[Issue 23797] Improve type-testing `is` expressions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23797

Nick Treleaven <nick at geany.org> changed:

           What    |Removed                     |Added
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                 CC|                            |nick at geany.org

--- Comment #2 from Nick Treleaven <nick at geany.org> ---
Just to note that `module` and `package` (already accepted), `return` and
visibility should not be part of an `is` expression, because an `is` expression
is supposed to test whether something is a valid type. It's not to test
symbols. From the spec:

"An IsExpression is evaluated at compile time and is used to check if an
expression is a valid type."

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