[clang] [Clang] Consider default template arguments when synthesizing CTAD guides (PR #147675)

[Younan Zhang via cfe-commits]

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@@ -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);
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zyn0217 wrote:

I finally figured out another way to address it: we don't need to bother looking at the injected template arguments.

https://github.com/llvm/llvm-project/pull/147675