fix: handle untyped shift expressions in SSA build

[cpunion]

Summary

This PR fixes the SSA sanity check failure on nested shift expressions like:

value << (1 << bytes[3])

Root Cause

The issue is that go/types returns untyped int for 1 << bytes[3], but go/ssa sanity check disallows untyped instruction results, causing a panic:

Error: instruction has 'untyped' result: t2 = 1:untyped int << t1 : untyped int
panic: SanityCheck failed

This was reported upstream: golang/go#77067

Solution

Added fixUntypedShiftTypes() function to convert untyped shift types to their default type (using types.Default()) before calling prog.CreatePackage. This applies the conversion that the Go spec requires:

"If the left operand of a non-constant shift expression is an untyped constant, it is first implicitly converted to the type it would assume if the shift expression were replaced by its left operand alone."

Changes

• internal/build/build.go: Added fixUntypedShiftTypes() workaround
• test/go/shift_untyped_test.go: Added test cases

Testing

• ✅ go test ./test/go passes
• ✅ go run ./cmd/llgo test ./test/go passes

Fixes #1530

[gemini-code-assist]

Summary of Changes

Hello @cpunion, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue where nested shift expressions involving untyped constants could cause a panic during the SSA build process. The fix introduces a preprocessing step to correctly type these expressions according to Go's specification, ensuring the SSA generator receives properly typed inputs and preventing runtime failures.

Highlights

• SSA Sanity Check Fix: Addresses a panic in go/ssa that occurred with nested shift expressions where go/types returned an "untyped int" for the inner shift, violating SSA's sanity checks.
• Untyped Shift Type Conversion: Implements fixUntypedShiftTypes() to explicitly convert the type of untyped shift expression results to their default type (e.g., int) before the SSA build process, aligning with Go's implicit conversion rules.
• New Test Cases: Adds test/go/shift_untyped_test.go with specific test cases to reproduce and verify the fix for nested shift expressions, including edge cases.

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[gemini-code-assist]

Code Review

This pull request introduces a workaround for an upstream bug in go/types that causes panics in the SSA build for certain nested shift expressions. The fix involves iterating through expression types and correcting untyped shift expression results to their default types. A comprehensive set of tests has been added to cover the buggy behavior and ensure the fix is effective. The implementation is correct, but I've suggested a small refactoring to improve readability by reducing nested conditions.

[xgopilot]

Code Review Summary

Excellent fix for the SSA sanity check failure! The implementation is clean, focused, and includes comprehensive test coverage.

Key Strengths:

• Clear problem documentation with upstream issue reference
• Well-structured test cases covering the original bug and edge cases
• Minimal, targeted workaround approach

Critical Issue:

• Map modification during iteration in fixUntypedShiftTypes - should use two-pass approach

Documentation Improvements:

• Add Go spec quote to function documentation
• Fix "pattern2" reference in test comments

No security or performance concerns identified.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.04%. Comparing base (bba9538) to head (f88f80d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1531   +/-   ##
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  Coverage   91.04%   91.04%           
=======================================
  Files          45       45           
  Lines       11999    11999           
=======================================
  Hits        10924    10924           
  Misses        899      899           
  Partials      176      176           

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