[Clang] Fix overloading for constrained variadic functions (llvm#93817)

Found by llvm#93667

clang/docs/ReleaseNotes.rst

@@ -816,6 +816,8 @@ Bug Fixes to C++ Support
 - Fix incorrect merging of modules which contain using declarations which shadow
   other declarations. This could manifest as ODR checker false positives.
   Fixes (`#80252 <https://github.com/llvm/llvm-project/issues/80252>`_)
+- Fix a regression introduced in Clang 18 causing incorrect overload resolution in the presence of functions only
+  differering by their constraints when only one of these function was variadic.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaOverload.cpp

@@ -10367,7 +10367,7 @@ static bool sameFunctionParameterTypeLists(Sema &S,
   FunctionDecl *Fn1 = Cand1.Function;
   FunctionDecl *Fn2 = Cand2.Function;
 
-  if (Fn1->isVariadic() != Fn1->isVariadic())
+  if (Fn1->isVariadic() != Fn2->isVariadic())
     return false;
 
   if (!S.FunctionNonObjectParamTypesAreEqual(

clang/test/SemaTemplate/concepts.cpp

@@ -1122,3 +1122,25 @@ template <typename d> concept f = c< d >;
 template <f> struct e; // expected-note {{}}
 template <f d> struct e<d>; // expected-error {{class template partial specialization is not more specialized than the primary template}}
 }
+
+
+namespace constrained_variadic {
+template <typename T = int>
+struct S {
+    void f(); // expected-note {{candidate}}
+    void f(...) requires true;   // expected-note {{candidate}}
+
+    void g(...);  // expected-note {{candidate}}
+    void g() requires true;  // expected-note {{candidate}}
+
+    consteval void h(...);
+    consteval void h(...) requires true {};
+};
+
+int test() {
+    S{}.f(); // expected-error{{call to member function 'f' is ambiguous}}
+    S{}.g(); // expected-error{{call to member function 'g' is ambiguous}}
+    S{}.h();
+}
+
+}