tagged unions

compiler/checker/checker.odin

@@ -1048,6 +1048,25 @@ validate_type_nodes :: proc(checker: ^Checker) {
 					)
 				}
 			}
+			// A tagged union (`item.child` set) must tag with an enum, and each variant
+			// must name a member of it. The synthesized `union(enum)` tag satisfies this
+			// by construction; the check guards the explicit `union(Enum)` form.
+			if item.kind == .Union && types.is_valid(item.child) {
+				if !types.is_enum(item.child, &checker.module.types) {
+					source.add(checker.diagnostics, source.Span{}, "a tagged union's tag must be an enum")
+				} else {
+					for field in types.fields_for(&checker.module.types, id) {
+						if _, ok := find_enum_member(checker, item.child, symbol.Id(field.name)); !ok {
+							source.addf(
+								checker.diagnostics,
+								source.Span{},
+								"union variant '%s' is not a member of the tag enum",
+								symbol_text(checker, symbol.Id(field.name)),
+							)
+						}
+					}
+				}
+			}
 		}
 		if item.kind == .Function {
 			if !item.c_abi {