[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context

Author: huqizhi

clang/docs/ReleaseNotes.rst

@@ -204,6 +204,8 @@ Bug Fixes to C++ Support
   parameter where we did an incorrect specialization of the initialization of
   the default parameter.
   Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
+- Fix a crash in codegen when lambdas declared in an unevaluated context.
+  Fixes (`#76674 <https://github.com/llvm/llvm-project/issues/76674>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1613,8 +1613,8 @@ namespace {
 bool TemplateInstantiator::AlreadyTransformed(QualType T) {
   if (T.isNull())
     return true;
-
-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  if (T->isInstantiationDependentType() || T->isVariablyModifiedType() ||
+      (SemaRef.getLangOpts().CPlusPlus20 && T->isDecltypeType()))
     return false;
 
   getSema().MarkDeclarationsReferencedInType(Loc, T);
@@ -2685,7 +2685,8 @@ QualType Sema::SubstType(QualType T,
 
   // If T is not a dependent type or a variably-modified type, there
   // is nothing to do.
-  if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType())
+  if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType() &&
+      (getLangOpts().CPlusPlus20 && !T->isDecltypeType()))
     return T;
 
   TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);

clang/test/CodeGen/PR76674.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -o - %s
+// expected-no-diagnostics
+
+template <class>
+struct A {
+    template <class U>
+    using Func = decltype([] {return U{};});
+};
+
+A<int>::Func<int> f{};
+int i{f()};