[Clang] skip alignment checks on incomplete types to avoid an assertion failure while parsing lambda used as default argument (#94542)

Fixes #93512

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -912,7 +912,8 @@ Bug Fixes to C++ Support
   between the addresses of two labels (a GNU extension) to a pointer within a constant expression. (#GH95366).
 - Fix immediate escalation bugs in the presence of dependent call arguments. (#GH94935)
 - Clang now diagnoses explicit specializations with storage class specifiers in all contexts.
-
+- Fix an assertion failure caused by parsing a lambda used as a default argument for the value of a
+  forward-declared class. (#GH93512).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaStmt.cpp

@@ -3355,7 +3355,7 @@ Sema::NamedReturnInfo Sema::getNamedReturnInfo(const VarDecl *VD) {
 
   // Variables with higher required alignment than their type's ABI
   // alignment cannot use NRVO.
-  if (!VD->hasDependentAlignment() &&
+  if (!VD->hasDependentAlignment() && !VDType->isIncompleteType() &&
       Context.getDeclAlign(VD) > Context.getTypeAlignInChars(VDType))
     Info.S = NamedReturnInfo::MoveEligible;
 

clang/test/SemaCXX/lambda-as-default-parameter.cpp

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+struct a; // expected-note {{forward declaration of 'a'}} \
+             expected-note {{forward declaration of 'a'}}
+void b(a c = [] { return c; }); // expected-error {{initialization of incomplete type 'a'}} \
+                                   expected-error {{variable has incomplete type 'a'}}