[Issue 19021] [REG 2.077] Type merging immutable -> mutable broken in AST

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19021

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Adding in the reference fixes immediate bug.

https://github.com/dlang/dmd/pull/8398


But now `pure nothrow @nogc` is no longer part of the symbol mangle.

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