Bug 10728 – A combination of implicit conversion and lambda call cannot be compiled

In the following code, function foo cannot be compiled but it should. --- foo.d string foo(char[] s) // It cannot be compiled { return s ? (){ return s[0..$].dup; }() : null; } string bar(char[] s) // It can be compiled { return s ? s[0..$].dup : null; } --- Output: $ dmd -main -run foo.d foo.d(3): Error: cannot implicitly convert expression (s ? delegate char[]() { return _adDupT(& _D11TypeInfo_Aa6__initZ, s[0LU..__dollar]); } () : null) of type char[] to string

It is reproduced by dmd v2.064-devel-f1c47fb on Linux 64bit.

In D2, use .idup to create a copy of an immutable string. The result of .dup will implicitly convert to a string in some circumstances as a convenience feature to ease migration from D1. It works because the compiler knows that although the result of .dup is mutable, it is also unique. However, this inference is limited by design.