[C++20] Fix crash with invalid concept requirement (#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

Author: AaronBallman

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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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;

clang/test/SemaCXX/concept-crash-on-diagnostic.cpp

@@ -48,3 +48,15 @@ concept is_foo_concept = __is_same(foo::bar, T);
 // expected-error@-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