[Issue 19180] New: Expose dmd.mtype.Type.isZeroInit as __traits(isZeroInit, T)

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19180

          Issue ID: 19180
           Summary: Expose dmd.mtype.Type.isZeroInit as
                    __traits(isZeroInit, T)
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: n8sh.secondary at hotmail.com

It's useful to identify at compile time types with 0 initializers. Doing this
with CTFE and templates is slow and memory-hungry[1] and runs into
errors[2][3]. The compiler already has its own check for this that we can
expose instead.

Proposed syntax is `__traits(isZeroInit, T)` where the second argument must
resolve to the type T itself instead of being any expression whose result is an
instance of T. This would be to prevent someone from mistakenly thinking he can
use `__traits(isInitZero, x)` to test whether some variable `x` was explicitly
initialized as zero.

[1] https://github.com/dlang/phobos/pull/6537
[2] https://github.com/dlang/phobos/pull/6670#issuecomment-414111649
[3] https://github.com/dlang/phobos/pull/6461

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