[Sema] Fix a crash when a BlockExpr appears in a default-member-initializer of a class template

[erik.pilkington]

Summary

Previously, clang would crash here:

template <class T> struct S {
  int (^p)() = ^{ return 0; };
};
S<int> x;

The problem was that InstantiateClass() was iterating over all the decls(), which included the BlockDecl. The template instantiator doesn't know how to instantiate BlockDecls though, it knows about BlockExprs. This is typically fine because BlockExprs must be the parent of the BlockDecl. To accommodate this, delay instantiating the BlockDecl until we're instantiating the default-member-initializer.

Thanks for taking a look!
Erik

[erik.pilkington]

(Forgot to add rdar://41200624)

[rjmccall]

I honestly don't know why the BlockDecl is in the members list in the first place; that seems wrong, for the same reason that we wouldn't (I assume?) consider a lambda's implicit record to be a member.

[erik.pilkington]

In D49688#1172269, @rjmccall wrote:

I honestly don't know why the BlockDecl is in the members list in the first place; that seems wrong, for the same reason that we wouldn't (I assume?) consider a lambda's implicit record to be a member.

Looks like we do actually treat a lambda's implicit record as a member. This loop is only iterating over decls that are in the pattern decl's context, and I think it makes sense that the BlockDecl is there, right?

[rjmccall]

It makes sense that the BlockDecl's parent DC is the class, although I think it would be even better if we took a page from Swift and make a special DC for initializer expressions. I don't think it makes sense for it to be listed in the DC's list of declarations.

Regardless, it's not something you have to fix. Patch LGTM.

lib/Sema/SemaTemplateInstantiate.cpp

for (auto *Member : Pattern->decls()) {
     //      class B *g;
     //    };
     // @endcode
     // 'class B' has the template as lexical context but semantically it is
     // introduced in namespace scope.
     if (Member->getDeclContext() != Pattern)
       continue;
 
+    // BlockDecls can appear in a default-member-initializer. They must be the
+    // child of a BlockExpr, so we only know how to instantiate them from there.
+    if (isa<BlockDecl>(Member))
+      continue;
+
     if (Member->isInvalidDecl()) {
       Instantiation->setInvalidDecl();
       continue;
     }
 
     Decl *NewMember = Instantiator.Visit(Member);
     if (NewMember) {
       if (FieldDecl *Field = dyn_cast<FieldDecl>(NewMember)) {

test/SemaCXX/instantiate-blocks.cpp

 
 int main(void)
 {
     foo(100, 'a');  // expected-note {{in instantiation of function template specialization 'foo<int, char>' requested here}}
 
    noret((float)0.0, double(0.0)); // expected-note {{in instantiation of function template specialization 'noret<float, double>' requested here}}
 }
 
+namespace rdar41200624 {
+template <class T>
+struct S {
+  int (^p)() = ^{ return 0; };
+  T (^t)() = ^{ return T{}; };
+  T s = ^{ return T{}; }();
+};
+S<int> x;
+}