https://issues.dlang.org/show_bug.cgi?id=12573
Steven Schveighoffer <[email protected]> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |[email protected] --- Comment #1 from Steven Schveighoffer <[email protected]> --- I think it is dangerous to allow this. Allowing the implicit casting of a return is OK, since you cannot modify the return via the mutable reference, but allowing arbitrary assignment inside the function allows modifying the mutable reference, breaking immutability. If we consider the trivial case: string foo2(in string s, ref string sout) pure nothrow { auto s2 = s.dup; sout = s2; // this would potentially be allowed auto s3 = sout.idup; // copy the data s2[0] = 'a'; // now modified immutable data referenced by sout. return sout.idup; // could be changed to return s3? } Basically, the compiler can make the legal assumption that since s3 and sout are immutable, and have not changed, calling idup on sout will reasonably result in the same value that s3 has. It would be a legal optimization. However, on return, sout has changed from what s3 contains, so the return value may not be equivalent to sout. A return does not have this vulnerability, since the function ends at a return statement, and the cast is effectively occurring after the return. In fact, you have no access to the return, so it's not possible to use it in a pure manner inside the function. I would recommend not allowing this, unless you could make more restrictive rules. I'm not sure if it's worth it. May be better to focus on multiple return values. --
