From 1ee02f9b6053617b741de542ee127c17c5a548be Mon Sep 17 00:00:00 2001
From: cor3ntin <corentinjabot@gmail.com>
Date: Thu, 30 May 2024 17:22:07 +0200
Subject: [PATCH] [Clang] Fix overloading for constrained variadic functions
 (#93817)

Found by #93667
---
 clang/docs/ReleaseNotes.rst          |  2 ++
 clang/lib/Sema/SemaOverload.cpp      |  2 +-
 clang/test/SemaTemplate/concepts.cpp | 22 ++++++++++++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 9dc93f53fe7168..f0cdccfcc3c237 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 6c5e8afbcfb6ee..6c4ce1022ae274 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/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(
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index 787cc809e25353..a4b42cad79abd4 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/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();
+}
+
+}