[clang] [clang-format] Fix a crash on parsing requires clause (PR #125021)

[Owen Pan via cfe-commits]

https://github.com/owenca created https://github.com/llvm/llvm-project/pull/125021

Fixes #124921.

>From 1ca8ed5646429368412eba2e856d8215e3ded1a2 Mon Sep 17 00:00:00 2001
From: Owen Pan <owenpiano at gmail.com>
Date: Wed, 29 Jan 2025 20:08:17 -0800
Subject: [PATCH] [clang-format] Fix a crash on parsing requires clause

Fixes #124921.
---
 clang/lib/Format/UnwrappedLineParser.cpp | 2 +-
 clang/unittests/Format/FormatTest.cpp    | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index 906fc11a07d5ee..120922d271aab7 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -3632,7 +3632,7 @@ void UnwrappedLineParser::parseRequiresClause(FormatToken *RequiresToken) {
   // It could be inlined into here.
   parseConstraintExpression();
 
-  if (!InRequiresExpression)
+  if (!InRequiresExpression && FormatTok->Previous)
     FormatTok->Previous->ClosesRequiresClause = true;
 }
 
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 57f12221cdc7e6..30f3533ac73f72 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -26478,6 +26478,9 @@ TEST_F(FormatTest, RequiresClauses) {
                "foo();\n"
                "#endif\n"
                "bar(requires);");
+
+  verifyNoCrash("template <class T>\n"
+                "    requires(requires { std::declval<T>()");
 }
 
 TEST_F(FormatTest, RequiresExpressionIndentation) {