[clang] [Clang] Do not consider a variadic function ellipsis part of a default arg (PR #153496)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/153496

When stashing the tokens of a parameter of a member function, we would munch an ellipsis, as the only considered terminal conditions were `,` and `)`.

Fixes #153445

>From e7fdb9f9c9d8a58658f89330dea99006b078c2e7 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Wed, 13 Aug 2025 22:54:07 +0200
Subject: [PATCH] [Clang] Do not consider a variadic function ellipsis part of
 a default arg

When stashing the tokens of a parameter of a member function,
we would munch an ellipsis, as the only considered terminal conditions
were `,` and `)`.

Fixes #153445
---
 clang/docs/ReleaseNotes.rst                       |  2 ++
 clang/lib/Parse/ParseCXXInlineMethods.cpp         |  6 ++++++
 clang/test/Parser/cxx-variadic-func.cpp           | 12 ++++++++++++
 clang/test/Parser/cxx2c-oxford-variadic-comma.cpp |  1 +
 4 files changed, 21 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d109518bca3f3..be72b040808b3 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -206,6 +206,8 @@ Bug Fixes to C++ Support
   casts that are guaranteed to fail (#GH137518).
 - Fix bug rejecting partial specialization of variable templates with auto NTTPs (#GH118190).
 - Fix a crash when using ``explicit(bool)`` in pre-C++11 language modes. (#GH152729)
+- Fix the parsing of variadic member functions when the ellipis immediately follows a default argument.(#GH153445)
+
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseCXXInlineMethods.cpp b/clang/lib/Parse/ParseCXXInlineMethods.cpp
index 9a010fb5f3427..74e25002e468b 100644
--- a/clang/lib/Parse/ParseCXXInlineMethods.cpp
+++ b/clang/lib/Parse/ParseCXXInlineMethods.cpp
@@ -1161,6 +1161,12 @@ bool Parser::ConsumeAndStoreInitializer(CachedTokens &Toks,
 
   while (true) {
     switch (Tok.getKind()) {
+    case tok::ellipsis:
+      // We found an elipsis at the end of the parameter list;
+      // it is not part of a parameter declaration.
+      if (ParenCount == 1 && NextToken().is(tok::r_paren))
+        return true;
+      goto consume_token;
     case tok::comma:
       // If we might be in a template, perform a tentative parse to check.
       if (!AngleCount)
diff --git a/clang/test/Parser/cxx-variadic-func.cpp b/clang/test/Parser/cxx-variadic-func.cpp
index 98a34d3e1bf67..d1b822e7c3f65 100644
--- a/clang/test/Parser/cxx-variadic-func.cpp
+++ b/clang/test/Parser/cxx-variadic-func.cpp
@@ -6,3 +6,15 @@ void f(...) {
 }
 
 void h(int n..., int m); // expected-error {{expected ')'}} expected-note {{to match}}
+
+
+namespace GH153445 {
+void f(int = {}...);
+
+struct S {
+  void f(int = {}...);
+  void g(int...);
+};
+
+void S::g(int = {}...) {}
+}
diff --git a/clang/test/Parser/cxx2c-oxford-variadic-comma.cpp b/clang/test/Parser/cxx2c-oxford-variadic-comma.cpp
index b8015b4815b2e..18ce770851a36 100644
--- a/clang/test/Parser/cxx2c-oxford-variadic-comma.cpp
+++ b/clang/test/Parser/cxx2c-oxford-variadic-comma.cpp
@@ -36,6 +36,7 @@ void o(int x, ...);
 
 struct S {
   void p(this S...) {} // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+  void f(int = {}...); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
 };
 
 template<class ...Ts>