[llvm-bugs] [Bug 45026] improve diagnostic for dependent using-declaration that was intended to inherit constructors but used invalid syntax

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45026

Richard Smith <richard-llvm at metafoo.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Incorrect "dependent using  |improve diagnostic for
                   |declaration resolved to     |dependent using-declaration
                   |type without 'typename'"    |that was intended to
                   |error                       |inherit constructors but
                   |                            |used invalid syntax
         Resolution|INVALID                     |---
             Status|RESOLVED                    |REOPENED

--- Comment #3 from Richard Smith <richard-llvm at metafoo.co.uk> ---
> 1) If I should always do it in this form, why clang still accepts past syntax in a lot of cases and breaks only with such complicated case, seems to be very confusing.

Per the above link, the language restriction only applies "when the
nested-name-specifier is dependent", which is to say when the part before the
final '::' involves a template parameter in some way.

Nonetheless it's good practice to always write the same name before and after
the '::', even though sometimes other syntaxes are sometimes valid.

> 2) Error message is not clear at all.

This is a fair point; it would be feasible to add a note to the diagnostic:

note: use the same name before and after the '::' to inherit constructors from
'ParentContainer<int>::NestedClass'
    using parent_t::NestedClass;
                    ^~~~~~~~~~~
                    parent_t

Reopening to track the possible diagnostic improvement.

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