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

[d-bugmail at puremagic.com]

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

Basile-z <b2.temp at gmx.com> changed:

           What    |Removed                     |Added
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             Blocks|                            |18436


Referenced Issues:

https://issues.dlang.org/show_bug.cgi?id=18436
[Issue 18436] broken opCast fails silently when used with std.conv.to
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