Assign default literal to a deconstruction

[jcouv]

@alrz commented on Mon May 15 2017

Version Used: master

Steps to Reproduce:

class C {
    void M(int i , int j) {
        (i, j) = default;
    }
}

Expected Behavior: compile with no error.

Actual Behavior: error CS8131: Deconstruct assignment requires an expression with a type on the right-hand-side.

---

@alrz commented on Mon May 15 2017

@jcouv Can this be special cased? would come in handy sometime.

[jcouv]

I had not thought about that. Thanks for the pointer to actual usage of this (copied that snippet below):

(constant, expression) =
                        IsConstant(expression1) ? (expression1, expression2) :
                        IsConstant(expression2) ? (expression2, expression1) :
                        default((TExpressionSyntax, TExpressionSyntax));

The current feature only supports either (1) a tuple literal (which will get some type inference), (2) a tuple type (which is trivial to deconstruct), or (3) some type with Deconstruct.
Adding a fourth case for default doesn't seem too hard, but we should discuss with LDM.

FYI @gafter

[svick]

@alrz

That specific case already seems to work. This code compiles fine on Roslyn master:

(constant, expression) =
    IsConstant(expression1) ? (expression1, expression2) :
    IsConstant(expression2) ? (expression2, expression1) :
    default;

[alrz]

@svick

Right. A ternary does have a type, whence the default is inferred, which in turn, satisfies the deconstruction requirement.

[alrz]

Still I think (x , y) = default; could be useful when you want to zero-out some variables, potentially out params, that are not of the same type (otherwise could be accomplished via chained assignments).

[artemious7]

Perhaps, it would be better to zero-out some variables like this: x , y = default; instead of (x , y) = default;. Otherwise, it would be confusing, because to me it means deconstructing from a Tuple, which is not really involved here. It should be merely setting default values to each of the existing variables.