2132 – CTFE: can't evaluate ~= at compile time, D2 only.

This triggers the behaviour shown in bugzilla #1947; but that's incidental.
Here, it's the error message which is the problem. It works OK on D1, and also works on D2 if you use ~ instead of ~=.
The problem could be related to #1768, but the workaround there (explicitly initialising the variable) doesn't work here.
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bug.d(10): Error: cannot evaluate func(";") at compile time
bug.d(10): Error: argument to mixin must be a string, not (func(";"))
Statement::blockExit(009989DC)
mixin(func(";"));
Assertion failure: '0' on line 123 in file 'statement.c'
abnormal program termination
-----------
---
char [] func(invariant char [] s)
{
 char [] u = "".dup;
 u ~= s; // ICE
 u = u ~ s; // OK
 return u; 
}
void main() {
 mixin(func(";"));
}

Fixed dmd 1.031 and 2.015