[Clang][Lex] Fix parsing of nested requirement to prevent flowing off…

… the end of token stream (#73691)

Currently when parsing a nested requirement we attempt to balance parens
if we have a parameter list. This will fail in some cases of ill-formed
code and keep going until we fall off the token stream and crash. This
fixes the hand parsing by using SkipUntil which will properly flag if we
don't find the expected tokens.

Fixes: #73112

clang/docs/ReleaseNotes.rst

@@ -790,6 +790,9 @@ Bug Fixes to C++ Support
   completes (except deduction guides). Fixes:
   (`#59827 <https://github.com/llvm/llvm-project/issues/59827>`_)
 
+- Fix crash when parsing nested requirement. Fixes:
+  (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Parse/ParseExprCXX.cpp

@@ -3635,10 +3635,12 @@ ExprResult Parser::ParseRequiresExpression() {
               auto Res = TryParseParameterDeclarationClause();
               if (Res != TPResult::False) {
                 // Skip to the closing parenthesis
-                // FIXME: Don't traverse these tokens twice (here and in
-                //  TryParseParameterDeclarationClause).
                 unsigned Depth = 1;
                 while (Depth != 0) {
+                  bool FoundParen = SkipUntil(tok::l_paren, tok::r_paren,
+                                              SkipUntilFlags::StopBeforeMatch);
+                  if (!FoundParen)
+                    break;
                   if (Tok.is(tok::l_paren))
                     Depth++;
                   else if (Tok.is(tok::r_paren))

clang/test/Parser/cxx2a-concepts-requires-expr.cpp

@@ -160,3 +160,11 @@ template <int N>
 requires requires {
  typename BitInt<N>; // ok
 } using r44 = void;
+
+namespace GH73112 {
+void f() {
+    requires { requires(int; } // expected-error {{expected ')'}} \
+                               // expected-error {{expected expression}} \
+                               // expected-note {{to match this '('}}
+}
+}