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Extensions: improve error recovery in older language versions #80206

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Merged
merged 1 commit into from
Sep 11, 2025
Merged

Extensions: improve error recovery in older language versions #80206

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4 changes: 3 additions & 1 deletion src/Compilers/CSharp/Portable/Parser/LanguageParser.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3241,7 +3241,9 @@ private MemberDeclarationSyntax ParseMemberDeclarationCore(SyntaxKind parentKind

private bool IsExtensionContainerStart()
{
return this.CurrentToken.ContextualKind == SyntaxKind.ExtensionKeyword && IsFeatureEnabled(MessageID.IDS_FeatureExtensions);
// For error recovery, we recognize `extension` followed by `<` even in older language versions
return this.CurrentToken.ContextualKind == SyntaxKind.ExtensionKeyword &&
(IsFeatureEnabled(MessageID.IDS_FeatureExtensions) || this.PeekToken(1).Kind == SyntaxKind.LessThanToken);
}

// if the modifiers do not contain async or replace and the type is the identifier "async" or "replace", then
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12 changes: 10 additions & 2 deletions src/Compilers/CSharp/Portable/Symbols/Source/SourceConstructorSymbol.cs
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Expand Up @@ -47,8 +47,16 @@ private SourceConstructorSymbol(

if (syntax.Identifier.ValueText != containingType.Name)
{
// This is probably a method declaration with the type missing.
diagnostics.Add(ErrorCode.ERR_MemberNeedsType, location);
if (syntax.Identifier.Text == "extension")
{
bool reported = !MessageID.IDS_FeatureExtensions.CheckFeatureAvailability(diagnostics, syntax);
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@jcouv jcouv Sep 10, 2025

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📝 I intentionally didn't introduce a dedicated error message for this niche scenario, where the user intends to write a constructor but accidentally named it "extension" or should have named the type "extension". I think it's safe to assume the user wants an extension block

Debug.Assert(reported);
}
else
{
// This is probably a method declaration with the type missing.
diagnostics.Add(ErrorCode.ERR_MemberNeedsType, location);
}
}

bool hasAnyBody = syntax.HasAnyBody();
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5 changes: 3 additions & 2 deletions src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs
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Expand Up @@ -1843,13 +1843,16 @@ internal override IEnumerable<Symbol> GetInstanceFieldsAndEvents()
protected void AfterMembersChecks(BindingDiagnosticBag diagnostics)
{
var compilation = DeclaringCompilation;
var location = GetFirstLocation();

if (IsInterface)
{
CheckInterfaceMembers(this.GetMembersAndInitializers().NonTypeMembers, diagnostics);
}
else if (IsExtension)
{
CheckExtensionMembers(this.GetMembers(), diagnostics);
MessageID.IDS_FeatureExtensions.CheckFeatureAvailability(diagnostics, compilation, location);
}

CheckMemberNamesDistinctFromType(diagnostics);
Expand All @@ -1869,8 +1872,6 @@ protected void AfterMembersChecks(BindingDiagnosticBag diagnostics)
ReportRequiredMembers(diagnostics);
}

var location = GetFirstLocation();

if (this.IsRefLikeType)
{
compilation.EnsureIsByRefLikeAttributeExists(diagnostics, location, modifyCompilation: true);
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