semantic model vs extension method group

[gafter]

When binding an instance method group, we try twice. First, assuming we will find something applicable, we apply the language rules correctly. Then, if that fails, we are more relaxed and gather even inapplicable and inaccessible candidates. This permits us to produce better diagnostics and information from SemanticModel for failed invocations.

However, we only do one pass for extension methods. As a result the SemanticModel has to do some work on its own to try to reconstruct things. It cannot always do a decent job. Below shows a regression in a test in a recent PR due to this. There was a line that used to read

            Assert.Equal(CandidateReason.Inaccessible, semanticInfo.CandidateReason);

that now reads

            Assert.Equal(CandidateReason.OverloadResolutionFailure, semanticInfo.CandidateReason);

That is because the compiler now avoids considering inaccessible methods when resolving extension methods (this was necessary to fix #25813)

The offending test is

        [WorkItem(542850, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/542850")]
        [Fact]
        public void InaccessibleExtensionMethod()
        {
            string sourceCode = @"
using System;
using System.Collections.Generic;
using System.Linq;

public static class Extensions
{
    private static int Goo(this string z) { return 3; }
}

class Program
{
    static void Main(string[] args)
    {
        args[0]./*<bind>*/Goo/*</bind>*/();
    }
}
";
            var semanticInfo = GetSemanticInfoForTest<IdentifierNameSyntax>(sourceCode);

            Assert.Null(semanticInfo.Type);
            Assert.Null(semanticInfo.ConvertedType);
            Assert.Equal(ConversionKind.Identity, semanticInfo.ImplicitConversion.Kind);

            Assert.Null(semanticInfo.Symbol);
            Assert.Equal(CandidateReason.OverloadResolutionFailure, semanticInfo.CandidateReason);
            Assert.Equal(1, semanticInfo.CandidateSymbols.Length);
            var sortedCandidates = semanticInfo.CandidateSymbols.OrderBy(s => s.ToTestDisplayString(), StringComparer.Ordinal).ToArray();
            Assert.Equal("System.Int32 System.String.Goo()", sortedCandidates[0].ToTestDisplayString());
            Assert.Equal(SymbolKind.Method, sortedCandidates[0].Kind);

            Assert.Equal(1, semanticInfo.MethodGroup.Length);
            var sortedMethodGroup = semanticInfo.MethodGroup.OrderBy(s => s.ToTestDisplayString(), StringComparer.Ordinal).ToArray();
            Assert.Equal("System.Int32 System.String.Goo()", sortedMethodGroup[0].ToTestDisplayString());

            Assert.False(semanticInfo.IsCompileTimeConstant);
        }