http://www.digitalmars.com/d/2.0/class.html
Inheritance is defined by the grammar to be allowed to take this form:
Protection Identifier
However, this is not mentioned anywhere, and the semantics are entirely undefined.
Comment #1 by issues.dlang — 2010-12-01T10:12:07Z
Well, that's exceptionally weird. It compiles too. D doesn't support any kind of inheritance but public. It seems like either it's a hold-over which was never removed (assuming that D supported private or protected inheritance in the past), or it was intended to make porting C++ code easier. But it seems to me that allowing and then ignoring (as the compiler presumably does) protection identifiers when inheriting classes is not good behavior. The fact that the compiler allows it though means that it's more than just a spec issue.
Comment #2 by smjg — 2011-07-24T09:38:50Z
Looking at (In reply to comment #1)
> Well, that's exceptionally weird. It compiles too. D doesn't
> support any kind of inheritance but public. It seems like either
> it's a hold-over which was never removed (assuming that D supported
> private or protected inheritance in the past),
According to the description and first comment on issue 177, private/protected inheritance once worked. But it doesn't work now (1.069, 2.054). But it never made sense to have the feature in D.
> or it was intended to make porting C++ code easier.
D was never intended to be source-compatible with C++, or even C. Consequently, such a tiny concession makes no sense at all, and even less sense considering the fundamental differences between C++ classes and D classes.
> But it seems to me that allowing and then ignoring (as the compiler
> presumably does) protection identifiers when inheriting classes is
> not good behavior. The fact that the compiler allows it though
> means that it's more than just a spec issue.
Issue 177 is "Remove inheritance protection". Essentially that has been done, but by ignoring attempts to use it rather than making it illegal. So that issue has been reduced to this one.
Comment #3 by smjg — 2011-07-24T09:44:36Z
*** Issue 177 has been marked as a duplicate of this issue. ***