See access_private_parent_member_in_unrelated_class_with_friend.carbon
for an example of this fix.
The problem with that example, prior to this fix, is that when
`Derived::F` is accessed, clang tries to find `F` via
`CarbonExternalASTSource::FindExternalVisibleDeclsByName`, but
`LookupQualifiedName` doesn't find anything because it (correctly) sees
that `F` is a private member of `Base`, so it's not accessible. What it
doesn't know is that it will be accessed from the context of `A`, which
is a friend of `Base`, and so privacy doesn't matter.
`FindExternalVisibleDeclsByName` is really supposed to report visible
items, regardless of whether they can be accessed. But changing it to
simply ignore access (e.g. by modifying `LookupQualifiedName` to
optionally ignore access) doesn't quite work here. This can make a
private member of a base class appear within the `DeclContext` of a
derived class with access set to private, when it should be set to none.
This causes an assert in clang (this case is tested in
fail_access_private_parent_member_in_unrelated_fn.carbon).
Instead, change `FindExternalVisibleDeclsByName` to call
`LookupNameInExactScope` instead of `LookupQualifiedName`. This prevents
extended scopes from being searched, so members of a base class are not
found when filling in the `DeclContext` of a derived class. Clang will
call `FindExternalVisibleDeclsByName` in the context of the base class,
and the base class members are added at that time.
Nicholas Bishop
ca1898506b