sx/issues/0070-forward-alias-global-annotation-before-fixpoint.md

0070 — forward alias in top-level global annotation reaches LLVM verifier

RESOLVED. Root cause: issue 0069's resolveForwardIdentifierAliases fixpoint runs at the END of Lowering.scanDecls, but the same scan loop resolved top-level var_decl global annotations (and typed module-constant annotations) via self.resolveType(ta) BEFORE that fixpoint ran — so a forward alias (A :: B; B :: i32; g : A = 7;) was still absent from type_alias_map, resolveType fabricated an empty-struct stub, and the global got a type mismatching its initializer at LLVM verification (the typed-const path silently mistyped the constant instead). Fix: split scanDecls into two passes. Pass 1 registers function/type/alias facts; then resolveForwardIdentifierAliases converges the aliases; then pass 2 registers top-level var_decl globals (registerTopLevelGlobal) and typed module constants (registerTypedModuleConst), so their annotations resolve against the converged alias map. Globals/typed-consts can't be named in a type position, so deferring them past type/alias registration is order-safe; the untyped module-const branch (no annotation to resolve) stays in pass 1. Regression: examples/0133-types-forward-alias-global.sx.

Symptom

A forward identifier type alias used as a top-level global's type annotation does not resolve before the global is registered, producing an LLVM verifier failure instead of compiling as the alias target type.

Observed:

LLVM verification failed: Global variable initializer type does not match global variable type!
ptr @g

Expected: A :: B; B :: i32; g : A = 7; should type g as i32 and compile/run the same way as the ordered alias form.

Reproduction

A :: B;
B :: i32;

g : A = 7;

main :: () -> i32 {
    return g;
}

Run:

./zig-out/bin/sx run .sx-tmp/probe-0069-forward-alias-global.sx

The repro is standalone; the inline source above is sufficient to recreate the scratch file under .sx-tmp/.

Investigation prompt

Fix issue 0070: a forward identifier type alias used in a top-level global annotation must resolve before that global's type is registered.

Context:

• Issue 0069 (49a383d) added Lowering.resolveForwardIdentifierAliases, a fixpoint post-pass at the end of scanDecls, to resolve top-level identifier-RHS aliases like A :: B; B :: i32;.
• That works for aliases used later in function bodies because the A2.4 unknown-type pass and body lowering run after scanDecls.
• But top-level var_decl annotations are resolved inside the same scanDecls loop before resolveForwardIdentifierAliases(decls) is called. So g : A = 7; can be typed while A is still absent from ProgramIndex.type_alias_map.
• Suspected area: src/ir/lower.zig, Lowering.scanDecls, especially the ordering between .const_decl alias collection, the new resolveForwardIdentifierAliases, and the .var_decl branch that calls self.resolveType(ta).

Likely fix:

• Split the scan ordering so all top-level type declarations and identifier aliases converge before any top-level global annotation is resolved.
• One possible shape: first scan/register function/type/alias facts, run the forward-alias fixpoint, then handle top-level var_decl global registration and literal module constants that require resolved annotation types.
• Do not reintroduce issue 0068: NotAType :: 123; v: NotAType must still emit unknown type 'NotAType'.
• Do not fabricate stubs while trying to resolve the forward alias. The alias facts should still come from ProgramIndex.type_alias_map and real TypeTable.findByName hits.

Verification:

• Add a focused regression, likely in the 01xx types block:

A :: B;
B :: i32;
g : A = 7;
main :: () -> i32 { return g; }

• Keep examples/0132-types-forward-type-alias.sx, examples/0116-types-type-alias-size-align.sx, examples/0201-generics-generic-struct.sx, and examples/1117-diagnostics-value-const-as-type-rejected.sx green.
• Run:

zig build
zig build test
bash tests/run_examples.sh

Expected result: the forward-alias global program exits 7, issue 0068 remains rejected with a diagnostic, and the full suite passes.