Fix #19905 - expand type tuples in delegate params during IFTI #22262
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When matching a lambda against a delegate type like `R delegate(Args)` during IFTI, parameter types that resolve to TypeTuples (e.g. from AliasSeq) were not being expanded before comparing parameter counts. This caused IFTI to fail for lambdas like `(a) => a.m()` when matched against `R delegate(AliasSeq!S)`, even though explicit instantiation `fun!void((a) => a.m())` worked correctly. The fix resolves and expands TypeTuple parameters before comparing counts, similar to what TypeFunction.typeSemantic does, but only for parameters that don't rely on template parameters being deduced. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <[email protected]>
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Thanks for your pull request, @CyberShadow! Bugzilla referencesYour PR doesn't reference any Bugzilla issue. If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.
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rikkimax
reviewed
Dec 18, 2025
| Type pt = pto.type; | ||
| if (!reliesOnTemplateParameters(pt, parameters[inferStart .. parameters.length])) | ||
| { | ||
| pt = pt.syntaxCopy().typeSemantic(e.loc, sc); |
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Something about that syntaxCopy call doesn't seem necessary.
Needs further review.
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Claude says:
syntaxCopy is needed because typeSemantic may modify the type in-place
(setting deco), and we don't want to contaminate the template's
parameter list for future instantiations.
CyberShadow
commented
Dec 19, 2025
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LLM disclosure: this entire PR is AI-generated. It might be OK, or it might be slop. The fix looks sensible on the surface, and it passes the test suite, however, so does most slop in general. Still, I'd really like this bug to be fixed, so off the chance that this is actually OK, it seems worth a shot.
When matching a lambda against a delegate type like
R delegate(Args)during IFTI, parameter types that resolve to TypeTuples (e.g. from AliasSeq) were not being expanded before comparing parameter counts.This caused IFTI to fail for lambdas like
(a) => a.m()when matched againstR delegate(AliasSeq!S), even though explicit instantiationfun!void((a) => a.m())worked correctly.The fix resolves and expands TypeTuple parameters before comparing counts, similar to what TypeFunction.typeSemantic does, but only for parameters that don't rely on template parameters being deduced.
🤖 Generated with Claude Code