @@ infer_expr :: proc(
 			case .Float:
 				last = types.F64
 				_ = pop(&stack)
-			case .String, .Array, .Null, .Undefined, .Address, .Deref, .Index, .Slice,
+			case .String, .Array, .Null, .Unreachable, .Undefined, .Address, .Deref, .Index, .Slice,
 			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed, .Enum_Literal, .Cast,
 			     .Comptime, .Bool, .Not, .Bit_Not, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left,
 			     .Shift_Right, .Shift_Left_Saturating, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
@@ -5274,7 +5295,7 @@ infer_expr :: proc(
 				}
 				if checker.template_diagnostics[template] != source.INVALID_DIAGNOSTIC {
 					declared := function_channel_type(checker, checker.ast_module.functions[template])
-					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
+					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) || types.is_noreturn(declared) else types.INVALID
 					_ = pop(&stack)
 					continue
 				}
@@ -5415,6 +5436,9 @@ infer_expr :: proc(
 					spec = ensure_spec(checker, frame.template, stack[frame_index].args, comptime_values)
 				} else {
 					spec = find_spec(checker, frame.template, stack[frame_index].args, comptime_values)
+					if spec == INVALID_SPEC {
+						spec = ensure_spec(checker, frame.template, stack[frame_index].args, comptime_values)
+					}
 					mark_spec_demanded(checker, spec, demanded)
 				}
 				if spec != INVALID_SPEC {
@@ -5424,7 +5448,7 @@ infer_expr :: proc(
 					checker.current_comptime_values = comptime_values
 					declared := function_channel_type(checker, function)
 					checker.current_comptime_values = previous_comptime
-					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
+					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) || types.is_noreturn(declared) else types.INVALID
 				}
 			} else {
 				declared := function_channel_type(checker, function)
@@ -5432,7 +5456,7 @@ infer_expr :: proc(
 				   !types.is_valid(function.error) {
 					last = types.I32
 				} else {
-					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
+					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) || types.is_noreturn(declared) else types.INVALID
 				}
 			}
 			delete(stack[frame_index].args, checker.allocator)
@@ -5957,7 +5981,7 @@ infer_spec_locals_and_result :: proc(
 	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
 		declared = types.I32
 	}
-	result_hint := declared if is_runtime_type(checker, declared) || declared == types.UINT else types.INVALID
+	result_hint := declared if is_runtime_type(checker, declared) || types.is_noreturn(declared) || declared == types.UINT else types.INVALID

 	locals: [dynamic]Infer_Local
 	locals.allocator = checker.allocator
@@ -6746,7 +6770,8 @@ find_build_local :: proc(locals: []Build_Local, name: symbol.Id) -> (Build_Local

 can_implicitly_convert_type :: proc(checker: ^Checker, actual, expected: types.Type) -> bool {
 	store := &checker.module.types
-	if types.equal(actual, expected) ||
+	if types.is_noreturn(actual) ||
+	   types.equal(actual, expected) ||
 	   types.can_widen(actual, expected) ||
 	   types.can_coerce_c_integer(actual, expected, checker.target) ||
 	   types.can_coerce_c_scalar(actual, expected, checker.target) ||
@@ -6777,6 +6802,9 @@ coerce_expr :: proc(
 		return expr_id
 	}
 	actual := checker.module.exprs[expr_id].type
+	if types.is_noreturn(actual) {
+		return expr_id
+	}
 	if types.equal(actual, expected) {
 		return expr_id
 	}
@@ -7782,6 +7810,11 @@ build_compound_expr :: proc(
 			kind=.Null, span=expr.span, type=expected, target=hir.INVALID_REF,
 			left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
 		})
+	case .Unreachable:
+		return add_hir_expr(checker, hir.Expr{
+			kind=.Unreachable, span=expr.span, type=types.NORETURN, target=hir.INVALID_REF,
+			left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 	case .Undefined:
 		id := source.add(
 			checker.diagnostics,
@@ -8703,7 +8736,7 @@ build_expr :: proc(
 				continue
 			}
 			switch expr.kind {
-			case .String, .Array, .Null, .Undefined, .Address, .Deref, .Index, .Slice,
+			case .String, .Array, .Null, .Unreachable, .Undefined, .Address, .Deref, .Index, .Slice,
 			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed,
 			     .Bool, .Cast, .Comptime, .Not, .Bit_Not, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left,
 			     .Shift_Right, .Shift_Left_Saturating, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range,
@@ -9413,12 +9446,16 @@ build_expr :: proc(
 				checker.current_comptime_values = previous_comptime
 			}
 			spec := INVALID_SPEC
-			if comptime_ok && frame.resolution >= 0 && frame.resolution < len(checker.call_resolutions) {
-				spec = find_spec(
-					checker, frame.template,
-					checker.call_resolutions[frame.resolution].runtime_types,
-					comptime_values,
-				)
+			if comptime_ok {
+				if frame.resolution >= 0 && frame.resolution < len(checker.call_resolutions) {
+					spec = find_spec(
+						checker, frame.template,
+						checker.call_resolutions[frame.resolution].runtime_types,
+						comptime_values,
+					)
+				} else {
+					spec = find_spec(checker, frame.template, stack[frame_index].arg_types, comptime_values)
+				}
 			}
 			delete(stack[frame_index].arg_types, checker.allocator)
 			stack[frame_index].arg_types = nil
@@ -10329,7 +10366,7 @@ build_block :: proc(
 				value_type = checker.module.exprs[value].type
 				if is_runtime_type(checker, declared) {
 					value = coerce_expr(checker, value, declared, statement.span)
-					value_type = checker.module.exprs[value].type
+					value_type = declared
 				} else if types.is_void(declared) {
 					id := source.add(checker.diagnostics, statement.span, "locals cannot have type void")
 					value = invalid_hir_expr(checker, statement.span, id)
@@ -10351,6 +10388,24 @@ build_block :: proc(
 				ctx.problematic^ = true
 				continue
 			}
+			if types.is_noreturn(value_type) {
+				id := source.addf(
+					checker.diagnostics,
+					statement.span,
+					"local '%s' cannot store a noreturn value",
+					symbol_text(checker, statement.name),
+				)
+				append(&body, hir.stmt_id(len(checker.module.statements)))
+				append(&checker.module.statements, hir.Stmt{
+					kind=.Trap, span=statement.span, expr=hir.INVALID_EXPR,
+					local=hir.INVALID_LOCAL, diagnostic=id,
+				})
+				if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); !found {
+					_ = append_build_local(ctx, statement.name, types.I64, !statement.immutable, statement.span)
+				}
+				ctx.problematic^ = true
+				continue
+			}
 			if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); found {
 				id := source.addf(
 					checker.diagnostics, statement.span,
@@ -10816,7 +10871,8 @@ build_block :: proc(
 					local = hir.INVALID_LOCAL, diagnostic = diagnostic,
 				})
 				ctx.problematic^ = true
-			} else if !types.is_void(checker.module.exprs[value].type) {
+			} else if !types.is_void(checker.module.exprs[value].type) &&
+			          !types.is_noreturn(checker.module.exprs[value].type) {
 				id := source.add(checker.diagnostics, statement.span, "non-void expression result must be consumed or assigned to '_'")
 				append(&body, hir.stmt_id(len(checker.module.statements)))
 				append(&checker.module.statements, hir.Stmt{
@@ -10926,7 +10982,8 @@ build_block :: proc(
 						if diagnostic == source.INVALID_DIAGNOSTIC {
 							diagnostic = checker.module.exprs[guard].diagnostic
 						}
-					} else if !types.is_bool(checker.module.exprs[guard].type) {
+					} else if !types.is_bool(checker.module.exprs[guard].type) &&
+					          !types.is_noreturn(checker.module.exprs[guard].type) {
 						diagnostic = source.add(
 							checker.diagnostics,
 							checker.ast_module.exprs[statement.guard].span,
@@ -10977,7 +11034,9 @@ build_block :: proc(
 				continue
 			}
 			condition := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
-			if checker.module.exprs[condition].kind != .Invalid && !types.is_bool(checker.module.exprs[condition].type) {
+			if checker.module.exprs[condition].kind != .Invalid &&
+			   !types.is_bool(checker.module.exprs[condition].type) &&
+			   !types.is_noreturn(checker.module.exprs[condition].type) {
 				id := source.add(checker.diagnostics, statement.span, "'if' condition must be a bool")
 				condition = invalid_hir_expr(checker, statement.span, id, types.BOOL)
 				ctx.problematic^ = true
@@ -11002,7 +11061,8 @@ build_block :: proc(
 				types.BOOL, ctx.pkg, ctx.file,
 			)
 			if checker.module.exprs[condition].kind != .Invalid &&
-			   !types.is_bool(checker.module.exprs[condition].type) {
+			   !types.is_bool(checker.module.exprs[condition].type) &&
+			   !types.is_noreturn(checker.module.exprs[condition].type) {
 				id := source.add(checker.diagnostics, statement.span, "'while' condition must be a bool")
 				condition = invalid_hir_expr(checker, statement.span, id, types.BOOL)
 				ctx.problematic^ = true
@@ -11797,7 +11857,8 @@ emit_value_if :: proc(
 			guard = build_expr(checker, if_stmt.guard, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
 			if checker.module.exprs[guard].kind == .Invalid {
 				ok = false
-			} else if !types.is_bool(checker.module.exprs[guard].type) {
+			} else if !types.is_bool(checker.module.exprs[guard].type) &&
+			          !types.is_noreturn(checker.module.exprs[guard].type) {
 				source.add(checker.diagnostics, checker.ast_module.exprs[if_stmt.guard].span, "'if' unwrap guard must be a bool")
 				ok = false
 			}
@@ -11814,7 +11875,9 @@ emit_value_if :: proc(
 		unwraps = unwrap_list[:]
 	} else {
 		condition = build_expr(checker, if_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
-		if checker.module.exprs[condition].kind != .Invalid && !types.is_bool(checker.module.exprs[condition].type) {
+		if checker.module.exprs[condition].kind != .Invalid &&
+		   !types.is_bool(checker.module.exprs[condition].type) &&
+		   !types.is_noreturn(checker.module.exprs[condition].type) {
 			id := source.add(checker.diagnostics, if_stmt.span, "'if' condition must be a bool")
 			condition = invalid_hir_expr(checker, if_stmt.span, id, types.BOOL)
 			ctx.problematic^ = true
@@ -13000,6 +13063,10 @@ enum_guards_return :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id, locals: []
 all_paths_return :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id, locals: []hir.Local = nil) -> bool {
 	for id in stmts {
 		statement := module.statements[id]
+		if statement.expr != hir.INVALID_EXPR && int(statement.expr) < len(module.exprs) &&
+		   types.is_noreturn(module.exprs[statement.expr].type) {
+			return true
+		}
 		#partial switch statement.kind {
 		case .Return, .Trap:
 			return true
@@ -13049,6 +13116,10 @@ loop_body_breaks :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id) -> bool {
 all_paths_exit :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id) -> bool {
 	for id in stmts {
 		statement := module.statements[id]
+		if statement.expr != hir.INVALID_EXPR && int(statement.expr) < len(module.exprs) &&
+		   types.is_noreturn(module.exprs[statement.expr].type) {
+			return true
+		}
 		#partial switch statement.kind {
 		case .Return, .Trap, .Break, .Continue:
 			return true
@@ -13078,7 +13149,7 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 	checker.current_comptime_values = spec.comptime_values
 	defer checker.current_comptime_values = previous_comptime
 	signature_diagnostic := source.INVALID_DIAGNOSTIC
-	unresolved_result := !types.is_void(spec.result) && !is_runtime_type(checker, spec.result)
+	unresolved_result := !types.is_void(spec.result) && !types.is_noreturn(spec.result) && !is_runtime_type(checker, spec.result)
 	if unresolved_result {
 		if types.is_comptime_only(spec.result, &checker.module.types) {
 			signature_diagnostic = source.addf(
@@ -1159,6 +1159,10 @@ ct_eval_expr :: proc(
 			return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "'null' requires an optional context")
 		}
 		return ct_add_value(state, Ct_Value{kind=.Null, type=expected}), ct_flow(.Normal), true
+	case .Unreachable:
+		return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(
+			state, .Not_Comptime, expr.span, "reached unreachable code during comptime evaluation",
+		)
 	case .Field:
 		if enum_type, enum_ok := enum_type_from_field_expr(checker, expr, state.pkg, state.file); enum_ok {
 			member, ok := find_enum_member(checker, enum_type, expr.name)
@@ -2697,6 +2701,7 @@ ct_typeinfo_value :: proc(state: ^Ct_State, target: types.Type, span: source.Spa
 	tag := "invalid"
 	if !item_ok {
 		if types.is_void(resolved) { tag = "void" }
+		else if types.is_noreturn(resolved) { tag = "noreturn" }
 		else if types.is_anyopaque(resolved) { tag = "anyopaque" }
 		else if types.is_bool(resolved) { tag = "bool" }
 		else if types.is_concrete_integer(resolved) { tag = "integer" }
@@ -83,6 +83,7 @@ Expr_Kind :: enum u8 {
 	Array,
 	Struct,
 	Null,
+	Unreachable,
 	Optional_Some,
 	Local,
 	Global,
@@ -50,7 +50,9 @@ keyword_kind :: proc(text: string) -> token.Kind {
 	case "true":          return .Keyword_True
 	case "false":         return .Keyword_False
 	case "void":          return .Keyword_Void
+	case "noreturn":      return .Keyword_Noreturn
 	case "anyopaque":     return .Keyword_Anyopaque
+	case "unreachable":   return .Keyword_Unreachable
 	case "bool":          return .Keyword_Bool
 	case "int":           return .Keyword_Int
 	case "uint":          return .Keyword_Uint
@@ -142,6 +142,11 @@ llvm_type :: proc(value: types.Type, store: ^types.Store = nil) -> string {
 	if types.is_void(resolved) {
 		return "void"
 	}
+	// `noreturn` has no value representation. Keep malformed storage recoverable with
+	// a byte carrier; function and call result positions map it to LLVM `void` below.
+	if types.is_noreturn(resolved) {
+		return "i8"
+	}
 	if types.is_bool(resolved) {
 		return "i1"
 	}
@@ -183,6 +188,9 @@ function_result_type :: proc(function: ir.Function, store: ^types.Store) -> stri
 	if function.is_main {
 		return "i32"
 	}
+	if types.is_noreturn(function.result) {
+		return "void"
+	}
 	if function.calling_convention == .C {
 		return c_abi_result_type(function.result, store)
 	}
@@ -2121,7 +2129,7 @@ emit_instruction_stream :: proc(
 					strings.write_string(&emitter.builder, "  ")
 				} else if result_abi.kind != .None && result_abi.kind != .Homogeneous_Float {
 					fmt.sbprintf(&emitter.builder, "  %%abi_result%d = ", instruction_index)
-				} else if !types.is_void(instruction.type) {
+				} else if !types.is_void(instruction.type) && !types.is_noreturn(instruction.type) {
 					fmt.sbprintf(&emitter.builder, "  %%v%d = ", instruction_index)
 				} else {
 					strings.write_string(&emitter.builder, "  ")
@@ -2137,7 +2145,10 @@ emit_instruction_stream :: proc(
 					}
 					strings.write_string(&emitter.builder, c_abi_result_type(function_item.child, &emitter.module.types))
 				} else {
-					strings.write_string(&emitter.builder, llvm_type(function_item.child, &emitter.module.types))
+					strings.write_string(
+						&emitter.builder,
+						"void" if types.is_noreturn(function_item.child) else llvm_type(function_item.child, &emitter.module.types),
+					)
 				}
 				if function_item.variadic {
 					strings.write_string(&emitter.builder, " (")
@@ -2187,6 +2198,10 @@ emit_instruction_stream :: proc(
 					wrote_arg = true
 				}
 				strings.write_string(&emitter.builder, ")\n")
+				if types.is_noreturn(instruction.type) {
+					strings.write_string(&emitter.builder, "  unreachable\n")
+					after_terminator = true
+				}
 				if result_abi.kind == .Indirect {
 					fmt.sbprintf(&emitter.builder, "  %%v%d = load %s, ptr %%abi_result_slot%d\n", instruction_index, llvm_type(function_item.child, &emitter.module.types), instruction_index)
 				} else if result_abi.kind != .None && result_abi.kind != .Homogeneous_Float {
@@ -2248,7 +2263,7 @@ emit_instruction_stream :: proc(
 				strings.write_string(&emitter.builder, "  ")
 			} else if result_abi.kind != .None && result_abi.kind != .Homogeneous_Float {
 				fmt.sbprintf(&emitter.builder, "  %%abi_result%d = ", instruction_index)
-			} else if !types.is_void(instruction.type) {
+			} else if !types.is_void(instruction.type) && !types.is_noreturn(instruction.type) {
 				fmt.sbprintf(&emitter.builder, "  %%v%d = ", instruction_index)
 			} else {
 				strings.write_string(&emitter.builder, "  ")
@@ -2304,6 +2319,10 @@ emit_instruction_stream :: proc(
 				wrote_arg = true
 			}
 			strings.write_string(&emitter.builder, ")\n")
+			if types.is_noreturn(instruction.type) {
+				strings.write_string(&emitter.builder, "  unreachable\n")
+				after_terminator = true
+			}
 			if result_abi.kind == .Indirect {
 				fmt.sbprintf(&emitter.builder, "  %%v%d = load %s, ptr %%abi_result_slot%d\n", instruction_index, llvm_type(target.result, &emitter.module.types), instruction_index)
 			} else if result_abi.kind != .None && result_abi.kind != .Homogeneous_Float {
@@ -2355,8 +2374,11 @@ emit_instruction_stream :: proc(
 			fmt.sbprintf(&emitter.builder, ", label %%bro_block_%d, label %%bro_block_%d\n", instruction.integer, u32(instruction.target))
 			after_terminator = true
 		case .Trap:
-			message := diagnostic_message(emitter, instruction.diagnostic, instruction.span, "invalid recovered source")
+			fallback := "reached unreachable code" if instruction.integer != 0 else "invalid recovered source"
+			message := diagnostic_message(emitter, instruction.diagnostic, instruction.span, fallback)
 			emit_trap_call(emitter, message)
+			strings.write_string(&emitter.builder, "  unreachable\n")
+			after_terminator = true
 		case .Return:
 			if global_initializer {
 				return_value = instruction.a
@@ -2681,10 +2703,18 @@ emit_functions :: proc(emitter: ^Emitter) {
 			strings.write_string(&emitter.builder, "...")
 		}
 		if function.implementation == .Declaration {
-			strings.write_string(&emitter.builder, ")\n\n")
+			strings.write_string(&emitter.builder, ")")
+			if types.is_noreturn(function.result) {
+				strings.write_string(&emitter.builder, " noreturn")
+			}
+			strings.write_string(&emitter.builder, "\n\n")
 			continue
 		}
-		strings.write_string(&emitter.builder, ") {\nentry:\n")
+		strings.write_string(&emitter.builder, ")")
+		if types.is_noreturn(function.result) {
+			strings.write_string(&emitter.builder, " noreturn")
+		}
+		strings.write_string(&emitter.builder, " {\nentry:\n")
 		emit_entry_allocas(emitter, function.instructions)
 		if function.calling_convention == .C {
 			for param_type, index in function.param_types {
@@ -716,6 +716,13 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 					diagnostic=source.INVALID_DIAGNOSTIC,
 				})
 				_ = pop(&stack)
+			case .Unreachable:
+				last = append_instruction(state, ir.Instruction{
+					op=.Trap, span=expr.span, type=types.NORETURN, integer=1,
+					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+				_ = pop(&stack)
 			case .String, .Array, .Struct, .Range, .Null, .Optional_Some, .Address, .Deref,
 			     .Index, .Slice, .Field, .Union_Tag, .Length, .Slice_Ptr, .Unwrap, .Orelse,
 			     .Try, .Catch, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
@@ -1037,6 +1044,9 @@ lower_statements :: proc(state: ^State, statements: []hir.Stmt_Id) {
 				})
 			} else {
 				value := lower_expr(state, statement.expr)
+				if types.is_noreturn(state.hir_module.exprs[statement.expr].type) {
+					continue
+				}
 				append_instruction(state, ir.Instruction{
 					op=.Return,
 					span=statement.span,
@@ -1680,11 +1690,22 @@ lower_body :: proc(hir_module: ^hir.Module, function: hir.Function, allocator: m
 	}

 	lower_statements(&state, function.body)
-	if len(state.instructions) == 0 ||
-	   (state.instructions[len(state.instructions)-1].op != .Return &&
-	    state.instructions[len(state.instructions)-1].op != .Return_Void) {
+	last_terminates := false
+	if len(state.instructions) > 0 {
+		last_instruction := state.instructions[len(state.instructions)-1]
+		last_terminates = last_instruction.op == .Return || last_instruction.op == .Return_Void ||
+		                  last_instruction.op == .Trap ||
+		                  last_instruction.op == .Call && types.is_noreturn(last_instruction.type)
+	}
+	if !last_terminates {
 		if types.is_void(function.result) {
 			append_instruction(&state, ir.Instruction{op=.Return_Void, type=types.VOID, target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC})
+		} else if types.is_noreturn(function.result) {
+			append_instruction(&state, ir.Instruction{
+				op=.Trap, type=types.NORETURN, integer=1,
+				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+				diagnostic=source.INVALID_DIAGNOSTIC,
+			})
 		} else {
 			value := append_instruction(&state, ir.Instruction{
 				op=.Const,
@@ -156,7 +156,7 @@ is_type_token :: proc(kind: token.Kind) -> bool {
 	     .Keyword_C_Short, .Keyword_C_Ushort, .Keyword_C_Int, .Keyword_C_Uint,
 	     .Keyword_C_Long, .Keyword_C_Ulong, .Keyword_C_Longlong, .Keyword_C_Ulonglong,
 	     .Keyword_C_Float, .Keyword_C_Double, .Keyword_C_Longdouble,
-	     .Keyword_Void, .Keyword_Anyopaque, .Keyword_Bool, .Keyword_Func, .Keyword_C_Func,
+	     .Keyword_Void, .Keyword_Noreturn, .Keyword_Anyopaque, .Keyword_Bool, .Keyword_Func, .Keyword_C_Func,
 	     .Identifier, .Question, .At, .Star, .Left_Bracket:
 		return true
 	}
@@ -373,6 +373,9 @@ parse_type_atom :: proc(parser: ^Parser) -> ast.Type_Syntax {
 	case .Keyword_Void:
 		advance(parser)
 		return types.VOID
+	case .Keyword_Noreturn:
+		advance(parser)
+		return types.NORETURN
 	case .Keyword_Anyopaque:
 		advance(parser)
 		return types.ANYOPAQUE
@@ -851,7 +854,7 @@ parse_integer_magnitude :: proc(text: string) -> (u64, bool) {
 parse_primary :: proc(parser: ^Parser, nesting: int) -> ast.Expr_Id {
 	tok := current(parser)
 	#partial switch tok.kind {
-	case .Keyword_Int, .Keyword_Uint, .Keyword_Float, .Keyword_Range, .Keyword_Void, .Keyword_Anyopaque, .Keyword_Bool:
+	case .Keyword_Int, .Keyword_Uint, .Keyword_Float, .Keyword_Range, .Keyword_Void, .Keyword_Noreturn, .Keyword_Anyopaque, .Keyword_Bool:
 		start := tok
 		target := parse_type_atom(parser)
 		return add_expr(parser, ast.Expr{
@@ -975,6 +978,15 @@ parse_primary :: proc(parser: ^Parser, nesting: int) -> ast.Expr_Id {
 			right=ast.INVALID_EXPR,
 			diagnostic=source.INVALID_DIAGNOSTIC,
 		})
+	case .Keyword_Unreachable:
+		advance(parser)
+		return add_expr(parser, ast.Expr{
+			kind=.Unreachable,
+			span=tok.span,
+			left=ast.INVALID_EXPR,
+			right=ast.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 	case .Keyword_Undefined:
 		advance(parser)
 		return add_expr(parser, ast.Expr{
@@ -96,7 +96,9 @@ Kind :: enum u8 {
 	Keyword_True,
 	Keyword_False,
 	Keyword_Void,
+	Keyword_Noreturn,
 	Keyword_Anyopaque,
+	Keyword_Unreachable,
 	Keyword_Bool,
 	Keyword_Int,
 	Keyword_Uint,
@@ -45,6 +45,7 @@ FLOAT :: Type(30)
 RANGE :: Type(31)
 ANYOPAQUE :: Type(32)
 UINT :: Type(33)
+NORETURN :: Type(34)

 DYNAMIC_START :: Type(64)

@@ -58,6 +59,7 @@ Numeric_Category :: enum u8 {
 Kind :: enum u8 {
 	Invalid,
 	Void,
+	Noreturn,
 	Anyopaque,
 	Int_Constraint,
 	Uint_Constraint,
@@ -599,6 +601,8 @@ kind :: proc(value: Type, store: ^Store = nil) -> Kind {
 		return .Invalid
 	case VOID:
 		return .Void
+	case NORETURN:
+		return .Noreturn
 	case ANYOPAQUE:
 		return .Anyopaque
 	case INT:
@@ -643,6 +647,10 @@ is_void :: proc(value: Type) -> bool {
 	return value == VOID
 }

+is_noreturn :: proc(value: Type) -> bool {
+	return value == NORETURN
+}
+
 is_anyopaque :: proc(value: Type) -> bool {
 	return value == ANYOPAQUE
 }
@@ -1717,6 +1725,12 @@ can_coerce_c_scalar :: proc(from, to: Type, selected := target.DEFAULT) -> bool
 }

 widest :: proc(a, b: Type) -> Type {
+	if is_noreturn(a) {
+		return b
+	}
+	if is_noreturn(b) {
+		return a
+	}
 	if equal(a, b) && is_concrete_scalar(a) {
 		return a
 	}
@@ -1759,6 +1773,7 @@ name :: proc(value: Type) -> string {
 	switch value {
 	case INVALID:       return "<invalid>"
 	case VOID:          return "void"
+	case NORETURN:      return "noreturn"
 	case ANYOPAQUE:     return "anyopaque"
 	case BOOL:          return "bool"
 	case INT:           return "int"
@@ -14271,6 +14271,131 @@ std_meta_tests_compile_and_run :: proc(t: ^testing.T) {
 	testing.expect_value(t, state.exit_code, 0)
 }

+@(test)
+noreturn_functions_function_pointers_and_peer_types_compile_and_run :: proc(t: ^testing.T) {
+	directory := "/tmp/brolang-test-noreturn"
+	main_path := "/tmp/brolang-test-noreturn/main.bro"
+	output := "/tmp/brolang-test-noreturn-output"
+	text := `meta :: import "@std/meta"
+
+die func() noreturn {
+	unreachable
+}
+
+forward func() noreturn {
+	die()
+}
+
+choose func(flag bool) i32 {
+	return if flag { yield 42 } else { yield die() }
+}
+
+reflects_bottom func($T type) bool {
+	match typeinfo!(T) {
+		.noreturn: return true
+		else: return false
+	}
+}
+
+main func() i32 {
+	callback @func() noreturn = forward
+	_ = callback
+	if choose(true) != 42 { return 1 }
+	if !reflects_bottom(noreturn) { return 2 }
+	return 0
+}
+`
+	_ = os2.remove_all(directory)
+	defer _ = os2.remove_all(directory)
+	defer _ = os.remove(output)
+	testing.expect(t, os.make_directory(directory) == nil)
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)text))
+	testing.expect_value(t, compiler_core.compile_package(
+		directory, output, nil, target.DEFAULT, cimport.Options{}, ".",
+	), 0)
+	state := run_executable(output)
+	testing.expect_value(t, state.exit_code, 0)
+}
+
+@(test)
+unreachable_traps_at_runtime_and_fails_at_comptime :: proc(t: ^testing.T) {
+	directory := "/tmp/brolang-test-unreachable"
+	main_path := "/tmp/brolang-test-unreachable/main.bro"
+	output := "/tmp/brolang-test-unreachable-output"
+	_ = os2.remove_all(directory)
+	defer _ = os2.remove_all(directory)
+	defer _ = os.remove(output)
+	testing.expect(t, os.make_directory(directory) == nil)
+	runtime_text := `main func() void {
+	unreachable
+}
+`
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)runtime_text))
+	testing.expect_value(t, compiler_core.compile_package(directory, output), 0)
+	state, stdout, stderr, err := os2.process_exec(
+		os2.Process_Desc{command=[]string{output}}, context.allocator,
+	)
+	defer delete(stdout)
+	defer delete(stderr)
+	testing.expect(t, err == nil)
+	testing.expect(t, !state.success)
+	testing.expect(t, strings.contains(string(stderr), "runtime trap: reached unreachable code"))
+
+	comptime_text := `value :: $unreachable
+main func() void {}
+`
+	source_file := source.Source{path="unreachable_comptime.bro", text=comptime_text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	found := false
+	for diagnostic in diagnostics.items {
+		found = found || strings.contains(diagnostic.message, "reached unreachable code during comptime evaluation")
+	}
+	testing.expect(t, found)
+}
+
+@(test)
+noreturn_storage_c_abi_and_fallthrough_are_rejected :: proc(t: ^testing.T) {
+	text := `Bad :: struct { value noreturn }
+foreign c_func() noreturn
+fallthrough func() noreturn {}
+parameter func(value noreturn) void { _ = value }
+main func() void {
+	if false { fallthrough() }
+	stored noreturn = unreachable
+}
+`
+	source_file := source.Source{path="invalid_noreturn.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+
+	field, c_abi, missing_return, parameter, storage := false, false, false, false, false
+	for diagnostic in diagnostics.items {
+		field = field || strings.contains(diagnostic.message, "record fields must have runtime value types")
+		c_abi = c_abi || strings.contains(diagnostic.message, "noreturn is not supported across the C ABI")
+		missing_return = missing_return || strings.contains(diagnostic.message, "does not return a value")
+		parameter = parameter || strings.contains(diagnostic.message, "only valid as a native function result type")
+		storage = storage || strings.contains(diagnostic.message, "cannot store a noreturn value")
+	}
+	testing.expect(t, field && c_abi && missing_return && parameter && storage)
+}
+
@(test)
 anonymous_records_and_struct_type_report_targeted_errors :: proc(t: ^testing.T) {
 	directory := "/tmp/brolang-test-bad-struct-type"
@@ -17,6 +17,7 @@

 [
  (null)
+  (unreachable)
  (undefined)
 ] @constant.builtin

@@ -23,6 +23,7 @@ EnumInfo :: struct {
 TypeInfo :: union(enum) {
 	invalid void
 	void void
+	noreturn void
 	anyopaque void
 	bool void
 	integer void
@@ -242,7 +242,7 @@ module.exports = grammar({
    _type_constant: $ => seq(optional('-'), choice($.integer, $.character)),

    builtin_type: _ => choice(
-      'void', 'type', 'anyopaque', 'bool', 'int', 'uint', 'float', 'range',
+      'void', 'noreturn', 'type', 'anyopaque', 'bool', 'int', 'uint', 'float', 'range',
      'i8', 'i16', 'i32', 'i64', 'u8', 'u16', 'u32', 'u64',
      'isize', 'usize', 'f32', 'f64',
      'c_char', 'c_schar', 'c_uchar', 'c_short', 'c_ushort',
@@ -462,6 +462,7 @@ module.exports = grammar({
      $.character,
      $.boolean,
      $.null,
+      $.unreachable,
      $.undefined,
    ),

@@ -645,6 +646,7 @@ module.exports = grammar({

    boolean: _ => choice('true', 'false'),
    null: _ => 'null',
+    unreachable: _ => 'unreachable',
    undefined: _ => 'undefined',
    sink: _ => '_',

@@ -653,10 +655,10 @@ module.exports = grammar({
      alias(choice(
        'test', 'func', 'c_func', 'struct', 'c_struct', 'opaque', 'union', 'enum',
        'distinct', 'alias', 'import', 'hide', 'return', 'try', 'catch', 'mut',
-        'null', 'undefined', 'orelse', 'and', 'or', 'xor', 'if', 'while', 'for',
+        'null', 'unreachable', 'undefined', 'orelse', 'and', 'or', 'xor', 'if', 'while', 'for',
        'expand', 'break', 'continue', 'defer', 'errdefer', 'yield', 'match', 'else',
        'true', 'false',
-        'void', 'type', 'anyopaque', 'bool', 'int', 'uint', 'float', 'range',
+        'void', 'noreturn', 'type', 'anyopaque', 'bool', 'int', 'uint', 'float', 'range',
        'i8', 'i16', 'i32', 'i64', 'u8', 'u16', 'u32', 'u64',
        'isize', 'usize', 'f32', 'f64',
        'c_char', 'c_schar', 'c_uchar', 'c_short', 'c_ushort',
@@ -17,6 +17,7 @@

 [
  (null)
+  (unreachable)
  (undefined)
 ] @constant.builtin

@@ -1577,6 +1577,10 @@
          "type": "STRING",
          "value": "void"
        },
+        {
+          "type": "STRING",
+          "value": "noreturn"
+        },
        {
          "type": "STRING",
          "value": "type"
@@ -3215,6 +3219,10 @@
          "type": "SYMBOL",
          "name": "null"
        },
+        {
+          "type": "SYMBOL",
+          "name": "unreachable"
+        },
        {
          "type": "SYMBOL",
          "name": "undefined"
@@ -5003,6 +5011,10 @@
      "type": "STRING",
      "value": "null"
    },
+    "unreachable": {
+      "type": "STRING",
+      "value": "unreachable"
+    },
    "undefined": {
      "type": "STRING",
      "value": "undefined"
@@ -5094,6 +5106,10 @@
                  "type": "STRING",
                  "value": "null"
                },
+                {
+                  "type": "STRING",
+                  "value": "unreachable"
+                },
                {
                  "type": "STRING",
                  "value": "undefined"
@@ -5170,6 +5186,10 @@
                  "type": "STRING",
                  "value": "void"
                },
+                {
+                  "type": "STRING",
+                  "value": "noreturn"
+                },
                {
                  "type": "STRING",
                  "value": "type"
@@ -882,6 +882,10 @@
        {
          "type": "undefined",
          "named": true
+        },
+        {
+          "type": "unreachable",
+          "named": true
        }
      ]
    }
@@ -2356,6 +2360,11 @@
      ]
    }
  },
+  {
+    "type": "unreachable",
+    "named": true,
+    "fields": {}
+  },
  {
    "type": "variable_declaration",
    "named": true,
@@ -2924,6 +2933,10 @@
    "type": "mut",
    "named": false
  },
+  {
+    "type": "noreturn",
+    "named": false
+  },
  {
    "type": "null",
    "named": false
@@ -3004,6 +3017,10 @@
    "type": "union",
    "named": false
  },
+  {
+    "type": "unreachable",
+    "named": false
+  },
  {
    "type": "usize",
    "named": false
@@ -22,3 +22,9 @@ clear func($K, $V type) void {

 Generated :: alias struct_type!(.auto, {"value"}, {i32}, {null})
 #                  ^^^^^^^^^^^^ function.builtin
+
+fail func() noreturn {
+#           ^^^^^^^^ type.builtin
+  unreachable
+# ^^^^^^^^^^^ constant.builtin
+}