DocumentationCommentId.GetSymbolsForDeclarationId does not work for local functions, even though CreateDeclarationId does

[svick]

Version Used: master

Steps to Reproduce:

Execute the following test:

[Fact]
public void LocalFunction()
{
    string code = @"
class C
{
    void M() { void Local() {} }
}";

    var comp = CreateCompilation(code);

    var tree = comp.SyntaxTrees.Single();
    var syntax = tree.GetRoot().DescendantNodes().OfType<LocalFunctionStatementSyntax>().Single();
    var symbol = comp.GetSemanticModel(tree).GetDeclaredSymbol(syntax)!;

    var actualDocId = DocumentationCommentId.CreateDeclarationId(symbol);

    string expectedDocId = "M:C.M.Local";

    Assert.Equal(expectedDocId, actualDocId);

    var foundSymbols = DocumentationCommentId.GetSymbolsForDeclarationId(expectedDocId, comp);

    Assert.Equal(new[] { symbol }, foundSymbols);
}

Actual Behavior:

The second asert fails, because GetSymbolsForDeclarationId returns an empty collection.

Expected Behavior:

The whole test succeeds. Or possibly the first Assert does not succeed, if local functions are not supported in documentation comment IDs.

[RikkiGibson]

At a glance it seems like you should not be able to create a declaration id for a local function.

[Youssef1313]

Related to: dotnet/csharplang#2110