[Issue 19448] New: Add a trait to test template arguments without instantiation

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19448

          Issue ID: 19448
           Summary: Add a trait to test template arguments without
                    instantiation
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

Designing interfaces with the assumption that a template will compile if the
arguments match leads to silent failure. std.conv.text is one such offender. If
a templated toString exists for the type but causes compiler errors on
instantiation, text() will incorrectly fall back on some other method.

Example:

```
void main() {
        writeln(X().text); //X(false)? That's not what I wanted!
}

struct X {
        bool a;
        void toString(T)(T sink) {
                easilyMistypedMethod(sink);
        }
        void easilyMisspelledMethod(T)(T sink) {
                put(sink, "Hello!");
        }
}
```

I'd like to propose a simple trait to resolve this.

Example:

```
void x(T : int)() {
        nonexistent();   
}

static assert(!__traits(compiles, x!bool()));
static assert(__traits(templateMatches, x, int));
static assert(__traits(templateMatches, x, short));
static assert(!__traits(templateMatches, x, string));

void doSomething(alias T)() {
        static if(__traits(templateMatches, T, int)) {
                T!int();
        }
}

static assert(!__traits(compiles, doSomething!x())); //x!int exists, but
doesn't compile, so this is expected

```

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