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Discard pattern in switch and is #30650

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gafter opened this issue Oct 22, 2018 · 1 comment
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Discard pattern in switch and is #30650

gafter opened this issue Oct 22, 2018 · 1 comment
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Area-Compilers Feature - Pattern Matching Pattern Matching Feature Request
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16.0

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gafter commented Oct 22, 2018

From dotnet/csharplang#1054

Is the discard pattern permitted at the top level of a switch statement? If so, it could change the meaning of existing code. I suggest we require you write case var _ to avoid any ambiguity.

Resolution 2018-10-10: A discard pattern in the presence of a constant will bind to the constant only in a switch statement case, with a warning given under a warning wave. Elsewhere in a pattern it is a discard pattern. A discard pattern in an is-expression in the presence of a type will bind to the type, with a warning under a warning wave.
@gafter gafter added this to the 16.0 milestone Oct 22, 2018
@gafter gafter self-assigned this Oct 22, 2018
gafter added a commit to gafter/roslyn that referenced this issue Oct 30, 2018
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Discard pattern versus symbol named _
2ccc4d5 
- A discard pattern is not accepted at the top level in a switch statement. In that case `_` binds to a constant in scope with a warning WRN_CaseConstantNamedUnderscore that is included in a warning wave.
- A discard pattern is not accepted at the top level in an is-expression. In that case `_` binds to a type in scope with a warning WRN_IsTypeNamedUnderscore that is included in a warning wave.
- Elsewhere a pattern `_` is a discard without warning no matter what definition for `_` might be in scope.

Fixes dotnet#30650
See also dotnet#30846
@gafter gafter mentioned this issue Oct 30, 2018
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gafter pushed a commit that referenced this issue Nov 6, 2018
Discard pattern versus symbol named _ (#30847)
3a260c7 
* Discard pattern versus symbol named `_`

- A discard pattern is not accepted at the top level in a switch statement. In that case `_` binds to a constant in scope with a warning WRN_CaseConstantNamedUnderscore that is included in a warning wave.
- A discard pattern is not accepted at the top level in an is-expression. In that case `_` binds to a type in scope with a warning WRN_IsTypeNamedUnderscore that is included in a warning wave.
- Elsewhere a pattern `_` is a discard without warning no matter what definition for `_` might be in scope.

Fixes #30650
See also #30846
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gafter commented Nov 6, 2018

Fixed in #30847

@gafter gafter closed this as completed Nov 6, 2018
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