[Issue 13448] Class specification misses template classes in base classes list

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13448

--- Comment #4 from Kenji Hara <k.hara.pg at gmail.com> ---
(In reply to hsteoh from comment #3)
> I'm not sure about basing the spec on the quirks of the current
> implementation. Conceptually speaking, the base class list consists of zero
> or one base classes, followed by zero or more interfaces. Just because the
> implementation currently parses it as BasicType doesn't mean that that's the
> way the spec should be written.

Grammar definition cannot define all D semantic specification.

> What does it mean, for example, to write
> "class C : int, string, float", which would be valid according to the
> BasicType definition? Obviously, the *intention* is that only valid base
> classes / interfaces (including any respective template instantiations) are
> included in the list, even if the compiler currently implements this as a
> list of BasicTypes and a post-parsing type check. I think the spec would be
> much clearer if written according to intention rather than the quirks of the
> current implementation.

The post-parsing type check cannot be avoidable even when the "much cleaner
grammar" is defined. See:

class C : int {} // The semantic error can be detected in parsing phase

alias B = int;
class C : B {}   // The semantic error cannot be detected in parsing phase

Of course, we can reject the syntax `class C : int {}` in parsing by the more
strict grammar definition. But it will increase grammar complexity and will
make more difficult to implement D parser.

Therefore, it's is reasonable that using `BasicType` in the `SuperClass` and
`Interfaces` definition to simplify grammar definition.

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