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Grammar rule *parameter_modifier* disallows valid V7 constructs #1209

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Grammar rule *parameter_modifier* disallows valid V7 constructs #1209

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3 changes: 2 additions & 1 deletion standard/classes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2130,7 +2130,7 @@ default_argument

parameter_modifier
: parameter_mode_modifier
| 'this'
| 'this' parameter_mode_modifier?
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The ordering is flexible:

Suggested change
| 'this' parameter_mode_modifier?
| 'this' parameter_mode_modifier?
| parameter_mode_modifier? 'this'

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My test shows that the order is indeed flexible. Given Nigel's proposal, this on its own would be ambiguous, but I think ANTLR chooses the lexical-first rule that matches, right, Nigel?

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@RexJaeschke – Yes, ANTLR resolves such ambiguity by picking the first alternative. We have noted elsewhere where the grammar relies on this, however in this case as the two alternatives represent exactly the same thing I don’t think a note is necessary.

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It was not originally so flexible, see https://github.com/dotnet/csharplang/blob/8bd52cca31b40d9720a98629aae9a0cd89bb92ec/meetings/2017/LDM-2017-12-04.md. But I'm not sure which version of C# got that change.

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The Roslyn change was dotnet/roslyn#23643. According to dotnet/csharplang#1022, this change is in C# 7.3. So OK for the ECMA C# 7 spec.

;

parameter_mode_modifier
Expand Down Expand Up @@ -3056,6 +3056,7 @@ When the first parameter of a method includes the `this` modifier, that method i

- It may only be an input parameter if it has a value type
- It may only be a reference parameter if it has a value type or has a generic type constrained to struct
- It shall not be an output parameter.
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While we're here, is it worth being consistent within this bullet list as to whether we end with a period or not?

- It shall not be a pointer type.

> *Example*: The following is an example of a static class that declares two extension methods:
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