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.
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.
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.
*** Issue 177 has been marked as a duplicate of this issue. ***
grammar part fixed in https://github.com/D-Programming-Language/d-programming-language.org/commit/55b8905dd1f251fd54a8306bc1cce353bfa0d09e
Deprecated it: https://github.com/D-Programming-Language/dmd/commit/3ab6ab40e457e8f32539be38b14ea3839f38d409
Pull to turn deprecation message into an error. https://github.com/D-Programming-Language/dmd/pull/3911
Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/5ffc7d3784dc416769ff1a4aa0ab959ca42cd47e Fix Issue 5299 - Protected inheritance is semantically undefined Deprecation -> Error https://github.com/D-Programming-Language/dmd/commit/486e92da11fd567c32634a3085a31fa266634d77 Merge pull request #3911 from yebblies/issue5299 Issue 5299 - Turn base class protection into an error