sx/issues/0062-generic-failable-return-not-monomorphized.md

0062 — generic function with a value-carrying ! return miscompiles

✅ RESOLVED (2026-06-01) — NOT A BUG (invalid repro syntax). The repro used the non-generic form (T: type, …) / (T: Type, …) — a plain value param of type Type, NOT a generic type parameter. Per specs.md (the $ sigil introduces a generic type parameter), a function generic type param must be $T: Type. With the correct form, generic value-carrying failable composition (ERR E5.1 sub-feature 8) works fully:

wrap :: ($T: Type, f: Closure() -> (T, !E)) -> (T, !E) { return try f(); }
wrap(s32, closure(() -> (s32, !E) { return 7; })) catch e -1   // 7
r, err := wrap(s32, closure(() -> (s32, !E) { return 9; }))    // r=9
wrap(s32, closure(() -> (s32, !E) { raise error.Bad; })) catch e -1  // -1

The only real (separate, orthogonal) defect found: a NON-$ T: Type function param used as a type silently resolves to an empty {} (renders T{}) instead of erroring — tracked as issue 0064, deferred (not ERR-scoped).

Symptom

A generic function whose return type is a value-carrying failable in the generic type param — wrap :: (T: type, …) -> (T, !E) — does not substitute T in the failable return tuple during monomorphization. Observed two ways:

• Consumed via catch: LLVM verification failed: PHI node operands are not the same type as the result! — the success branch carries {} (an unsubstituted / empty value) while the handler branch carries the real success type.
• Consumed via destructure: the success value renders as T{} (the literal generic type name) instead of the concrete value, and the error slot is wrong.

Expected: T is bound to the concrete monomorphization type (s32), the success value flows through as 7, and the error slot is 0 on success.

Reproduction

#import "modules/std.sx";
E :: error { Bad }
wrap :: (T: type, f: Closure() -> (T, !E)) -> (T, !E) { return try f(); }

main :: () -> s32 {
    // catch form → LLVM phi type mismatch:
    r := wrap(s32, closure(() -> (s32, !E) { return 7; })) catch e -1;
    print("{}\n", r);          // want 7
    return 0;
}

Destructure form (same root cause, different surfacing):

r, err := wrap(s32, closure(() -> (s32, !E) { return 7; }));
print("{} {}\n", r, xx err);   // prints "T{} s64"; want "7 0"

Investigation prompt

The bug is in monomorphizing a value-carrying failable return type in src/ir/lower.zig. monomorphizeFunction (~10259) / resolveReturnType2 (~8309) resolve the return type under type_bindings ($T → concrete). For a plain -> T this works; for -> (T, !E) the value slot T of the failable tuple appears NOT to be substituted — the success value stays the unsubstituted generic type (rendering as T{} / an empty {} in IR), so lowerFailableSuccessReturn / extractSuccessValue and the try success path produce a value of the wrong type, which the catch merge phi then rejects.

Likely fix: ensure the failable-tuple return type is re-resolved through type_bindings during monomorphization (the tuple's value fields, not just a top-level $T), and that failableSuccessType / the try/catch success extraction use the substituted tuple. Verify with both repros above (catch → prints 7; destructure → prints "7 0"). This is ERR E5.1 sub-feature 8 (generic functions with ! returns); the program-wide shape-union slice deliberately excluded generic shapes pending this fix.