void fillWithZero(T)(T[] arr) { foreach(ref x; arr) x = 0; } T[4] f(T)() { T[4] stackSpace = void; fillWithZero(stackSpace[]); return stackSpace; } static assert(f!int() == [0,0,0,0]); Fails with: bug6XXX.d(11): Error: variable stackSpace is used before initialization bug6XXX.d(11): Error: cannot evaluate fillWithZero(stackSpace[]) at compile time bug6XXX.d(16): Error: cannot evaluate f() at compile time bug6XXX.d(16): Error: static assert (cast(int[])f() == [0,0,0,0]) is not evaluatable at compile time It's CTFE only issue and is a blocker for one overload of phobos insertInPlace.
That's difficult. Implementing this would be a major feature, since assignment from void has a high risk of undefined behaviour. The f() function is passing a reference to an uninitialized value, to another function. In this particular case, it does initialize every member without reading from any of them, but that's difficult to determine. I guess it could be implemented by introducing an 'uninitialized expression', but then there are problems with this: void fillWithZero(T)(T[] arr) { auto q = arr[0]; // consider this line foreach(ref x; arr) x = 0; } T[4] f(T)() { T[4] stackSpace = void; fillWithZero(stackSpace[]); return stackSpace; } static assert(f!int() == [0,0,0,0]); The assignment to q should generate a 'variable used before set' error. But to be comprehensible, it should also refer to the line where stackSpace was initialized.
I see, somehow I hadn't occured to me that it would require a lot of extra work (and storage) to keep track of all uninitialized slots of array. Apart from variadic overload of insertInPlace I wouldn't expect this pattern to be too common, so it won't be a big issue. It was propmted by: insertInPlace(a, x, [b,c]); //allocation here is plain ridiculous so I proposed a more generalized version that allowed multiple arguments (that also solves this particular problem): insertInPlace(a, x, b, c); // initializes tmp array with b, c on stack and inserts
Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/f152b09d8f59a4981bff924802bebaccb926f948 Fix issue 6438 - [CTFE] wrong error "value used before set" when slicing =void array Take advantage of the new VoidExp. https://github.com/D-Programming-Language/dmd/commit/b71836bbc5e83af949484401e6f97346e7d66a2d Merge pull request #816 from donc/voidctfe6438 Fix issue 6438 - [CTFE] wrong error "value used before set" with = void