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Add precise proposed spec for improved common type. (#386)

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Neal Gafter 2017-04-04 14:47:39 -07:00 committed by GitHub
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proposals/improved-common-type.md
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@ -28,6 +28,29 @@ This is expected to affect the following aspects of the language:
- implicitly types [array creation expression](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#array-creation-expressions)
- inferring the [return type of a lambda](https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#inferred-return-type) for type inference
More precisely, we change the following sections of the specification (insertions in bold, deletions in strikethrough):
> #### Output type inferences
>
> An *output type inference* is made *from* an expression `E` *to* a type `T` in the following way:
>
> * If `E` is an anonymous function with inferred return type `U` ([Inferred return type](expressions.md#inferred-return-type)) and `T` is a delegate type or expression tree type with return type `Tb`, then a *lower-bound inference* ([Lower-bound inferences](expressions.md#lower-bound-inferences)) is made *from* `U` *to* `Tb`.
> * Otherwise, if `E` is a method group and `T` is a delegate type or expression tree type with parameter types `T1...Tk` and return type `Tb`, and overload resolution of `E` with the types `T1...Tk` yields a single method with return type `U`, then a *lower-bound inference* is made *from* `U` *to* `Tb`.
> * Otherwise, if `E` is an expression with type `U`, then a *lower-bound inference* is made *from* `U` *to* `T`.
> * **Otherwise, if `E` is a constant expression with value `null`, then a *null bound* is added to `T`**
> * Otherwise, no inferences are made.
> #### Fixing
>
> An *unfixed* type variable `Xi` with a set of bounds is *fixed* as follows:
>
> * The set of *candidate types* `Uj` starts out as the set of all types in the set of bounds for `Xi`.
> * We then examine each bound for `Xi` in turn: For each exact bound `U` of `Xi` all types `Uj` which are not identical to `U` are removed from the candidate set. For each lower bound `U` of `Xi` all types `Uj` to which there is *not* an implicit conversion from `U` are removed from the candidate set. For each upper bound `U` of `Xi` all types `Uj` from which there is *not* an implicit conversion to `U` are removed from the candidate set.
> * If among the remaining candidate types `Uj` there is a unique type `V` from which there is an implicit conversion to all the other candidate types, then ~~`Xi` is fixed to `V`.~~
> - **If `V` is a value type and there is a *null bound* for `Xi`, then `Xi` is fixed to `V?`**
> - **Otherwise `Xi` is fixed to `V`**
> * Otherwise, type inference fails.
## Drawbacks
[drawbacks]: #drawbacks