[Clang] Fix the source location of default template arguments in placeholder constraints (llvm#158414)

We discovered this issue while working on the concept normalization
refactoring. We missed the source location when diagnosing the
instantiation point of the placeholder constraints, which is involved by
the substitution of default template arguments that happens before
constraint evaluation.

See the issue alive: https://godbolt.org/z/cWr9qP3E8

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -285,6 +285,9 @@ Improvements to Clang's diagnostics
 
 - Clang now looks through parenthesis for ``-Wundefined-reinterpret-cast`` diagnostic.
 
+- Fixed a bug where the source location was missing when diagnosing ill-formed
+  placeholder constraints.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5180,7 +5180,7 @@ static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
     TemplateArgs.addArgument(TypeLoc.getArgLoc(I));
 
   Sema::CheckTemplateArgumentInfo CTAI;
-  if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
+  if (S.CheckTemplateArgumentList(Concept, TypeLoc.getNameLoc(), TemplateArgs,
                                   /*DefaultArgs=*/{},
                                   /*PartialTemplateArgs=*/false, CTAI))
     return true;

clang/test/SemaTemplate/concepts.cpp

@@ -1251,6 +1251,27 @@ int i = SVGPropertyOwnerRegistry<SVGCircleElement>::fastAnimatedPropertyLookup()
 
 }
 
+namespace GH61824 {
+
+template<typename T, typename U = typename T::type> // #T_Type
+concept C = true;
+
+constexpr bool f(C auto) { // #GH61824_f
+  return true;
+}
+
+C auto x = 0;
+// expected-error@#T_Type {{type 'int' cannot be used prior to '::'}} \
+// expected-note@-1 {{in instantiation of default argument}}
+
+// This will be fixed when we merge https://github.com/llvm/llvm-project/pull/141776
+// Which makes us behave like GCC.
+static_assert(f(0));
+// expected-error@-1 {{no matching function for call}} \
+// expected-note@#GH61824_f {{constraints not satisfied}} \
+// expected-note@#T_Type {{type 'int' cannot be used prior to '::'}}
+
+}
 
 namespace GH149986 {
 template <typename T> concept PerfectSquare = [](){} // expected-note 2{{here}}