[clang] fix error recovery ICE on copy elision when returing invalid …

…variable

See PR51708.

Attempting copy elision in dependent contexts with invalid variable,
such as a variable with incomplete type, would cause a crash when attempting
to calculate it's alignment.

The fix is to just skip this optimization on invalid VarDecl, as otherwise this
provides no benefit to error recovery: This functionality does not try to
diagnose anything, it only calculates a flag which will affect where the
variable will be allocated during codegen.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rtrieu

Differential Revision: https://reviews.llvm.org/D109191

(cherry picked from commit d98c34f)

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1087,7 +1087,7 @@ Decl *TemplateDeclInstantiator::VisitVarDecl(VarDecl *D,
 
   SemaRef.BuildVariableInstantiation(Var, D, TemplateArgs, LateAttrs, Owner,
                                      StartingScope, InstantiatingVarTemplate);
-  if (D->isNRVOVariable()) {
+  if (D->isNRVOVariable() && !Var->isInvalidDecl()) {
     QualType RT;
     if (auto *F = dyn_cast<FunctionDecl>(DC))
       RT = F->getReturnType();

clang/test/CXX/class/class.init/class.copy.elision/p3.cpp

@@ -518,3 +518,37 @@ template <class T> X<T> test_dependent_invalid_decl() {
 template X<int> test_dependent_invalid_decl<int>(); // expected-note {{requested here}}
 
 } // namespace test_auto_variables
+
+namespace PR51708 {
+
+class a1;                  // expected-note 4 {{forward declaration of 'PR51708::a1'}}
+template <class> class A2; // expected-note 4 {{template is declared here}}
+using a2 = A2<int>;
+
+template <class b> b f() {
+  // expected-error@-1 {{incomplete result type 'PR51708::a1' in function definition}}
+  // expected-error@-2 {{implicit instantiation of undefined template 'PR51708::A2<int>}}
+
+  b d;
+  // expected-error@-1 {{variable has incomplete type 'PR51708::a1'}}
+  // expected-error@-2 {{implicit instantiation of undefined template 'PR51708::A2<int>}}
+
+  return d;
+}
+template a1 f<a1>(); // expected-note-re {{in instantiation {{.*}} requested here}}
+template a2 f<a2>(); // expected-note-re {{in instantiation {{.*}} requested here}}
+
+template <class b> b g() {
+  // expected-error@-1 {{incomplete result type 'PR51708::a1' in function definition}}
+  // expected-error@-2 {{implicit instantiation of undefined template 'PR51708::A2<int>}}
+
+  b d __attribute__((aligned(1)));
+  // expected-error@-1 {{variable has incomplete type 'PR51708::a1'}}
+  // expected-error@-2 {{implicit instantiation of undefined template 'PR51708::A2<int>}}
+
+  return d;
+}
+template a1 g<a1>(); // expected-note-re {{in instantiation {{.*}} requested here}}
+template a2 g<a2>(); // expected-note-re {{in instantiation {{.*}} requested here}}
+
+} // namespace PR51708