static assert( (""~"\©"[0]).length == 1 ); That passes on 2.051, but fails on 2.052 with "static assert (2u == 1u) is false" This is likely *not* related to issue 5717, since that test case works in 2.052.
Rather, this is *not the same as* issue 5717. They may be related.
Like bug 5717, this was caused by the fix to bug 4389 (char[]~dchar and wchar[]~dchar *never* worked). The problem is in constfold.c, Cat(). It erroneously assumes that all concatenation is equivalent to string ~ dchar. But this isn't true for char[]~char, wchar[]~wchar, (this happens during constant-folding optimization, which is how it manifests in the test case). In such cases the dchar encoding should not occur - it should just give an encoding length of 1, and do a simple memcpy. It applies to everything of the form (e2->op == TOKint64) in that function. (1) size_t len = es1->len + utf_codeLength(sz, v); s = mem.malloc((len + 1) * sz); memcpy(s, es1->string, es1->len * sz); (2) utf_encode(sz, (unsigned char *)s + , v); Lines (1) and (2) are valid for hetero concatenation, but when both types are the same the lines should be: (1) size_t len = es1->len + 1; (2) memcpy((unsigned char *)s + (sz * es1->len), &v, sz); This should definitely be factored out into a helper function -- it's far too repetitive already.
Patch by Don Clugston: https://github.com/D-Programming-Language/dmd/commit/41617e9ade97187467383a12314267cb0cd5df49 https://github.com/D-Programming-Language/dmd/commit/49e7f2a80ab2bdc042d33d63c4332767a0bf1911