================
@@ -1061,15 +1061,36 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
SmallVector<DeducedTemplateArgument> DeduceResults(
F->getTemplateParameters()->size());
+ // We don't have to deduce against the alias template specialization,
+ // if the source template is a synthesized alias deduction guide. This allows
+ // us to utilize the default template arguments from alias declaration.
+ //
+ // template <class T>
+ // using Foo = A<A<T>>;
+ //
+ // template <class U = int>
+ // using Bar = Foo<U>;
+ //
+ // In terms of Bar, we want U to appear in the synthesized deduction guide,
+ // but U would remain undeduced if we deduce against A<T> instead of T.
+ // Also note that since the deduced results are only used for synthesizing
+ // template parameters, they should not introduce unintended behavior in
+ // theory.
+ ArrayRef<TemplateArgument> Ps = FReturnType->template_arguments();
+ if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(F->getTemplatedDecl());
+ DG && DG->getSourceDeductionGuideKind() ==
+ CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias)
+ Ps = F->getInjectedTemplateArgs(Context);
----------------
mizvekov wrote:
>From what I gather about the context of this function, F is always a deduction
>guide, so the dyn_cast is superfluous.
Also, wouldn't we always be wanting to deduce against the injected template
parameters of F anyway, so you could instead just change
`FReturnType->template_arguments()` below to
`F->getInjectedTemplateArgs(Context)`? And you could then nuke the whole part
above about obtaining `FReturnType`, making this fix a simplification overall?
https://github.com/llvm/llvm-project/pull/147675
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