Crash on invalid on NVRO within template instantiation

[Weverything]

Bugzilla Link	51708
Resolution	FIXED
Resolved on	Sep 07, 2021 10:49
Version	trunk
OS	Linux
Blocks	#50580
CC	@zygoloid,@tstellar
Fixed by commit(s)	d98c34f c30b281

Extended Description

This crash was first observed at this revision:
f2d5fce

As seen in the test case below, the crash happens when the VarDecl being tested for NVRO is invalid during template instantiation.

Testing shows that in TemplateDeclInstantiator::VisitVarDecl, if the call to SemaRef.getNamedReturnInfo is skipped when Var->isInvalidDecl(), then the crash can be avoided. Although a better fix would be to handle invalid Decl's when checking alignment.

$ cat test.ii
template
class a
;
template
b f() {
b d;
return d;
}
auto e = {
f< a< int > >() };

$ clang test.ii
test.ii:10:3: error: implicit instantiation of undefined template 'a'
f< a< int > >() };
^
test.ii:2:7: note: template is declared here
class a
^
test.ii:5:3: error: implicit instantiation of undefined template 'a'
b f() {
^
test.ii:10:3: note: in instantiation of function template specialization 'f<a>' requested here
f< a< int > >() };
^
test.ii:2:7: note: template is declared here
class a
^
test.ii:6:5: error: implicit instantiation of undefined template 'a'
b d;
^
test.ii:2:7: note: template is declared here
class a
^
clang/lib/AST/RecordLayoutBuilder.cpp:3256: const clang::ASTRecordLayout &clang::ASTContext::getASTRecordLayout(const clang::RecordDecl *) const: Assertion `D && "Cannot get layout of forward declarations!"' failed.

[Weverything]

assigned to @mizvekov

[mizvekov]

FYI this also affects clang before that patch if the variable's alignment is explicitly specified:

template <class>
class a
;
template <typename b>
b f() {
  alignas(1) b d;
  return d;
}
auto e = {
  f< a< int > >() };

Example on clang-12: https://godbolt.org/z/E8zx1xn3T

[mizvekov]

Fix in review at https://reviews.llvm.org/D109191

[mizvekov]

Fixed by d98c34f

[tstellar]

Merged: c30b281