[Issue 13184] better error messages for incorrect constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=13184

--- Comment #4 from Manu <turkeyman at gmail.com> ---
I think it should definitely emit an error.
There may come some time in the future where it is decided that application of
the attribute may like to mean something new and useful, but if we leave this
situation as is, then that is a breaking change.
If we expect that meaningless attributes produce compile errors, then in the
future when we want to make it meaningful, we can do it without breaking any
code.

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