[Issue 19270] is expression fails with function types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19270

Basile-z <b2.temp at gmx.com> changed:

           What    |Removed                     |Added
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                 CC|                            |b2.temp at gmx.com

--- Comment #4 from Basile-z <b2.temp at gmx.com> ---
> writing a function type is only allowed as the target of an alias declaration.

Yes, the reson of this restriction is that in the past the old AliasDecl
syntax, i.e the one with out `=`, supported expressing a function type but not
the new one.

Moving the code to parse function types everywhere should be trivial but it is
true that it's not much required in usual D code.

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