[Issue 18558] Template alias spec incomplete

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18558

--- Comment #4 from Jonathan Marler <johnnymarler at gmail.com> ---
Maybe a better description...

template "alias" parameters can accept "symbols" and "values".  If a symbol is
passed to the template, i.e.

MyTemplate!someSymbol

Then the symbol is passed directly to the template without being treated as an
expression to evaluate.  Even if the symbol is a function, the function is not
called and then passed to the template, the function symbol is passed to it.

However, if an "non-symbol" expression is passed to the template, then the
expression is evaluated at "compile time" and the return value passed to it,
i.e.

MyTemplate!42
MyTemplate!(40 + 2)

enum x = 40;
MyTemplate!(x + 2)

auto return42() { return 42; }
MyTemplate!(return42())

All these represent the same template instantiation.

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