Fix ResolveToken for TypeSpec with MVAR

[josesimoes]

Description

• Now checking if caller or caller generic type is null.

Motivation and Context

How Has This Been Tested?

• Running MDP unit tests and sample app with implementation of Span<T>.
• Tested against Merge generics support in mscorlib CoreLibrary#245
  [build with MDP buildId 56369]

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 stability by adding additional checks when resolving certain generic method parameters, preventing potential errors in specific scenarios.

[coderabbitai]

Walkthrough

A null check was added in the CLR_RT_TypeDef_Instance::ResolveToken method for the TBL_TypeSpec case, ensuring that both the caller pointer and its genericType member are non-null before resolving a generic method parameter. The method now returns false if this check fails.

Changes

File(s)	Change Summary
src/CLR/Core/TypeSystem.cpp	Added null checks for caller and caller->genericType in ResolveToken method before resolving generic method parameters.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant TypeSystem

    Caller->>TypeSystem: ResolveToken(token, caller)
    alt token is TBL_TypeSpec and datatype is DATATYPE_MVAR
        TypeSystem->>TypeSystem: Check if caller and caller.genericType are not null
        alt Either is null
            TypeSystem-->>Caller: return false
        else Both are valid
            TypeSystem->>TypeSystem: Proceed with resolving generic method parameter
        end
    else Other cases
        TypeSystem->>TypeSystem: Handle as before
    end
    TypeSystem-->>Caller: return result

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📜 Recent review details

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Reviewing files that changed from the base of the PR and between e0c3cad and 0f2cd5a.

📒 Files selected for processing (1)

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

🧰 Additional context used

🧠 Learnings (2)

📓 Common learnings

Learnt from: josesimoes
PR: nanoframework/nf-interpreter#3172
File: src/CLR/Core/TypeSystem.cpp:4556-4589
Timestamp: 2025-05-14T17:33:51.546Z
Learning: When parsing TypeSpec signatures in nanoFramework, the first Advance() call consumes the VAR/MVAR token, followed by additional Advance() calls to walk to the specific generic parameter position.

Learnt from: josesimoes
PR: nanoframework/nf-interpreter#3190
File: src/CLR/Core/TypeSystem.cpp:0-0
Timestamp: 2025-06-26T09:16:55.184Z
Learning: In nanoFramework's CLR attribute parsing (src/CLR/Core/TypeSystem.cpp), the sentinel value 0xFFFF in string tokens represents a null string. When encountered, this should result in a true null reference (using SetObjectReference(nullptr)) rather than an empty string instance, and the boxing operation should be skipped via early return.

src/CLR/Core/TypeSystem.cpp (2)

undefined

<retrieved_learning>
Learnt from: josesimoes
PR: #3190
File: src/CLR/Core/TypeSystem.cpp:0-0
Timestamp: 2025-06-26T09:16:55.184Z
Learning: In nanoFramework's CLR attribute parsing (src/CLR/Core/TypeSystem.cpp), the sentinel value 0xFFFF in string tokens represents a null string. When encountered, this should result in a true null reference (using SetObjectReference(nullptr)) rather than an empty string instance, and the boxing operation should be skipped via early return.
</retrieved_learning>

<retrieved_learning>
Learnt from: josesimoes
PR: #3172
File: src/CLR/Core/TypeSystem.cpp:4556-4589
Timestamp: 2025-05-14T17:33:51.546Z
Learning: When parsing TypeSpec signatures in nanoFramework, the first Advance() call consumes the VAR/MVAR token, followed by additional Advance() calls to walk to the specific generic parameter position.
</retrieved_learning>

⏰ Context from checks skipped due to timeout of 90000ms (5)

• GitHub Check: nf-interpreter (Check_Code_Style)
• GitHub Check: nf-interpreter (Nightly build) (Build_STM32_targets ORGPAL_PALX)
• GitHub Check: nf-interpreter (Nightly build) (Build_STM32_targets ORGPAL_PALTHREE)
• GitHub Check: nf-interpreter (Nightly build) (Check_Build_Options)
• GitHub Check: nf-interpreter (Check_Build_Options)

🔇 Additional comments (1)

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

1049-1055: LGTM! Good defensive programming practice.

This null check addition mirrors the existing pattern used for the DATATYPE_VAR case above and prevents potential null pointer dereferences when resolving generic method parameters. The change improves code safety and consistency between VAR and MVAR handling.

✨ Finishing Touches

• 📝 Generate Docstrings

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[josesimoes]

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