JIT: Avoid folding returned null-stored Task in async versions

[Copilot]

Runtime-async await folding mistakes a null Task reference for a zero-initialized ValueTask, completing successfully instead of throwing NullReferenceException.

• Fix: Require the zero constant in impFoldAwaitedTopOfStack to be TYP_INT, rejecting TYP_REF nulls.
• Regression coverage: Add null Task and Task<int> cases using the parameter-store pattern that triggers the incorrect fold.

• Fixes JIT: (bug) runtime-async await of a null Task completes successfully instead of throwing NullReferenceException #133723

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[jakobbotsch]

@copilot Please rephrase the code and the PR description to more specifically refer to the fact that this has to do with the folding optimization, and not generally awaiting null tasks. General null task awaits should be throwing except in the very specific case where an async version ends with t = null; return t and we fold that.

[Copilot]

@copilot Please rephrase the code and the PR description to more specifically refer to the fact that this has to do with the folding optimization, and not generally awaiting null tasks. General null t...

Renamed the regression to NullTaskReturnMustNotFold and clarified the helper names/comment in 1b4252e. All three return tests passed with tiering enabled and disabled.

I couldn't update the live PR body with the available tools. Could you replace its summary with:

impFoldAwaitedTopOfStack incorrectly folds the store/return pattern in an async version ending in t = null; return t as a zero-initialized ValueTask return, suppressing the required NullReferenceException. Ordinary null Task awaits already throw. Require the zero constant to be TYP_INT to exclude TYP_REF nulls from this optimization. Regression tests cover this specific pattern for both Task and Task<int>.

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[jakobbotsch]

PTAL @EgorBo

There is a pattern match for return default(ValueTask<T>), but that match ended up also recognizing Task t = null; return t;.

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Fixes JIT folding of null Task references in async methods and adds regression coverage.

Changes:

• Restricts zero-value folding to TYP_INT constants.
• Adds null Task and Task<int> regression tests.

File summaries

File	Description
src/tests/async/returns/returns.cs	Adds null-task regression tests.
src/coreclr/jit/importer.cpp	Prevents folding typed null references.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite