From 13b6f31548784452990da6dba555af8d7a061958 Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Tue, 15 Feb 2022 17:55:35 -0800
Subject: [PATCH] Fix crash when deserializing a lambda expression in a
 decltype.

---
 clang/lib/AST/StmtProfile.cpp               | 12 ++++++++++--
 clang/test/PCH/cxx20-unevaluated-lambda.cpp | 17 +++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/PCH/cxx20-unevaluated-lambda.cpp

diff --git a/clang/lib/AST/StmtProfile.cpp b/clang/lib/AST/StmtProfile.cpp
index 09853e0f0e49..b590f4a00263 100644
--- a/clang/lib/AST/StmtProfile.cpp
+++ b/clang/lib/AST/StmtProfile.cpp
@@ -38,6 +38,10 @@ namespace {
 
     void VisitStmt(const Stmt *S);
 
+    void VisitStmtNoChildren(const Stmt *S) {
+      HandleStmtClass(S->getStmtClass());
+    }
+
     virtual void HandleStmtClass(Stmt::StmtClass SC) = 0;
 
 #define STMT(Node, Base) void Visit##Node(const Node *S);
@@ -218,7 +222,7 @@ namespace {
 void StmtProfiler::VisitStmt(const Stmt *S) {
   assert(S && "Requires non-null Stmt pointer");
 
-  HandleStmtClass(S->getStmtClass());
+  VisitStmtNoChildren(S);
 
   for (const Stmt *SubStmt : S->children()) {
     if (SubStmt)
@@ -1945,7 +1949,11 @@ StmtProfiler::VisitCXXTemporaryObjectExpr(const CXXTemporaryObjectExpr *S) {
 
 void
 StmtProfiler::VisitLambdaExpr(const LambdaExpr *S) {
-  VisitExpr(S);
+  // Do not recursively visit the children of this expression. Profiling the
+  // body would result in unnecessary work, and is not safe to do during
+  // deserialization.
+  VisitStmtNoChildren(S);
+
   // C++20 [temp.over.link]p5:
   //   Two lambda-expressions are never considered equivalent.
   VisitDecl(S->getLambdaClass());
diff --git a/clang/test/PCH/cxx20-unevaluated-lambda.cpp b/clang/test/PCH/cxx20-unevaluated-lambda.cpp
new file mode 100644
index 000000000000..29af5e61c330
--- /dev/null
+++ b/clang/test/PCH/cxx20-unevaluated-lambda.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -std=c++20 -include %s %s -o %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-pch %s -o %t
+// RUN: %clang_cc1 -std=c++20 -include-pch %t -verify %s
+
+// expected-no-diagnostics
+
+#ifndef HEADER
+#define HEADER
+
+template<typename T> auto f() -> decltype([]{ return T(42); });
+
+#else /*included pch*/
+
+static_assert(decltype(f<int>())()() == 42);
+
+#endif // HEADER