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

clang/docs/ReleaseNotes.rst

@@ -265,6 +265,8 @@ Bug Fixes to C++ Support
 - No longer reject valid use of the ``_Alignas`` specifier when declaring a
   local variable, which is supported as a C11 extension in C++. Previously, it
   was only accepted at namespace scope but not at local function scope.
+- 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

@@ -1614,7 +1614,17 @@ bool TemplateInstantiator::AlreadyTransformed(QualType T) {
   if (T.isNull())
     return true;
 
-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  bool DependentLambdaType = false;
+  if (CXXRecordDecl *RD = T->getAsCXXRecordDecl(); RD && RD->isLambda()) {
+    QualType LambdaCallType = RD->getLambdaCallOperator()->getType();
+    if (LambdaCallType->isInstantiationDependentType() ||
+        LambdaCallType->isVariablyModifiedType()) {
+      DependentLambdaType = true;
+    }
+  }
+
+  if (T->isInstantiationDependentType() || T->isVariablyModifiedType() ||
+      DependentLambdaType)
     return false;
 
   getSema().MarkDeclarationsReferencedInType(Loc, T);
@@ -2683,11 +2693,6 @@ QualType Sema::SubstType(QualType T,
          "Cannot perform an instantiation without some context on the "
          "instantiation stack");
 
-  // If T is not a dependent type or a variably-modified type, there
-  // is nothing to do.
-  if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType())
-    return T;
-
   TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);
   return Instantiator.TransformType(T);
 }

clang/test/CodeGen/PR76674.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -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()};