[clang] [Clang] Fixed a crash when __PRETTY_FUNCTION__ or __FUNCSIG__ (clang-cl) appears in the trailing return type of the lambda (PR #122611)

[via cfe-commits]

https://github.com/TilakChad created https://github.com/llvm/llvm-project/pull/122611

The (function) type of the lambda function is null while parsing trailing return type. The type is filled-in when the lambda body is entered. So, resolving `__PRETTY_FUNCTION__` before the lambda body is entered causes the crash. 

This resolves the crash: https://github.com/llvm/llvm-project/issues/121274. 

>From 4e5935f6631f0f0cd828559ec29ed931bc0333d3 Mon Sep 17 00:00:00 2001
From: Tilak Chad <tilakchad111 at gmail.com>
Date: Sun, 12 Jan 2025 00:03:32 +0545
Subject: [PATCH] [Clang] Fixed a crash when __PRETTY_FUNCTION__ or __FUNCSIG__
 (clang-cl) appears in the trailing return type of the lambda

---
 clang/lib/AST/Expr.cpp                |  7 ++++++-
 clang/test/SemaCXX/crash-GH121274.cpp | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/crash-GH121274.cpp

diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 5331357b5d1fef..0caff41c8a8cf6 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -774,7 +774,12 @@ std::string PredefinedExpr::ComputeName(PredefinedIdentKind IK,
     const FunctionDecl *Decl = FD;
     if (const FunctionDecl* Pattern = FD->getTemplateInstantiationPattern())
       Decl = Pattern;
-    const FunctionType *AFT = Decl->getType()->getAs<FunctionType>();
+
+    const Type *Ty = Decl->getType().getTypePtrOrNull();
+    if (!Ty)
+      return "";
+
+    const FunctionType *AFT = Ty->getAs<FunctionType>();
     const FunctionProtoType *FT = nullptr;
     if (FD->hasWrittenPrototype())
       FT = dyn_cast<FunctionProtoType>(AFT);
diff --git a/clang/test/SemaCXX/crash-GH121274.cpp b/clang/test/SemaCXX/crash-GH121274.cpp
new file mode 100644
index 00000000000000..28677a0949bf9e
--- /dev/null
+++ b/clang/test/SemaCXX/crash-GH121274.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++11 -verify %s
+// expected-no-diagnostics
+
+// Do not crash when __PRETTY_FUNCTION__ appears in the trailing return type of the lambda
+void foo() {
+	[]() -> decltype(static_cast<const char*>(__PRETTY_FUNCTION__)) {
+		return nullptr;
+	}();
+
+#ifdef MS
+	[]() -> decltype(static_cast<const char*>(__FUNCSIG__)) {
+		return nullptr;
+	}();
+#endif
+}