[Clang] Consider default template arguments when synthesizing CTAD guides

clang/docs/ReleaseNotes.rst

@@ -803,7 +803,7 @@ Bug Fixes to C++ Support
 - Clang no longer crashes when trying to unify the types of arrays with
   certain differences in qualifiers (this could happen during template argument
   deduction or when building a ternary operator). (#GH97005)
-- Fixed type alias CTAD issues involving default template arguments. (#GH134471)
+- Fixed type alias CTAD issues involving default template arguments. (#GH133132), (#GH134471)
 - Fixed CTAD issues when initializing anonymous fields with designated initializers. (#GH67173)
 - The initialization kind of elements of structured bindings
   direct-list-initialized from an array is corrected to direct-initialization.

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -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);
+
   // FIXME: DeduceTemplateArguments stops immediately at the first
   // non-deducible template argument. However, this doesn't seem to cause
   // issues for practice cases, we probably need to extend it to continue
   // performing deduction for rest of arguments to align with the C++
   // standard.
-  SemaRef.DeduceTemplateArguments(
-      F->getTemplateParameters(), FReturnType->template_arguments(),
-      AliasRhsTemplateArgs, TDeduceInfo, DeduceResults,
-      /*NumberOfArgumentsMustMatch=*/false);
+  SemaRef.DeduceTemplateArguments(F->getTemplateParameters(), Ps,
+                                  AliasRhsTemplateArgs, TDeduceInfo,
+                                  DeduceResults,
+                                  /*NumberOfArgumentsMustMatch=*/false);
 
   SmallVector<TemplateArgument> DeducedArgs;
   SmallVector<unsigned> NonDeducedTemplateParamsInFIndex;

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -207,13 +207,14 @@ namespace test15 {
 template <class T> struct Foo { Foo(T); };
 
 template<class V> using AFoo = Foo<V *>;
-template<typename> concept False = false;
+template<typename> concept False = false; // #test15_False
 template<False W>
-using BFoo = AFoo<W>; // expected-note {{candidate template ignored: constraints not satisfied [with V = int]}} \
-                      // expected-note {{cannot deduce template arguments for 'BFoo' from 'Foo<int *>'}} \
-                      // expected-note {{implicit deduction guide declared as 'template <class V> requires __is_deducible(AFoo, Foo<V *>) && __is_deducible(test15::BFoo, Foo<V *>) BFoo(V *) -> Foo<V *>}} \
-                      // expected-note {{candidate template ignored: could not match 'Foo<V *>' against 'int *'}} \
-                      // expected-note {{template <class V> requires __is_deducible(AFoo, Foo<V *>) && __is_deducible(test15::BFoo, Foo<V *>) BFoo(Foo<V *>) -> Foo<V *>}}
+using BFoo = AFoo<W>; // expected-note {{candidate template ignored: constraints not satisfied [with W = int]}} \
+                      // expected-note@-1 {{because 'int' does not satisfy 'False'}} \
+                      // expected-note@#test15_False {{because 'false' evaluated to false}} \
+                      // expected-note {{implicit deduction guide declared as 'template <False<> W> requires __is_deducible(AFoo, Foo<W *>) && __is_deducible(test15::BFoo, Foo<W *>) BFoo(W *) -> Foo<W *>}} \
+                      // expected-note {{candidate template ignored: could not match 'Foo<W *>' against 'int *'}} \
+                      // expected-note {{template <False<> W> requires __is_deducible(AFoo, Foo<W *>) && __is_deducible(test15::BFoo, Foo<W *>) BFoo(Foo<W *>) -> Foo<W *>}}
 int i = 0;
 AFoo a1(&i); // OK, deduce Foo<int *>
 
@@ -539,3 +540,23 @@ using C = Proxy< A<T> >;
 C test{ 42 }; // expected-error {{no viable constructor or deduction guide for deduction of template arguments}}
 
 } // namespace GH125821
+
+namespace GH133132 {
+
+template <class T>
+struct A {};
+
+template <class T>
+using Foo = A<A<T>>;
+
+template <class T = int>
+using Bar = Foo<T>;
+
+template <class T = int, class U = int>
+using Baz = Bar<T>;
+
+Bar a{};
+
+Baz b{};
+
+} // namespace GH133132