Always match the union payload with a property pattern in the STJ source generator

[akoeplinger]

Follow-up to the review feedback on #131569.

That PR fixed infinite recursion in the generated union deconstructor for recursive union types by matching the payload with a { Value: T x } property pattern, but only for the case whose type is the union type itself; every other case kept a bare type pattern.

Per @AlekseyTs's suggestion, this generates all cases that way unconditionally, which removes the special case from the emitter and is more robust:

• It does not rely on the compiler's union-unwrapping rule for type patterns. Per the unions proposal, a type pattern T applied to a union is equivalent to T or { Value: T }, so a bare type pattern happens to work for cases that are not the union type itself only because the union instance can never match them.
• It always verifies the payload rather than the union instance, so no arm can become a trivially-matching catch-all that shadows the arms after it.

Generated code before:

return value switch
{
    null => ((Type?)null, (object?)null),
    bool caseValue0 => (typeof(bool), (object?)caseValue0),
    { Value: RecursiveNat caseValue1 } => (typeof(RecursiveNat), (object?)caseValue1),
};

After:

return value switch
{
    null => ((Type?)null, (object?)null),
    { Value: bool caseValue0 } => (typeof(bool), (object?)caseValue0),
    { Value: RecursiveNat caseValue1 } => (typeof(RecursiveNat), (object?)caseValue1),
};

No behavior change is expected, so no new tests are added; #131569 already added recursive-union coverage in both case-declaration orders.

Testing

• System.Text.Json.SourceGeneration.Roslyn4.4.Tests — 10932 passed, 0 failed
• System.Text.Json.SourceGeneration.Roslyn3.11.Tests — 475 passed, 0 failed
• System.Text.Json.SourceGeneration.Roslyn4.4.Unit.Tests — 282 passed, 0 failed

cc @eiriktsarpalis @AlekseyTs

Note

This PR was created with the assistance of GitHub Copilot.

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Pull request overview

Updates the System.Text.Json source generator’s union metadata emitter so the generated union deconstructor switch arms always match the union payload via a { Value: ... } property pattern, rather than sometimes using a bare type pattern on the union itself.

Changes:

• Removes the special-case branch for “recursive union” cases in the union deconstructor emitter.
• Emits { Value: <case-type> <local> } patterns for all union deconstructor arms unconditionally.
• Updates the inline rationale comment explaining why payload property patterns are preferred.

[akoeplinger]

hmm the tests pass. I'll defer to area experts.