Fix typed select pushing Type::Any instead of expected type

[sumleo]

Summary

• The typed select instruction's type checker validates operands against the expected type but forgets to set result_type, leaving it as Type::Any.
• Fix adds result_type = expected[0] in the typed select branch.

Details

In TypeChecker::OnSelect (type-checker.cc), result_type is initialized to Type::Any. The untyped select path (when expected is empty) correctly sets result_type = type1 on line 1012. However, the typed select path (when expected is non-empty) validates both operands against expected[0] but never assigns result_type.

The typed select instruction (select t) exists specifically to provide precise type information for cases where the operand types alone are insufficient (e.g., reference types). Pushing Type::Any as the result type defeats this purpose and can cause downstream type checking to be less precise than intended.

The fix adds result_type = expected[0] after the CheckType calls in the typed select branch, mirroring the pattern used in the untyped path.

Tests

• typed-select-result-type.txt: Verifies that typed select with funcref/externref annotations properly propagates the result type, allowing the result to be used in contexts expecting that specific reference type (e.g., global.set to a funcref global, return from an externref function).
• bad-typed-select-type-mismatch.txt: Verifies that using a typed select (result funcref) where externref is expected correctly produces a type mismatch error.

[zherczeg]

This looks valid. Even without GC there are some reference types, so it is possible to add a test for this. With GC, unknown reference types in dead code could be an interesting test as well.

[sbc100]

@zherczeg should we land this now or ask for a test for be added?

[sumleo]

Added two tests:

• test/typecheck/typed-select-result-type.txt — positive test that typed select with funcref and externref annotations properly propagates the result type (e.g., storing the result in a (mut funcref) global, returning it from an externref function).
• test/typecheck/bad-typed-select-type-mismatch.txt — negative test that using a typed select (result funcref) where externref is expected correctly produces a type mismatch error.