[clang] [Clang][Lex] Fix parsing of nested requirement to prevent flowing off the end of token stream (PR #73691)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik created https://github.com/llvm/llvm-project/pull/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: https://github.com/llvm/llvm-project/issues/73112

>From 3f8b552ccc78c0d5ca98ed529ce28ac8c95f0656 Mon Sep 17 00:00:00 2001
From: Shafik Yaghmour <shafik.yaghmour at intel.com>
Date: Tue, 28 Nov 2023 11:20:12 -0800
Subject: [PATCH] [Clang][Lex] Fix parsing of nested requirement to prevent
 flowing off the end of token stream

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: https://github.com/llvm/llvm-project/issues/73112
---
 clang/lib/Parse/ParseExprCXX.cpp                   | 4 ++++
 clang/test/Parser/cxx2a-concepts-requires-expr.cpp | 8 ++++++++
 2 files changed, 12 insertions(+)

diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 79db094e098f8e6..a0903eb3a932490 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -3639,6 +3639,10 @@ ExprResult Parser::ParseRequiresExpression() {
                 //  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))
diff --git a/clang/test/Parser/cxx2a-concepts-requires-expr.cpp b/clang/test/Parser/cxx2a-concepts-requires-expr.cpp
index a18a54c7fad0690..971591afb08dba2 100644
--- a/clang/test/Parser/cxx2a-concepts-requires-expr.cpp
+++ b/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 '('}}
+}
+}