Fix closed-generic .cctor ordering causing NullReferenceException

[josesimoes]

Description

• Add EnsureGenericCctorCompleted() to force a closed generic type .cctor to run before its first use.
• Gate CEE_NEWOBJ, static CEE_CALL, CEE_LDSFLD, CEE_LDSFLDA, and CEE_STSFLD on the new check, removed the old post-access reschedule-on-null fallback.
• Extend the gate to open declaring TypeSpecs (e.g. SZGenericArrayEnumerator<!!0>) that resolve to a closed instantiation via the caller's method generic context.
• Update docs with the generic .cctor lifecycle and call contract.

Motivation and Context

• Resolves Null reference exception thrown when using List.Add<T> Home#1821
• Related with Support for generics <T> Home#782.

How Has This Been Tested?

• Running unit tests in System.Collections.
• [tested against Merge generics support in mscorlib CoreLibrary#245].

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).

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📝 Walkthrough

Walkthrough

The interpreter centralizes generic static constructor completion in EnsureGenericCctorCompleted, invokes it for generic calls, construction, and static field access, and documents the resulting lifecycle, scheduling states, and control-flow contract.

Changes

Generic static constructor enforcement

Layer / File(s)	Summary
Demand gate and lifecycle contract src/CLR/Core/Interpreter.cpp, src/CLR/Core/CLAUDE.md	Adds EnsureGenericCctorCompleted to track generic .cctor state, schedule or push constructor frames, and document interpreter and crawler behavior.
Call and construction integration src/CLR/Core/Interpreter.cpp	Enforces generic .cctor completion before static calls and generic object construction, handling restart and reschedule results.
Static field access integration src/CLR/Core/Interpreter.cpp	Enforces completion before generic static field operations, removes null-resolution rescheduling, and narrows generic tracing output.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant ILInterpreter
  participant EnsureGenericCctorCompleted
  participant GenericCctorExecution
  participant StaticFieldResolver
  ILInterpreter->>EnsureGenericCctorCompleted: process generic call, newobj, or static field access
  EnsureGenericCctorCompleted->>GenericCctorExecution: inspect or update .cctor state
  GenericCctorExecution-->>ILInterpreter: return completion, restart, or reschedule status
  ILInterpreter->>StaticFieldResolver: resolve static field after completion

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🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the PR’s main fix for closed-generic .cctor ordering and NullReferenceException.
Description check	✅ Passed	The description is directly related to the code and doc changes in this PR.
Linked Issues check	✅ Passed	The changes address issue #1821 by ensuring generic .cctor completion before relevant list and static operations.
Out of Scope Changes check	✅ Passed	The code and documentation changes all support the same generic .cctor bug fix and contain no obvious unrelated scope.

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[josesimoes]

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/CLR/Core/Interpreter.cpp`:
- Around line 3066-3077: In the NEWOBJ .cctor type-selection block, reverse the
priority so valid stack->m_call.genericType is selected before
calleeInst.genericType. Preserve the existing validity checks and fallback
behavior, aligning this gate with the caller-first context selection used by the
object-creation path.
- Around line 1099-1118: Roll back the generic constructor record flags when
initialization or frame scheduling fails. In the flow that sets c_Scheduled and
c_Executed before CLR_RT_MethodDef_Instance::InitializeFromIndex and
CLR_RT_StackFrame::Push, clear both flags before returning either failure
HRESULT so a later trigger can retry the .cctor.

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Fix all unresolved CodeRabbit comments on this PR:

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Configuration used: Repository YAML (base), Central YAML (inherited)

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Run ID: dbf60a7e-eda4-411e-903a-6b09bb305ab2

📥 Commits

Reviewing files that changed from the base of the PR and between 699e1b3 and 378e623.

📒 Files selected for processing (2)

• src/CLR/Core/CLAUDE.md
• src/CLR/Core/Interpreter.cpp