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

[a-tarasyuk]

Fixes #93512

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #93512

---

Full diff: https://github.com/llvm/llvm-project/pull/94542.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaStmt.cpp (+1-1)
• (added) clang/test/SemaCXX/lambda-as-default-parameter.cpp (+6)

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'}}

[AaronBallman]

LGTM

[a-tarasyuk]

@AaronBallman Thank you for your feedback. Do you think this needs to be reviewed by someone else? I can't add reviewers myself :)

[AaronBallman]

@AaronBallman Thank you for your feedback. Do you think this needs to be reviewed by someone else? I can't add reviewers myself :)

Nope, it's ready to land; sorry for not asking earlier, do you need me to commit this on your behalf?

[a-tarasyuk]

@AaronBallman Yes, I do. I don't have access to merge…