[Issue 21138] New: Difficult to treat UDAs given to parameters.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21138

          Issue ID: 21138
           Summary: Difficult to treat UDAs given to parameters.
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: zan77137 at nifty.com

Function arguments can be given UDAs, but they are very difficult to treat.
----------------------------------------------------------
import std;

template Temp1(alias params) { alias Temp1 = params; }
template Temp2(params...) { alias Temp2 = params; }
template Foo(alias f) {
    // OK1
    alias params = Parameters!(f);
    pragma(msg, params); // (@(10) int, @(15) long)

    // OK2
    pragma(msg, params[0..1]); // (@(10) int)
    pragma(msg, __traits(getAttributes, params[0..1])); // tuple(10)

    // NG1: Lack UDAs
    alias param2  = params[0];
    pragma(msg, params[0]); // int
    //pragma(msg, __traits(getAttributes, params[0])); // Error

    // NG2: Error
    //pragma(msg, Temp1!(params));

    // NG3: This is able to pass, but lack UDAs
    pragma(msg, Temp2!(params));
}
void main() {
    alias X = Foo!( (@(10) int x, @(15) long b) => x );
}
----------------------------------------------------------
In NG1 and NG3, UDAs should not lack.

NG2 should allow them to be passed to an alias parameter, as is possible in
OK1. But maybe this should be considered separately from this issue. (Allow
tuple to be passed to an alias template parameter)

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