build cache: cache hits drop generic local ABI type definitions

[cpunion]

Summary

A package can build, link, and run successfully from a forced cold build, then fail to link on the immediately following LLGo package-cache hit. The cached path loses definitions for generic local runtime type descriptors while the synthetic root module still references them from init$abitypes$array.

This is reproducible with build parallelism fixed at one, so it is not explained by concurrent package workers.

Discovered while validating #2296, but reproduced unchanged on main without that PR.

Reproducer

At main commit c454c0e65b7161c833da54643968b06eede93660:

go build -o /tmp/llgo-cache-repro ./cmd/llgo

# Force a serial rebuild while leaving cache publication enabled: PASS.
LLGO_ROOT="$PWD" /tmp/llgo-cache-repro test \
  -a -p=1 -count=1 -timeout=5m \
  -run "^TestGeneric(Local|Nested)" ./test/go

# Same source and serial build setting, now loading the package cache: link failure.
LLGO_ROOT="$PWD" /tmp/llgo-cache-repro test \
  -p=1 -count=1 -timeout=5m \
  -run "^TestGeneric(Local|Nested)" ./test/go

The second command reports missing symbols such as:

undefined symbol: *_llgo_github.com/goplus/llgo/test/go.local[string].p7240776
undefined symbol: *_llgo_github.com/goplus/llgo/test/go.local[int].p7240776
undefined symbol: *_llgo_github.com/goplus/llgo/test/go.T[int;int].p7241770
undefined symbol: *_llgo_github.com/goplus/llgo/test/go.T[int;github.com/goplus/llgo/test/go.U[int;int]·3].p7241770

All are referenced by the generated init$abitypes$array in the synthetic main object.

Disabling LLGo package-cache reads and writes makes the same source pass:

LLGO_BUILD_CACHE=off LLGO_ROOT="$PWD" /tmp/llgo-cache-repro test \
  -p=1 -count=1 -timeout=5m \
  -run "^TestGeneric(Local|Nested)" ./test/go

Additional checks

• -a -p=1 followed by -p=1 reproduces on unmodified main.
• A different test selector reproduces the same link failure because the entire test/go package is linked; running the generic-local tests directly confirms the symbols involved.
• A cold build with LLGO_BUILD_CACHE=off passes the full focused finalizer and reflect test set.

Expected behavior

The cache-hit build should contain the same ABI type definitions as the cold build and link successfully. Package build parallelism should not change cache correctness.

Environment

• Darwin 25.2.0 arm64
• Go 1.26.5 darwin/arm64
• LLGo (devel) darwin/arm64
• Homebrew Clang 19.1.7

Workaround

Use LLGO_BUILD_CACHE=off or force every build with -a until cache restoration preserves the synthesized type definitions.

[cpunion]

Bisect result and causal check:

• last good: 0531ea2e0 (ssa: distinguish generic local runtime types)
• first bad: 853895d04 (cl: address generic local type review feedback)
• these commits are adjacent in the ssa: distinguish generic local runtime types #1819 branch

Using Go 1.24.11 and the same minimal generic-local llgo test fixture:

Commit	forced cold build	immediate cache hit
0531ea2e0	PASS	PASS
853895d04	PASS	link failure

The relevant change in 853895d04 is:

-obj := types.NewTypeName(token.NoPos, t.Obj().Pkg(), ...)
+obj := types.NewTypeName(t.Obj().Pos(), t.Obj().Pkg(), ...)

The synthesized TypeName is detached from the package scope. ssa/abi.scopeIndices therefore cannot derive stable scope indices and falls back to .p<token.Pos>. Raw token.Pos values change across package-loading invocations. A cold build creates definitions and references with the same value, but the cached archive keeps the previous .p... definitions while the next synthetic main generates references with a new .p... value.

Causal validation on current main (c454c0e65): changing only that synthesized type position back to token.NoPos made both the cold build and immediate cache hit pass. The repository existing TestGenericLocalRuntimeTypesIncludeOuterArgs and TestGenericNestedLocalRuntimeTypeNames also passed on both runs, so the one-line experiment preserved the current identity/name assertions.

No fix has been committed as part of #2296.