feat: expand composed error-set inline match arms

README.md

@@ -122,13 +122,13 @@ optional `if`/`while` captures also execute in type-producing bodies: `if count
 
 type-producing bodies also execute scalar/enum `match` statements and type-valued match expressions in initializers, assignments and returns, such as `return match wide { true: i16, false: i8 }`. selected expression arms may themselves be matches. an ordinary checked IR match returns the selected arm index, preserving subject/pattern typing, range boundaries, overlap, unreachable-arm and exhaustiveness diagnostics without checking unexecuted bodies. statement arms retain scoped locals, assignments, early returns and enclosing-loop break/continue; expression selection shares the type-control step bound.
 
-matches also select nested types, array lengths, generic arguments, callback signatures and field defaults in type-producing bodies. for example, `return struct { values [2](match wide { true: i16, false: i8 }) }` generates concrete storage without elaborating the unselected arm. snapshots preserve loop-local type/value bindings, selected literals retain their spelling and source provenance, and nominal results are cached before their recursive fields are expanded. deferred type selection shares the producer's execution bound; failed elaboration retains its original diagnostic without leaving unchecked generated types behind. general type-valued execution and inline-match syntax remain open.
+matches also select nested types, array lengths, generic arguments, callback signatures and field defaults in type-producing bodies. for example, `return struct { values [2](match wide { true: i16, false: i8 }) }` generates concrete storage without elaborating the unselected arm. snapshots preserve loop-local type/value bindings, selected literals retain their spelling and source provenance, and nominal results are cached before their recursive fields are expanded. deferred type selection shares the producer's execution bound; failed elaboration retains its original diagnostic without leaving unchecked generated types behind. general type-valued execution remains open.
 
 tagged-union matches in type-producing bodies support immutable by-value payload captures in statements, expressions and nested type/storage positions. for example, `match choice { .count |length|: return [length]i16, .empty: return bool }` uses a captured `usize` to generate storage. grouped variants require identical nonvoid payload types; invalid captures in unselected arms are still diagnosed. selected payloads become owned typed constants, preserving narrow integers, nested aggregates and strings across generated lengths/defaults, loop snapshots and repeatable compilation. pointer/slice/callback-bearing active payloads retain the constant-materialization escape boundary below.
 
 ordinary procedures may compute integer, boolean, float and string compile-time arguments and return nested records/tuples, fixed arrays, tagged unions and explicit fallible results from constant inputs, such as `Fixed(i16, length(2))`. calls reuse the ordinary checker, typed IR and interpreter, preserving local type inference, checked arithmetic, mutable locals, range/while loops, break/continue and recursive or specialized helper calls. aggregate and fallible arguments receive their declared context, including dependent `[N]T` parameters; cached specializations compare normalized values, not literal spelling or evaluation identity. returned constants become owned source literals before temporary interpreter storage is released, preserving by-value copies, exact integer types, string bytes, enum/optional fields and IEEE float bits. tagged unions retain their actual discriminant and only the active payload, including nested aggregates and void/zero-cell variants. fallible constants retain success/error identity, composed or payload-bearing errors and void success; `catch` can consume them in generated lengths, defaults and runtime expressions.
 
-`$(expression)` explicitly evaluates a constant expression, including an ordinary procedure call, even in a package without templates. use a typed expression when its shape cannot be inferred independently, such as `$(Point { x = 40 })`; integer literals retain contextual typing. the resulting value works in runtime expressions, bindings, expression statements and array lengths, while the erased expression still receives ordinary checking. each interpreter evaluation is isolated and limited to 100,000 IR instructions/terminators and 128 active frames; failures retain their originating source locations. runtime parameters and mutable local/global dependencies cannot supply compile-time inputs. data pointers, slices, callbacks and untagged unions cannot escape temporary evaluation as generated constants; active payloads or aggregate fields containing unsupported values are diagnosed rather than retaining interpreter handles. untagged unions lack an active-member identity, so selecting a source variant could discard observable bytes. executed foreign calls remain interpreter traps. general type-valued execution, inline-match syntax, metadata reflection and existing generic-library acceptance remain open.
+`$(expression)` explicitly evaluates a constant expression, including an ordinary procedure call, even in a package without templates. use a typed expression when its shape cannot be inferred independently, such as `$(Point { x = 40 })`; integer literals retain contextual typing. the resulting value works in runtime expressions, bindings, expression statements and array lengths, while the erased expression still receives ordinary checking. each interpreter evaluation is isolated and limited to 100,000 IR instructions/terminators and 128 active frames; failures retain their originating source locations. runtime parameters and mutable local/global dependencies cannot supply compile-time inputs. data pointers, slices, callbacks and untagged unions cannot escape temporary evaluation as generated constants; active payloads or aggregate fields containing unsupported values are diagnosed rather than retaining interpreter handles. untagged unions lack an active-member identity, so selecting a source variant could discard observable bytes. executed foreign calls remain interpreter traps. general type-valued execution, metadata reflection and existing generic-library acceptance remain open.
 
 `if $(condition) { ... } else { ... }` evaluates a boolean guard before expanding or checking either body, including calls to ordinary constant-input procedures. concrete generic specializations also select branches whose guards use constants, immutable constant bindings, type comparisons or substituted compile-time parameters. only the selected body is specialized and checked; both bodies are still parsed and name-resolved. selected bodies retain their scope, cleanup and return behavior, and erased guards retain ordinary type checking and source diagnostics. ordinary calls in an implicit guard stay runtime; use `$()` to request their evaluation. optional captures remain runtime, and ordinary nongeneric `if (true)` still checks both bodies.
 
@@ -192,7 +192,9 @@ untagged `Bits :: union { integer u64, real f64 }` overlays its members in share
 
 `match value { .number |n|: n + 2, .empty: 0 }` selects one arm after evaluating its subject once. matches support enums, tagged unions, composed error sets, numeric/boolean scalars, comma-separated patterns, numeric ranges, final `else` arms, arm-local payload captures, statement bodies and expression results. checking rejects missing coverage, duplicate/overlapping patterns, unreachable arms and incompatible captures/results. block arms can return or transfer loop control; yielding value blocks and pointer captures belong to later gates.
 
-a final `inline |tag|:` arm on a nominal enum, or `inline |payload[, tag]|:` on a tagged union, expands into one ordinary arm for each variant not covered earlier. dispatch remains runtime and evaluates the subject once. enum captures and optional union tags are immutable compile-time enum values; payloads are immutable by-value captures with each variant's exact type, including a `void` value for empty variants. either capture may be discarded with `_`. static tag matches/conditions select variant-specific operations before checking, including generic calls, nested inline arms and type-producing procedure bodies. ordinary payloads remain runtime values, and return/break/continue/deferred cleanup retain ordinary match semantics. redundant/nonfinal inline arms, invalid captures and scalar/untagged subjects are diagnosed. expansion shares the 4096-step type-control bound. composed error-set inline arms and metadata reflection remain open.
+a final `inline |tag|:` arm on an enum, or `inline |payload[, tag]|:` on a tagged union, expands into one ordinary arm for each variant not covered earlier. dispatch remains runtime and evaluates the subject once. enum captures and optional union tags are immutable compile-time enum values; payloads are immutable by-value captures with each variant's exact type, including a `void` value for empty variants. either capture may be discarded with `_`. static tag matches/conditions select variant-specific operations before checking, including generic calls, nested inline arms and type-producing procedure bodies. ordinary payloads remain runtime values, and return/break/continue/deferred cleanup retain ordinary match semantics. redundant/nonfinal inline arms, invalid captures and scalar/untagged subjects are diagnosed. expansion shares the 4096-step type-control bound.
+
+composed error sets use the same inline arms, including through `catch |err| match err { ... }`. sets containing only void variants use `inline |tag|:`; sets with any payload use `inline |payload[, tag]|:`, with void values for their empty members. nested compositions, transparent aliases and repeated members expand each remaining variant once. named struct errors use their declaration name as the variant tag, and payloads retain their exact struct type and by-value isolation. for example, `Missing | union(enum) { failed i8 }` can use `inline |payload, tag|: match tag { .failed: i16(payload), .missing: 0 }`. constant inputs and type-producing bodies also select generated storage from composed payloads. pointer captures and metadata reflection remain open.
 
 `while` repeats a block or single statement while its `bool` condition holds, re-evaluating the condition on every iteration. guards follow the same braced/braceless convention as `if`. loop bodies introduce child scopes; their locals, including locals in nested conditional arms, begin uninitialized on each scope entry.