A syntax error is missing for a misplaced ColonColon token in explicit interface implementation.

[AlekseyTs]

using N = NS1;

public class C : NS1.I
{
    void N::I::M() // Expect an error on a :: before M
    {
    }
}

namespace NS1
{
    interface I
    {
        void M();
    }
}

Observed: No errors
Expected: A syntax error

[Youssef1313]

Fixing this bug would be a breaking change. Is the breaking change acceptable in this case? If it's acceptable, let me know and I can debug it.

With a quick look - I suspect the following condition should be moved up outside its containing if:

roslyn/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Lines 5974 to 5978 in c30efad

	if (separator.Kind != SyntaxKind.DotToken)
	{
	separator = WithAdditionalDiagnostics(separator, GetExpectedTokenError(SyntaxKind.DotToken, separator.Kind, separator.GetLeadingTriviaWidth(), separator.Width));
	separator = ConvertToMissingWithTrailingTrivia(separator, SyntaxKind.DotToken);
	}

i.e, currently the above check doesn't flow in the following path:

roslyn/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Line 6024 in c30efad

explicitInterfaceOpt = _syntaxFactory.ExplicitInterfaceSpecifier(explicitInterfaceName, separator);

(Not much confident without debugging it)

[AlekseyTs]

I think the fix is actually to report ERR_AliasQualAsExpression here

roslyn/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Line 5964 in c30efad

? this.ConvertToMissingWithTrailingTrivia(this.EatToken(), SyntaxKind.DotToken)

as we do here

roslyn/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Line 5927 in c30efad

separator = this.AddError(separator, ErrorCode.ERR_AliasQualAsExpression);

.

[AlekseyTs]

However, it looks like ERR_UnexpectedAliasedName would be more appropriate for the N1::N2::I.M scenario.