[Issue 20670] immutable template specialization pattern matches immutable struct, strips immutable

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20670

--- Comment #5 from Dennis <dkorpel at live.nl> ---
Looking at https://github.com/dlang/dmd/pull/6958 and
https://github.com/dlang/dmd/pull/13312, it seems like intended behavior.

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