[Clang] Only compare template params of potential overload after checking their decl context

clang/lib/Sema/SemaOverload.cpp

@@ -1259,6 +1259,40 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
   if ((OldTemplate == nullptr) != (NewTemplate == nullptr))
     return true;
 
+  // Is the function New an overload of the function Old?
+  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
+  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
+
+  // Compare the signatures (C++ 1.3.10) of the two functions to
+  // determine whether they are overloads. If we find any mismatch
+  // in the signature, they are overloads.
+
+  // If either of these functions is a K&R-style function (no
+  // prototype), then we consider them to have matching signatures.
+  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
+      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
+    return false;
+
+  const auto *OldType = cast<FunctionProtoType>(OldQType);
+  const auto *NewType = cast<FunctionProtoType>(NewQType);
+
+  // The signature of a function includes the types of its
+  // parameters (C++ 1.3.10), which includes the presence or absence
+  // of the ellipsis; see C++ DR 357).
+  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
+    return true;
+
+  // For member-like friends, the enclosing class is part of the signature.
+  if ((New->isMemberLikeConstrainedFriend() ||
+       Old->isMemberLikeConstrainedFriend()) &&
+      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
+    return true;
+
+  // Compare the parameter lists.
+  // This can only be done once we have establish that friend functions
+  // inhabit the same context, otherwise we might tried to instantiate
+  // references to non-instantiated entities during constraint substitution.
+  // GH78101.
   if (NewTemplate) {
     // C++ [temp.over.link]p4:
     //   The signature of a function template consists of its function
@@ -1296,34 +1330,6 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
       return true;
   }
 
-  // Is the function New an overload of the function Old?
-  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
-  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
-
-  // Compare the signatures (C++ 1.3.10) of the two functions to
-  // determine whether they are overloads. If we find any mismatch
-  // in the signature, they are overloads.
-
-  // If either of these functions is a K&R-style function (no
-  // prototype), then we consider them to have matching signatures.
-  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
-      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
-    return false;
-
-  const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
-  const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);
-
-  // The signature of a function includes the types of its
-  // parameters (C++ 1.3.10), which includes the presence or absence
-  // of the ellipsis; see C++ DR 357).
-  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
-    return true;
-
-  // For member-like friends, the enclosing class is part of the signature.
-  if ((New->isMemberLikeConstrainedFriend() ||
-       Old->isMemberLikeConstrainedFriend()) &&
-      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
-    return true;
   const auto *OldMethod = dyn_cast<CXXMethodDecl>(Old);
   const auto *NewMethod = dyn_cast<CXXMethodDecl>(New);
 

clang/test/CXX/over/over.load/p2-0x.cpp

@@ -55,4 +55,31 @@ static_assert(not test<type<2>&>);
 static_assert(test<type<2>&&>);
 
 }
+
+namespace GH78101 {
+
+template<typename T, typename U, int i>
+concept True = true;
+
+template<typename T, int I>
+struct Template {
+    static constexpr int i = I;
+    friend constexpr auto operator+(True<T, i> auto f) {
+        return i;
+    }
+};
+
+template<int I>
+struct Template<float, I> {
+    static constexpr int i = I;
+    friend constexpr auto operator+(True<float, i> auto f) {
+        return i;
+    }
+};
+
+Template<void, 4> f{};
+static_assert(+Template<float, 5>{} == 5);
+
+}
+
 #endif

clang/test/Modules/GH77953.cpp

@@ -0,0 +1,28 @@
+// From https://github.com/llvm/llvm-project/issues/77953
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/a.cppm -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -fmodule-file=a=%t/a.pcm %t/b.cppm
+
+//--- a.cppm
+export module a;
+
+template<typename, typename>
+concept c = true;
+
+export template<typename... Ts>
+struct a {
+	template<typename... Us> requires(... and c<Ts, Us>)
+	friend bool operator==(a, a<Us...>) {
+		return true;
+	}
+};
+
+template struct a<>;
+
+//--- b.cppm
+import a;
+
+template struct a<int>;