1685 – Array index is evaluated twice

Under certain circumstances, an array index is evaluated twice:
$ cat test1.d 
import std.stdio;
int f() {
 writefln("f()");
 return 0;
}
void main() {
 int[1] a;
 a[f()] += 42L;
}
$ dmd test1.d -oftest1
gcc test1.o -o test1 -m32 -Xlinker -L/opt/dmd/bin/../lib -lphobos2 -lpthread -lm 
$ ./test1
f()
f()
$
If the line "a[f()] += 42L;" is replaced by "a[f()] = 42L;" or "a[f()] += 42;" then f() is called only once.

Can be reproduce on Windows with DMD too. Both v1.023 & v2.007 are affected. 
It seems that the line is interpreted as
/*1*/ a[f()]
/*2*/ + cast(int)42L
/*3*/ -> a[f()]
f() is evaluated twice only when a downward cast is needed, so replacing 42L by 42.0 will trigger the 2nd f(), but simpler types such as bool and char won't.
Also affects other *= methods, and associative arrays.

The bug is still present in version 2.011.
It's especially dangerous with float arrays, e.g.
f[i++] += 0.1;
will result in i being incremented by 2 (and the wrong element of f[] being incremented by 0.1).

f[i++] += 0.1; example is really critical for "float[] f". Please add this to DStress.
I have many codes in template versions (both for float and double), but most literals and functions returns double which then need implicit cast for floats. This bug makes version for floats very very broken code. I have no idea in what places this bug was silently corrupting my data.

Fixed dmd 1.038 amd 2.022