[Issue 10945] New: Type extracted in an 'is' expression in a constraint should be visible in the template body

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10945

           Summary: Type extracted in an 'is' expression in a constraint
                    should be visible in the template body
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: druntime
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2013-09-01 16:50:44 PDT ---
Here's some interesting code another user has provided, with a reduced
test-case:

-----
struct Templ(Args...)
{
}

struct WillWork(alias T)
    if (is(T : Templ!Args, Args...))
{
    static if (is(T : Templ!Args, Args...))
        alias A = Args[0];
    else
        static assert(0);
}

struct WontWork(alias T)
    if (is(T : Templ!Args, Args...))
{
    // Error: undefined identifier Args, did you mean alias T?
    alias A = Args[0];
}

void main()
{
    alias C = Templ!int;
    alias B = WillWork!C;
    B.A a;

    alias B2 = WontWork!C;  // ng
}
-----

Note how the 'WillWork' template has to redundantly duplicate the constraint
into a static if condition. It would be simpler if the body of the template had
access to the types which were extracted with the 'is' expression in the
template constraint.

I can't think of any harm this would do, it would be beneficial. If the
template is extracting the types like that, it probably wants something to do
with those types in the template body.

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