fix: type guard narrowing for union functions

[lewis6991]

Fixes #696

[gemini-code-assist]

Summary of Changes

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This pull request resolves a bug in the type inference system where type guards for union types, particularly those involving functions, were not correctly narrowing the types. Previously, this could lead to erroneous ParamTypeMismatch diagnostics. The changes introduce a crucial check to ensure that if a union type is narrowed down to an empty set of types, the narrowing process correctly indicates no valid type, thereby preventing incorrect type assertions and improving the accuracy of the code analysis.

Highlights

• Fix Type Guard Narrowing for Union Functions: This pull request addresses an issue where type guards for union types, specifically those involving functions, were not correctly narrowing the types. This led to false positive ParamTypeMismatch diagnostics.
• Improved Union Type Narrowing Logic: The type narrowing logic has been enhanced to correctly handle scenarios where a union type becomes empty after narrowing. A check if union_types.is_empty() { return None; } was added to ensure that if all types in a union are filtered out, the result correctly indicates no valid type.
• New Test Case Added: A dedicated test case, test_issue_696, has been introduced to reproduce and validate the fix for this specific type guard narrowing problem, ensuring no regressions occur.

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Changelog

• crates/emmylua_code_analysis/src/diagnostic/test/param_type_check_test.rs
  • Added test_issue_696 to specifically test the scenario where a union type containing a function is narrowed by a type guard, ensuring no ParamTypeMismatch is reported when the types are compatible.
• crates/emmylua_code_analysis/src/semantic/infer/narrow/narrow_type/mod.rs
  • Introduced a check if union_types.is_empty() { return None; } after filtering union types. This ensures that if all types in a union are filtered out during narrowing, the result is None, indicating no valid type, rather than an an empty union. This applies to both LuaType::Union and LuaType::MultiLineUnion processing.

Activity

• The pull request was created by lewis6991 to address issue bad type narrow with type() #696, which describes the type guard narrowing problem for union functions.

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Code Review

This pull request correctly fixes a bug in type narrowing for union types. Previously, when narrowing a union type where no member types matched the target, the operation would incorrectly result in a nil type instead of failing. This could lead to incorrect type inference downstream. The fix ensures that the narrowing operation returns None in such cases, which is the correct behavior. A new test case is added to verify the fix and prevent future regressions. The changes are logical and well-tested. I have added a couple of minor suggestions to improve code conciseness by using more idiomatic Rust.