Bug 6519 – Problem with inout and type inference of polysemous types

D2 code: inout(int[]) foo(inout int[] data) { return data; } void main() { enum int[] a = [1, 2]; const r1 = foo(a); // OK enum r2 = foo(a); // Not OK } DMD 2.055beta gives: test.d(7): Error: variable test2.main.r2 only fields, parameters or stack based variables can be inout test.d(7): Error: cannot evaluate foo([1,2]) at compile time

This is an enum bug, not a CTFE problem. Here's a reduced test case: inout(int) foo(inout int data) { return data; } enum int e1 = foo(7); // OK enum e1 = foo(7); // fails! It thinks that you're trying to declare an enum of type 'inout(int)'.

Here's an example which doesn't involve CTFE at all. inout(int) foo(inout int data) { return 7; } void main() { pragma(msg, typeof(foo(7)).stringof); // ---> inout(int) } The problem may be in expression.c, functionParameters(). If at parameter matches an inout parameter with implicit conversion, the inout stays unresolved. That's necessary to allow things like: foo(A, B)(inout(A) a, inout(B) b) when B is int; it should work when A is immutable, and also when it is mutable. But... array literals are a problem. You can write: int[] a = [1,2,3]; and also immutable(int)[] b = [1,2,3]; In the first case, a sort of implicit .dup gets added. Suppose we define inout(int[]) foo(inout(int[]) x) { return x; } Should the following compile? int[] x = foo([1,2,3]); immutable(int[]) y = foo([1,2,3]); const(int[]) z = foo([1,2,3]); Currently only z compiles. The others say you cannot convert from inout(int[]) to int[]. One solution might be to say that if _all_ inout parameters are polysemous value types, so that the return constness remains ambiguous, a tie-breaking rule is applied to all of the parameters. There are two reasonable options: (a) always mutable. This would mean that x would compile, but z would stop working in existing code. y would continue to be rejected. That is, the type of foo([1,2,3]) would be typeof([1,2,3]). (b) always const. No change to what compiles. This gives more efficient code, since array literals don't need to be duped. A third option would be that the return type propagates to the parameters. Then, x, y, and z would all work, and we'd have perfect forwarding. Implicit conversion of the return type of a call to such a function, would mean implicit conversion of all the ambiguous parameters to such a function. Note that this is recursive: a parameter of an inout function could itself be the return value of another inout function. This would be optimally efficient; there would never be an unnecessary implicit .dup of array literals. It's a bit scary though -- I worry that that there might be unintended consequences of such an idea.

I think this is a duplication of bug 3748. With my patch (https://github.com/D-Programming-Language/dmd/pull/359), the sample code in comment #0 works correctly. (In reply to comment #2) > Should the following compile? > int[] x = foo([1,2,3]); > immutable(int[]) y = foo([1,2,3]); > const(int[]) z = foo([1,2,3]); D's literals work like polysemous value, but basically they have mutable types. static assert(is(typeof([1, 2]) == int[])); Therefore typeof(foo([1, 2])) equals to int[], and only y shouldn't compile, others should.

*** This issue has been marked as a duplicate of issue 3748 ***