Local functions reuse method parameter and type parameter symbols, which add diagnostics to declaration diagnostics

[agocke]

This can cause inconsistencies between the semantic model and standard binding, e.g.

        public void FetchLocalFunctionSymbolThroughLocal()
        {
            var tree = SyntaxFactory.ParseSyntaxTree(@"
using System;
class C
{
    public void M()
    {
        void Local<[A, B, CLSCompliant, D]T>() 
        {
            var x = new object(); 
        }
        Local<int>();
    }
}");
            var comp = CreateCompilationWithMscorlib(tree);
            comp.DeclarationDiagnostics.Verify();
            comp.VerifyDiagnostics(
                // (7,20): error CS8204: Attributes are not allowed on local function parameters or type parameters
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_AttributesInLocalFuncDecl, "[A, B, CLSCompliant, D]").WithLocation(7, 20),
                // (7,21): error CS0246: The type or namespace name 'AAttribute' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "A").WithArguments("AAttribute").WithLocation(7, 21),
                // (7,21): error CS0246: The type or namespace name 'A' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "A").WithArguments("A").WithLocation(7, 21),
                // (7,24): error CS0246: The type or namespace name 'BAttribute' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "B").WithArguments("BAttribute").WithLocation(7, 24),
                // (7,24): error CS0246: The type or namespace name 'B' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "B").WithArguments("B").WithLocation(7, 24),
                // (7,41): error CS0246: The type or namespace name 'DAttribute' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "D").WithArguments("DAttribute").WithLocation(7, 41),
                // (7,41): error CS0246: The type or namespace name 'D' could not be found (are you missing a using directive or an assembly reference?)
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "D").WithArguments("D").WithLocation(7, 41),
                // (7,27): error CS7036: There is no argument given that corresponds to the required formal parameter 'isCompliant' of 'CLSCompliantAttribute.CLSCompliantAttribute(bool)'
                //         void Local<[A, B, CLSCompliant, D]T>() { }
                Diagnostic(ErrorCode.ERR_NoCorrespondingArgument, "CLSCompliant").WithArguments("isCompliant", "System.CLSCompliantAttribute.CLSCompliantAttribute(bool)").WithLocation(7, 27));


            var model = comp.GetSemanticModel(tree);

            var x = tree.GetRoot().DescendantNodes().OfType<VariableDeclaratorSyntax>().Where(v => v.Identifier.ValueText == "x").Single();
            var localSymbol = (LocalFunctionSymbol)model.GetDeclaredSymbol(x).ContainingSymbol;
            var typeParam = localSymbol.TypeParameters.Single();
            var attrs = typeParam.GetAttributes();

            Assert.True(attrs[0].AttributeClass.IsErrorType());
            Assert.True(attrs[1].AttributeClass.IsErrorType());
            Assert.False(attrs[2].AttributeClass.IsErrorType());
            Assert.Equal(comp.GlobalNamespace
                             .GetMember<NamespaceSymbol>("System")
                             .GetMember<NamedTypeSymbol>("CLSCompliantAttribute"),
                attrs[2].AttributeClass);
            Assert.True(attrs[3].AttributeClass.IsErrorType());
            comp.DeclarationDiagnostics.Verify();
        }

In addition, local functions should never add diagnostics to DeclarationDiagnostics.

This may require creating new symbols for local function parameters and type parameters.

[agocke]

Note: Removing GetDefaultSyntaxValue should be reworked as part of this, as per #16736 (diff)