Fixes for type descriptor with generics

[josesimoes]

Description

• Initializing a type descriptor from a signature token now properly handles a VAR in the signature (now uses the assembly from the caller generic type).
• Checking the match for type descriptors now looks into the correct type handler.

Motivation and Context

How Has This Been Tested?

• Corelib unit tests and new Sys.Collection unit tests with support for generic types.
• Tested against Merge generics support in mscorlib CoreLibrary#245.
  [build with MDP buildId 57148]

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue with code or algorithm)
• New feature (non-breaking change which adds functionality to code)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dev Containers (changes related with Dev Containers, has no impact on code or features)
• Dependencies/declarations (update dependencies or assembly declarations and changes associated, has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist

• My code follows the code style of this project (only if there are changes in source code).
• My changes require an update to the documentation (there are changes that require the docs website to be updated).
• I have updated the documentation accordingly (the changes require an update on the docs in this repo).
• I have read the CONTRIBUTING document.
• I have tested everything locally and all new and existing tests passed (only if there are changes in source code).

Summary by CodeRabbit

• Bug Fixes
  • Improved detection of closed generic classes during matching, reducing runtime mismatches and boxing-related errors.
  • Corrected resolution of generic type parameters across assemblies to prevent incorrect type selection.
  • Fewer edge-case failures when using generics in user code and libraries.
  • Enhances compatibility and stability with third-party libraries that heavily use generics.

[coderabbitai]

Walkthrough

Adjusts generic handling in the runtime: updates TypeSpec VAR resolution to use the caller’s generic assembly context, and modifies TypeDescriptorsMatch to promote CLASS to GENERICINST based on NANOCLR_INDEX_IS_VALID for the generic handler.

Changes

Cohort / File(s)	Summary
Generic type resolution (TypeSpec VAR) src/CLR/Core/TypeSystem.cpp	Resolves VAR (!T) via the caller’s generic type assembly context instead of the current assembly; delegates FindGenericParamAtTypeSpec to g_CLR_RT_TypeSystem.m_assemblies[caller->genericType->Assembly() - 1].
Type match promotion (CLASS→GENERICINST) src/CLR/Core/CLR_RT_HeapBlock.cpp	In TypeDescriptorsMatch, changes promotion condition for CLASS to GENERICINST from checking handler token non-empty to NANOCLR_INDEX_IS_VALID(actualType.m_handlerGenericType).

Sequence Diagram(s)

sequenceDiagram
  autonumber
  actor Caller
  participant TypeSystem
  participant TypeSpec
  participant Assemblies as g_CLR_RT_TypeSystem.m_assemblies[caller.genericType.Assembly()-1]

  Caller->>TypeSystem: Resolve TypeSpec
  TypeSystem->>TypeSpec: Parse next element
  alt VAR (!T) encountered
    TypeSystem->>Assemblies: FindGenericParamAtTypeSpec(...)
    Assemblies-->>TypeSystem: Generic param mapping (concrete type)
    TypeSystem-->>Caller: Resolved type (from caller's generic context)
  else Non-VAR path
    TypeSystem-->>Caller: Resolve via existing flow
  end

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sequenceDiagram
  autonumber
  participant Matcher as TypeDescriptorsMatch
  participant Actual as actualType
  participant Expected as expectedType

  Matcher->>Actual: Inspect DataType
  alt actualDataType == DATATYPE_CLASS and handler index valid
    note right of Matcher: NANOCLR_INDEX_IS_VALID(Actual.m_handlerGenericType)
    Matcher->>Matcher: Promote CLASS -> GENERICINST
  end
  Matcher->>Expected: Proceed with generic-inst matching
  Matcher-->>Matcher: Return match result

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The title clearly indicates that the pull request addresses fixes in the type descriptor logic related to generics, which aligns with the main changes to descriptor matching and generic parameter resolution in the codebase. It is concise and directly references the core area modified without unnecessary noise.

✨ Finishing touches

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• Post copyable unit tests in a comment

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Reviewing files that changed from the base of the PR and between f187be0 and a2dd00f.

📒 Files selected for processing (2)

• src/CLR/Core/CLR_RT_HeapBlock.cpp (1 hunks)
• src/CLR/Core/TypeSystem.cpp (1 hunks)

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🔇 Additional comments (1)

src/CLR/Core/CLR_RT_HeapBlock.cpp (1)

794-794: LGTM! Improved generic type validation.

The change from direct field comparison (actualType.m_handlerGenericType.data != CLR_EmptyToken) to using the NANOCLR_INDEX_IS_VALID macro is a solid improvement. This provides better encapsulation and likely includes additional validation beyond just checking for an empty token, ensuring that closed-generic types boxed as CLASS are correctly promoted to GENERICINST for proper type matching.

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[josesimoes]

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