Fix Issue 21975 - is expression ignores implicit conversion of struct via alias this when pattern matching

[RazvanN7]

The issue was that when the template types are deduced, implicit conversions were not taken into account. This worked for classes because it would not return immediately upon a no match [1]. I made use of the same check to fix this.

[1] https://github.com/dlang/dmd/blob/master/src/dmd/dtemplate.d#L4436

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21975	normal	is expression ignores implicit conversion of struct via alias this when pattern matching

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13738"

[pbackus]

Thanks!