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Field-backed properties: report diagnostic for variable named field declared in accessor #76671

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@cston cston commented Jan 8, 2025

Report an error for a declaration of a local or parameter named field within an accessor, since field will not bind to that variable. See item 2 in #76031.

Based on the variable declaration checks reverted in #74164.

@dotnet-issue-labeler dotnet-issue-labeler bot added Area-Compilers untriaged Issues and PRs which have not yet been triaged by a lead labels Jan 8, 2025
@cston cston force-pushed the variable-named-field branch from 0ada606 to b40643e Compare January 9, 2025 00:58
@@ -6886,8 +6886,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
<data name="WRN_FieldIsAmbiguous_Title" xml:space="preserve">
<value>The 'field' keyword binds to a synthesized backing field for the property.</value>
</data>
<data name="INF_IdentifierConflictWithContextualKeyword_Title" xml:space="preserve">
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INF_IdentifierConflictWithContextualKeyword_Title

Unused.

@cston cston marked this pull request as ready for review January 9, 2025 01:07
@cston cston requested a review from a team as a code owner January 9, 2025 01:07
@RikkiGibson RikkiGibson self-assigned this Jan 9, 2025
@cston cston force-pushed the variable-named-field branch from ca99664 to e89a75b Compare January 9, 2025 22:30
@cston cston requested review from 333fred and RikkiGibson January 10, 2025 16:42
@cston cston force-pushed the variable-named-field branch from aad5168 to bd70829 Compare January 10, 2025 16:54
<data name="INF_IdentifierConflictWithContextualKeyword_Title" xml:space="preserve">
<value>Identifier is a contextual keyword, with a specific meaning, in a later language version.</value>
<data name="ERR_VariableDeclarationNamedField" xml:space="preserve">
<value>In language version {0}, the 'field' keyword binds to a synthesized backing field for the property. Unescaped references to 'field' within the scope of this variable will refer to the synthesized field rather than this variable.</value>
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I think a message like the following would be better:

field is a keyword in this context. Rename the variable or use the identifier @field instead.

IMO, this gets the point across more effectively, and focuses the message on what the user needs to know (how to unblock). I think users who get this error on upgrade will already see that this is a new thing in the new language version.

@@ -561,6 +561,8 @@ private BoundStatement BindLocalFunctionStatement(LocalFunctionStatementSyntax n
var hasErrors = localSymbol.ScopeBinder
.ValidateDeclarationNameConflictsInScope(localSymbol, diagnostics);

ReportFieldContextualKeywordConflictIfAny(localSymbol, node, node.Identifier, diagnostics);
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I was curious if there was any more "central" location to do this check, like where local symbols are gathered up into a scope, or some such. It looks like probably not. Places like LocalScopeBinder.MakeLocal are not really designed for reporting diagnostics.

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I haven't found a central location to catch these cases.

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