Collection expressions: prefer ReadOnlySpan<T> over Span<T> in overload resolution #7591
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CyrusNajmabadi
approved these changes
Oct 10, 2023
333fred
approved these changes
Oct 12, 2023
RikkiGibson
reviewed
Oct 12, 2023
| > * **`E` is a *collection expression* and one of the following holds:** | ||
| > * **`T₁` is `System.ReadOnlySpan<E₁>`, and `T₂` is `System.Span<E₂>`, and an implicit conversion exists from `E₁` to `E₂`** | ||
| > * **`T₁` is `System.ReadOnlySpan<E₁>` or `System.Span<E₁>`, and `T₂` is an *array_or_array_interface_or_string_type* with *iteration type* `E₂`, and an implicit conversion exists from `E₁` to `E₂`** | ||
| > * **`T₁` is not a *span_type*, and `T₂` is not a *span_type*, and an implicit conversion exists from `T₁` to `T₂`** |
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nit: I wasn't sure why the qualifier "not a span_type" was needed here. Maybe without it, we would consider array to be "more derived" than span and therefore better?
Have we fully tested the difference in the betterness rules for a user-defined span type versus System.Span? e.g. if a user-defined ref struct collection were implicitly convertible from array, then array would be a better conversion from collection-expression, right?
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Yes, "not a span_type" is needed to avoid allowing an implicit conversion from span to array.
Added tests to dotnet/roslyn#70328 for a ref struct collection that is implicitly convertible to array. Thanks.
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See LDM-2023-10-11.md
See dotnet/roslyn#70318