openspec: add codegen-ir-correctness-and-gate change proposal

[devin-ai-integration]

Summary

Planning-only OpenSpec proposal (no code changes). It re-plans the next step toward a mainstream-usable Sengoo after core-language-correctness (PR #13) was merged and CI went green.

Why: building the toolchain and exercising the real sgc CLI on the toolchain this repo pins for developers (clang-14) shows the "done" core forms still fail to compile. The previous gate is green only because it compiles through the in-crate #[cfg(test)] compile_source() helper and runs under CI's clang-19.

Reproduced on clang 14.0.0 / rustc 1.94.0 (the pinned rust-toolchain.toml):

Form	Command	Observed
Array index	sgc run examples/04_array.sg	'%u_4' defined with type '[3 x i64]*' but expected 'i64*'
Array write/for	sgc run examples/05_loop.sg, conformance/03_array_write.sg	same element-pointer mismatch
Closure capture	sgc run examples/06_lambda.sg, conformance/04_closure_multi_capture.sg	'%u_5' defined with type '[1 x i64]*' but expected 'i64*'
Enum-returning fn	function with -> EnumType	@get typed { i64, [8 x i8] }(i64)* but call site expects i64(i64)*
Multi-payload match	arm Variant(b) => ... followed by any arm	parse error: invalid pattern: expected identifier
Local gate	cargo test -p sgc core_conformance_examples_compile_link_and_run	FAILED (the array case, inside the test)

Root cause: codegen states pointer operand types that disagree with the value's defined type, e.g.

%u_4 = alloca [3 x i64]                          ; %u_4 : [3 x i64]*
%u_4.elem.0 = getelementptr i64, i64* %u_4, i64 0  ; operand stated i64* — mismatch

Typed-pointer LLVM (clang <= 14) rejects this; opaque-pointer LLVM (clang >= 15, every pointer is ptr) drops the pointee type so the mismatch disappears and the address still computes correctly. CI uses clang-19, so it never sees the error. Separately, the match-arm parser only terminates a Variant(bindings) pattern correctly when it is the last arm; every committed conformance example places its single payload arm last, hiding the defect.

What the proposal owns

• Emit type-consistent IR for array places, closure-captured slots, and aggregate (enum/struct) function results — the type a value is defined with and the type it is used with must agree and pass the pinned toolchain's verifier.
• Fix the multi-payload match parse so a payload-binding arm parses in first/middle/last position.
• Make the conformance gate drive the real sgc CLI (sgc build/sgc run), not the in-crate helper, and add the two forms the old gate could not catch (multi-payload match, enum-returning fn).
• Pin the LLVM/clang contract: CI and the developer blueprint share one major version; sgc reports a clear error below contract instead of a raw IR verifier error.

This is a correctness/gate-integrity change; it explicitly does not add traits/Option-Result/as-casts/generic collections (those are separate proposed directions). For fairness, let mut, immutable-assignment rejection, and match exhaustiveness from core-language-correctness were independently verified as genuinely working.

Contents (openspec/changes/codegen-ir-correctness-and-gate/): proposal.md, design.md (reproductions + root cause + two acceptable pointer-model fixes), tasks.md (7 phases + archive gate), specs/.../spec.md (4 ADDED requirements, WHEN/THEN scenarios). openspec validate codegen-ir-correctness-and-gate --strict passes (CLI @fission-ai/openspec 1.4.1).

Link to Devin session: https://app.devin.ai/sessions/01c0d55188c64bef8f68fbe3c905738f
Requested by: @Hyper66666

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