[clang] [C++20] Fix crash with invalid concept requirement (PR #138877)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/138877

We were previously recovering a bit too hard; consumeClose() would skip to a recovery point, then we would call skipToEnd() to skip to another recovery point. Needless to say, the follow-on diagnostics were not great. But also, follow-on diagnostics were crashing due to unexpected null constraint expressions.

Now we only recover once.

Fixes #138820

>From 2f2f9ba95a1a633a0262ce15a64d6b0d7e14b600 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Wed, 7 May 2025 09:55:43 -0400
Subject: [PATCH] [C++20] Fix crash with invalid concept requirement

We were previously recovering a bit too hard; consumeClose() would skip
to a recovery point, then we would call skipToEnd() to skip to another
recovery point. Needless to say, the follow-on diagnostics were not
great. But also, follow-on diagnostics were crashing due to unexpected
null constraint expressions.

Now we only recover once.

Fixes #138820
---
 clang/docs/ReleaseNotes.rst                        |  2 ++
 clang/lib/Parse/ParseExprCXX.cpp                   |  4 +++-
 clang/test/SemaCXX/concept-crash-on-diagnostic.cpp | 12 ++++++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 55f774f5a672e..350244e3054cf 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -660,6 +660,8 @@ Bug Fixes to C++ Support
 - Fixed a crash when forming an invalid function type in a dependent context. (#GH138657) (#GH115725) (#GH68852)
 - No longer crashes when instantiating invalid variable template specialization
   whose type depends on itself. (#GH51347), (#GH55872)
+- Improved parser recovery of invalid requirement expressions. In turn, this
+  fixes crashes from follow-on processing of the invalid requirement. (#GH138820)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 32b08a12a3bb6..546c228a30513 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -3706,8 +3706,10 @@ ExprResult Parser::ParseRequiresExpression() {
           SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
           break;
         }
+        // If there's an error consuming the closing bracket, consumeClose()
+        // will handle skipping to the nearest recovery point for us.
         if (ExprBraces.consumeClose())
-          ExprBraces.skipToEnd();
+          break;
 
         concepts::Requirement *Req = nullptr;
         SourceLocation NoexceptLoc;
diff --git a/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
index c38f8888075de..1efed72522fef 100644
--- a/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
+++ b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
@@ -48,3 +48,15 @@ concept is_foo_concept = __is_same(foo::bar, T);
 // expected-error at -1 {{'bar' is a private member of 'GH131530::foo'}}
 
 }
+
+namespace GH138820 {
+int a;
+template<typename T>
+concept atomicish = requires() {
+  {    // expected-note {{to match this '{'}}
+    a
+   ... // expected-error {{expected '}'}}
+  };
+};
+atomicish<int> f(); // expected-error {{expected 'auto' or 'decltype(auto)' after concept name}}
+} // namespace GH138820