[clang] [Clang] Fix error recovery for default arguments (PR #208868)

[via cfe-commits]

https://github.com/gtrong created https://github.com/llvm/llvm-project/pull/208868


Fixes  #205718 
### Description
This PR fixes an issue where Clang incorrectly skips the creation of a parameter when an error occurs in a default argument that involves a GNU statement expression (`({ ... })`). 

Previously, if the default expression evaluation failed within a statement expression, the error recovery was not handled properly, leading to the parameter being dropped. This fix ensures that the parameter is correctly created and error recovery proceeds as expected.



### Testing
Added/Updated test cases in `clang/test/SemaCXX/default-arg-error-recovery.cpp` to cover invalid default arguments inside statement expressions.



>From 682b529a483be3a6c36afff725f9b41cfe116ff7 Mon Sep 17 00:00:00 2001
From: gtrong <1619664008 at qq.com>
Date: Fri, 10 Jul 2026 21:32:11 +0800
Subject: [PATCH] Backup my fixes before forcing LF

---
 clang/lib/Parse/ParseDecl.cpp                     |  5 +++--
 clang/test/SemaCXX/default-arg-error-recovery.cpp | 11 +++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/SemaCXX/default-arg-error-recovery.cpp

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 3f41e7c5c6f0d..d31de39259c1a 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -7777,9 +7777,10 @@ void Parser::ParseParameterDeclarationClause(
                                                      /*DefaultArg=*/nullptr);
               // Skip the statement expression and continue parsing
               SkipUntil(tok::comma, StopBeforeMatch);
-              continue;
+              DefArgResult = ExprError();
+            } else {
+              DefArgResult = ParseAssignmentExpression();
             }
-            DefArgResult = ParseAssignmentExpression();
           }
           if (DefArgResult.isInvalid()) {
             Actions.ActOnParamDefaultArgumentError(Param, EqualLoc,
diff --git a/clang/test/SemaCXX/default-arg-error-recovery.cpp b/clang/test/SemaCXX/default-arg-error-recovery.cpp
new file mode 100644
index 0000000000000..77d49cd818d4b
--- /dev/null
+++ b/clang/test/SemaCXX/default-arg-error-recovery.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++14 -verify %s
+
+struct S {};
+enum E { e1, e2 };
+
+template<typename T>
+auto foo(E = ({ ; }) ? 0 : 1, E = e2) {  // expected-error {{default argument may not use a GNU statement expression}}
+  return 42;
+}
+
+static_assert(foo<S>(e1) == 42, ""); // expected-error {{no matching function for call to 'foo'}}
\ No newline at end of file