[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 #17 from Paul Backus <snarwin+bugzilla at gmail.com> ---
Introduced by the fix for issue 17582:

https://github.com/dlang/dmd/pull/6958

So, it seems like the "implicit qualifier" behavior has always been intended,
but was inadvertently disabled for a 5-year period, starting from version 2.056
and ending in version 2.076.

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