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

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/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

>From 5a25cad3b3b3afcd50bc576544e5ec029e9f57a6 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 13 Sep 2025 16:10:39 +0800
Subject: [PATCH] [Clang] Fix the source location of default template arguments
 in placeholder constraints

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 happened in the substitution of
default template arguments that happened earlier than constraint evaluation.
---
 clang/lib/Sema/SemaTemplateDeduction.cpp |  2 +-
 clang/test/SemaTemplate/concepts.cpp     | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 64be2aab259f5..62e867c44ad14 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/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;
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index d63ad01b35800..209e7dc69797d 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/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 at -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 at -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}}