[SemaCXX] Fix ICE with structure bindings to members of template

[ddiproietto]

Summary

Trying to use structure binding with a structure that doesn't implement
std::tuple_size, should unpack the data members. When the struct is a
template though, clang might hit an assertion (if the type has not been
completed before), because CXXRecordDecl::DefinitionData is nullptr.

This commit fixes the problem by completing the type while trying to
decompose the structured binding.

The ICE happens in real world code, for example, when trying to iterate
a protobuf generated map with a range-based for loop and structure
bindings (because google::protobuf::MapPair is a template and doesn't
support std::tuple_size).

Reported-by: [e-mail]

[Rakete1111]

Yeah, seems like we don't instantiate pair<int, int> at all in that specific case. LGTM but you might want to wait for someone else to confirm.

[rsmith]

LGTM, thanks.

[ddiproietto]

Sorry, I don't have commit access, can someone push this, please?

Also, should this be backported to 8.x? I'm actually interested in a backport up to 5.x, if possible. If not possible, that's fine too. Thanks!

Cheers,

Daniele

[Rakete1111]

Backporting to 8.0.0 is possibly possible, you'll need to ping Hans. I don't know about previous versions though.

clang/lib/Sema/SemaDeclCXX.cpp

static DeclAccessPair findDecomposableBaseClass(Sema &S, SourceLocation Loc,
   }
 
   return DeclAccessPair::make(const_cast<CXXRecordDecl*>(ClassWithFields), AS);
 }
 
 static bool checkMemberDecomposition(Sema &S, ArrayRef<BindingDecl*> Bindings,
                                      ValueDecl *Src, QualType DecompType,
                                      const CXXRecordDecl *OrigRD) {
+  if (S.RequireCompleteType(Src->getLocation(), DecompType,
+                            diag::err_incomplete_type))
+    return true;
+
   CXXCastPath BasePath;
   DeclAccessPair BasePair =
       findDecomposableBaseClass(S, Src->getLocation(), OrigRD, BasePath);
   const CXXRecordDecl *RD = cast_or_null<CXXRecordDecl>(BasePair.getDecl());
   if (!RD)
     return true;
   QualType BaseType = S.Context.getQualifiedType(S.Context.getRecordType(RD),
                                                  DecompType.getQualifiers());

clang/test/SemaCXX/cxx1z-decomposition.cpp

for (auto [a,b,c] : t)
     a,b,c;
 }
 
 struct PR37352 {
   int n;
   void f() { static auto [a] = *this; } // expected-error {{cannot be declared 'static'}}
 };
 
+namespace instantiate_template {
+
+template <typename T1, typename T2>
+struct pair {
+  T1 a;
+  T2 b;
+};
+
+const pair<int, int> &f1();
+
+int f2() {
+  const auto &[a, b] = f1();
+  return a + b;
+}
+
+} // namespace instantiate_template
+
 // FIXME: by-value array copies