fixes #23765; Method calling proc with return value overlapping decla…

[ringabout]

…red generic type generates invalid C

fixes #2376

This pull request refines type comparison logic in the compiler to be more precise by considering backend alias types, and adds a regression test for a previously reported bug. The main changes are grouped below:

Compiler: Improved Type Comparison

• Updated sameInstantiation in compiler/seminst.nim to include the PickyBackendAliases flag when comparing concrete types, ensuring backend type aliases are treated strictly during instantiation checks.
• Updated searchInstTypes in compiler/semtypinst.nim to add the PickyBackendAliases flag when comparing generic parameters, improving correctness for type searches involving backend aliases.

Testing: Regression Coverage

• Added a test case for bug Method calling proc with return value overlapping declared generic type generates invalid C #23765 in tests/ccgbugs2/tcodegen.nim, covering type instantiation and backend alias handling in generic and method contexts.

[Copilot]

Pull request overview

This pull request tightens the compiler’s type-instantiation equality logic so that backend alias types are compared more strictly, preventing incorrect cache hits that can lead to invalid C code generation in certain generic/method scenarios (regression: #23765).

Changes:

• Updated sameInstantiation (generic proc instantiation cache) to compare concrete types with PickyBackendAliases.
• Updated searchInstTypes (generic type instantiation cache lookup) to compare generic parameters with PickyBackendAliases.
• Added a C/C++ codegen regression test reproducing issue #23765.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/ccgbugs2/tcodegen.nim	Adds regression coverage for #23765 (generic instantiation + method call leading to invalid C previously).
compiler/semtypinst.nim	Makes cached generic type instantiation lookup stricter by considering backend alias types.
compiler/seminst.nim	Makes generic proc instantiation equivalence stricter by considering backend alias types.

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