[Issue 18670] compiler segfault if `new` on a union type with dip1000

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18670

Citrus <bitter.taste at gmx.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bitter.taste at gmx.com

--- Comment #4 from Citrus <bitter.taste at gmx.com> ---
I can reproduce the segfault on master, the easy fix is to add a `arg !is null`
before calling `escapeByValue` to take into account the union types but I'm not
sure if the change is sound wrt the escape analysis.

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