# 0071 — global initialized from module const silently zero-initializes

> **RESOLVED.** Root cause: `Lowering.registerTopLevelGlobal`'s init_val switch
> serialized only literal / array-literal / struct-literal initializers; an
> identifier initializer (`g : A = K;`) fell through to `else => null`, so the
> global was emitted with no payload and silently zero-initialized.
> Fix: extracted the initializer serialization into `Lowering.globalInitValue`
> and added an `.identifier` arm that materializes the global's static value from
> `ProgramIndex.module_const_map` (typed module consts are registered in the same
> pass-2 just before, via `registerTypedModuleConst`). An identifier that names no
> usable constant now emits a diagnostic instead of silently zeroing. Other
> initializer shapes (enum-literal shorthand, etc.) keep their established
> static-lowering behavior — this pass only closes the identifier/module-const
> hole. Regression: `examples/0134-types-global-init-from-module-const.sx`
> (`g=42` / exit 42).

## Symptom

A top-level global initialized from a module constant compiles but is
zero-initialized instead of receiving the constant's value.

Observed:

```text
g=0
```

Expected: `g` should be initialized to `42`, or the compiler should reject the
initializer loudly if identifier/module-const global initializers are not
supported.

## Reproduction

```sx
#import "modules/std.sx";

A :: B;
B :: i32;

K : A : 42;
g : A = K;

main :: () -> i32 {
    print("g={}\n", g);
    return g;
}
```

Run:

```sh
./zig-out/bin/sx run .sx-tmp/probe-0070-global-init-from-const.sx
```

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

## Investigation prompt

Fix issue 0071: a top-level global initialized from a module constant must not
silently become zero.

Context:
- This surfaced during Codex re-review of `932cdfa`, the issue-0070 fix.
- `932cdfa` correctly defers top-level global and typed-module-const annotation
  resolution until after the forward-alias fixpoint.
- The remaining bug is in the global initializer path, not the annotation path:
  `K : A : 42; g : A = K;` resolves `A` correctly, registers `K` in
  `ProgramIndex.module_const_map`, but `g` is emitted as zero.

Suspected area:
- `src/ir/lower.zig`, `Lowering.registerTopLevelGlobal`.
- Its `init_val` switch serializes literal / array / struct-literal initializers,
  but an identifier initializer falls through to `else => null`, and the global
  is emitted with no initializer payload. That silently becomes zero-initialized.
- Related facts: `ProgramIndex.module_const_map` already records typed module
  constants via `registerTypedModuleConst`, now in pass 2 after alias convergence.

Likely fix:
- Add explicit handling for identifier initializers that name a module constant,
  converting the recorded constant value into the global's `ConstantValue` with
  the global's declared type.
- If some initializer shape cannot be represented as a global constant yet, emit
  a diagnostic instead of returning `null` / zero-initializing.
- Do not regress issue 0070: `A :: B; B :: i32; g : A = 7;` and
  `K : A : 35;` must still resolve through the converged alias map.
- Preserve literal, array literal, struct literal, and extern-global behavior.

Verification:
- Add a focused regression, likely in the `01xx` types block:

```sx
#import "modules/std.sx";
A :: B;
B :: i32;
K : A : 42;
g : A = K;
main :: () -> i32 { print("g={}\n", g); return g; }
```

- Keep these green:
  - `examples/0133-types-forward-alias-global.sx`
  - `examples/0132-types-forward-type-alias.sx`
  - `examples/0116-types-type-alias-size-align.sx`
  - `examples/0201-generics-generic-struct.sx`
  - `examples/1117-diagnostics-value-const-as-type-rejected.sx`
- Run:

```sh
zig build
zig build test
bash tests/run_examples.sh
```

Expected result: the new regression prints `g=42` and exits `42`; unsupported
global initializer shapes no longer silently zero-initialize; the full suite
passes.
