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

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk updated https://github.com/llvm/llvm-project/pull/94542

>From da4df73607a9edefc8db721818eff50e974a0637 Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Thu, 6 Jun 2024 01:55:54 +0300
Subject: [PATCH] [Clang] skip alignment checks on incomplete types to avoid an
 assertion failure while parsing lambda used as default argument

---
 clang/docs/ReleaseNotes.rst                        | 2 ++
 clang/lib/Sema/SemaStmt.cpp                        | 2 +-
 clang/test/SemaCXX/lambda-as-default-parameter.cpp | 6 ++++++
 3 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/lambda-as-default-parameter.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 39a9013c75a41..819fe1811ddaf 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -840,6 +840,8 @@ Bug Fixes to C++ Support
 - Fix a crash caused by improper use of ``__array_extent``. (#GH80474)
 - Fixed several bugs in capturing variables within unevaluated contexts. (#GH63845), (#GH67260), (#GH69307),
   (#GH88081), (#GH89496), (#GH90669) and (#GH91633).
+- 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 57465d4a77ac2..56f69dd50f361 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/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;
 
diff --git a/clang/test/SemaCXX/lambda-as-default-parameter.cpp b/clang/test/SemaCXX/lambda-as-default-parameter.cpp
new file mode 100644
index 0000000000000..1f07a7f5644b7
--- /dev/null
+++ b/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'}}