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Explicitly specify the order of evaluation for implicit Index/Range support (#5380)

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This is a follow up on an LDM decision, see https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-11-01.md for more information.
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AlekseyTs 2021-11-05 06:40:36 -07:00 committed by GitHub
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proposals/csharp-8.0/ranges.md
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@ -154,6 +154,12 @@ The new indexer will be implemented by converting the argument of type `Index` i
- When the argument is of the form `^expr2` and the type of `expr2` is `int`, it will be translated to `receiver.Length - expr2`.
- Otherwise, it will be translated as `expr.GetOffset(receiver.Length)`.
Regardless of the specific conversion strategy, the order of evaluation should be equivalent to the following:
1. `receiver` is evaluated;
2. `expr` is evaluated;
3. `length` is evaluated, if needed;
4. the `int` based indexer is invoked.
This allows for developers to use the `Index` feature on existing types without the need for modification. For example:
``` csharp
@ -221,7 +227,13 @@ This value will be re-used in the calculation of the second `Slice` argument. Wh
- When `expr` is of the form `expr1..` (where `expr1` can be omitted), then it will be emitted as `receiver.Length - start`.
- Otherwise, it will be emitted as `expr.End.GetOffset(receiver.Length) - start`.
The `receiver`, `Length`, and `expr` expressions will be spilled as appropriate to ensure any side effects are only executed once. For example:
Regardless of the specific conversion strategy, the order of evaluation should be equivalent to the following:
1. `receiver` is evaluated;
2. `expr` is evaluated;
3. `length` is evaluated, if needed;
4. the `Slice` method is invoked.
The `receiver`, `expr`, and `length` expressions will be spilled as appropriate to ensure any side effects are only executed once. For example:
``` csharp
class Collection {