[Issue 1715] New: Template specialization checks for equality rather than convertibility

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1715

           Summary: Template specialization checks for equality rather than
                    convertibility
           Product: D
           Version: 2.008
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Keywords: rejects-valid, spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: dhasenan at gmail.com


This is related to #1714: the workaround for 1714 is prevented by this, and
vice versa.

When specializing a template based on the template parameters of its argument,
a check is performed for equality, not convertibility. For every other type
specialization, a check is performed for convertibility rather than equality.
(Equality would be redundant in those situations, and pretty much every
situation.)

Example:
---
class Foo(T){}
class Bar : Foo!(int) {}
template GetFooArg (T : Foo!(U), U) {
   alias U GetFooArg;
}

static assert (is (GetFooArg!(Foo!(int)) == int)); // works
static assert (is (GetFooArg!(Bar) == int)); // false
pragma (msg, GetFooArg!(Bar).stringof); // template instance GetFooArg!(Bar) 
// does not match any template declaration
---


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