Fix ValueOption optional args with ?param caller syntax#19742
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When the SupportValueOptionsAsOptionalParameters language feature is enabled, do not force option<_> on caller-side ?name = expr arguments during overload inference. Leave the type as a fresh inference variable and let AdjustCalledArgTypeForOptionals (CalleeSide) reconcile against option<_> or voption<_>. Pre-LangVersion-10 behaviour is preserved. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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May 18, 2026
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Clean, well-scoped fix. The root cause was that TcMethodApplication_SplitSynArguments eagerly constrained the caller argument type to option<_> when the ?param = expr syntax was used, preventing ValueOption from ever unifying correctly.
The fix correctly gates on SupportValueOptionsAsOptionalParameters to leave the type as a fresh inference variable when the feature is enabled, letting later unification in AdjustCalledArgTypeForOptionals (CalleeSide branch) match the actual declared parameter type — whether option<_> or voption<_>.
Key points verified:
- Correctness: The CalleeSide branch at line 382 passes through
calledArgTyunchanged (which already handles both option/voption), so the deferred unification naturally resolves to the right wrapping type. - Backwards compat: LangVersion < 10 preserves old behavior (early
option<_>constraint). - Test coverage: Methods, constructors, both option variants, and langversion gating are all exercised.
- No inference regressions: Since
None/SomeandValueNone/ValueSomeare unambiguous on their own, removing the early constraint doesn't create inference ambiguity for practical cases.
LGTM ✅
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Fix caller-side
?name = exprsyntax for ValueOption optional parameters whenSupportValueOptionsAsOptionalParametersis enabled.Fixes #19711