Catch block not hit in unittest

[Lucia Cojocaru via Digitalmars-d-learn]

Hello,

I am trying to assert the fact that a UnicodeException is thrown 
by the piece of code added in this PR:
https://github.com/dlang/druntime/pull/1696/files
The unittest is in druntime/src/rt/lifetime.d:

unittest
{
    import core.exception : UnicodeException;

    try
    {
        string ret;
        int i = -1;
        ret ~= i;
    }
    catch(UnicodeException e)
    {
        assert(e.msg == "Invalid UTF-8 sequence");
    }
}

However running the tests for druntime fails because the 
exception is not caught:
****** FAIL release64 rt.lifetime
core.exception.UnicodeException@src/rt/lifetime.d(2031): Invalid 
UTF-8 sequence

I tried to use the same code as in the unittest above in a 
regular D source file and the UnicodeException was properly 
caught there.

Do you know what the problem might be?

Thank you,
Lucia