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Merge pull request #1721 from 333fred/null-coalescing-updates

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Update null-coalescing-assignment proposal with changes from LDM.
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Fred Silberberg 2018-07-19 11:06:39 -07:00 committed by GitHub
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proposals/null-coalescing-assignment.md
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@ -10,6 +10,8 @@
Simplifies a common coding pattern where a variable is assigned a value if it is null.
As part of this proposal, we will also loosen the type requirements on `??` to allow an expression whose type is an unconstrained type parameter to be used on the left-hand side.
## Motivation
[motivation]: #motivation
@ -37,7 +39,22 @@ assignment_operator
;
```
Which follows the [existing semantic rules for compound assignment operators](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#compound-assignment). What that means is that an operation of the form `x ??= y` is processed as if written `x = (x ?? y)` (except `x` shall be computed once). However, we permit the assignment to `x` (of its existing value) to be elided in the case that `x` is not null.
Which follows the [existing semantic rules for compound assignment operators](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#compound-assignment), except that we elide the assignment if the left-hand side is non-null. The rules for this feature are as follows.
Given `a ??= b`, where `A` is the type of `a`, `B` is the type of `b`:
1. If `A` does not exist or is a non-nullable value type, a compile-time error occurs.
2. If `B` is not implicitly convertible to `A`, a compile-time error occurs.
3. The type of `a ??= b` is `A`.
4. `a ??= b` is evaluated at runtime as `a ?? (a = b)`, except that `a` is only evaluated once.
For the relaxation of the type requirements of `??`, we update the spec where it currently states that, given `a ?? b`, where `A` is the type of `a`:
> 1. If A exists and is not a nullable type or a reference type, a compile-time error occurs.
We relax this requirement to:
1. If A exists and is a non-nullable value type, a compile-time error occurs.
## Drawbacks
[drawbacks]: #drawbacks
@ -47,7 +64,7 @@ As with any language feature, we must question whether the additional complexity
## Alternatives
[alternatives]: #alternatives
The programmer can write `(x = x ?? y)` or `if (x == null) x = y;` by hand.
The programmer can write `(x = x ?? y)`, `if (x == null) x = y;`, or `x ?? (x = y)` by hand.
## Unresolved questions
[unresolved]: #unresolved-questions