[clang] [clang] Fix a crash when default argument is passed to an explicit object parameter (PR #177534)

[via cfe-commits]

https://github.com/hax0kartik created https://github.com/llvm/llvm-project/pull/177534

This is an invalid usecase and we should mark it as such after outputting a Diagnostic. Fixes #176639.

>From 4176433ebb0822720f4f84fce6ddc8cad86dba0f Mon Sep 17 00:00:00 2001
From: hax0kartik <agarwala.kartik at gmail.com>
Date: Fri, 23 Jan 2026 12:05:46 +0530
Subject: [PATCH] [clang] Fix a crash when default argument is passed to an
 explicit object parameter

This is an invalid usecase and we should mark it as such after
outputting a Diagnostic.
---
 clang/docs/ReleaseNotes.rst                |  1 +
 clang/lib/Sema/SemaDeclCXX.cpp             |  1 +
 clang/test/SemaCXX/cxx2b-deducing-this.cpp | 15 +++++++++++++++
 3 files changed, 17 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 791212dafd342..4cd25b767320a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -173,6 +173,7 @@ Bug Fixes to Attribute Support
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a crash when instantiating ``requires`` expressions involving substitution failures in C++ concepts. (#GH176402)
+- Fixed a crash when a default argument is passed to an explicit object parameter. (#GH176639)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index c96189172a010..5837ecd6b9163 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -11600,6 +11600,7 @@ void Sema::CheckExplicitObjectMemberFunction(Declarator &D,
     Diag(ExplicitObjectParam->getLocation(),
          diag::err_explicit_object_default_arg)
         << ExplicitObjectParam->getSourceRange();
+    D.setInvalidType();
   }
 
   if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static ||
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this.cpp b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
index a9e31c3d06676..99f41bbc99020 100644
--- a/clang/test/SemaCXX/cxx2b-deducing-this.cpp
+++ b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
@@ -1400,3 +1400,18 @@ a void Bar(this int) { // expected-note {{candidate function}}
 }
 
 }
+
+namespace GH176639 {
+
+struct S {
+  void operator()(this S =) // expected-error {{the explicit object parameter cannot have a default argument}}
+          // expected-error at -1 {{expected ';' at end of declaration list}}
+          // expected-error at -2 {{expected expression}}
+};
+
+void foo() {
+  S s{};
+  s(0); // expected-error {{no matching function for call to object of type 'S'}}
+}
+
+}