No associated IFieldSymbol is provided for a field-like event

[jnm2]

If you create a compilation with the following syntax:

public class Foo
{
    public int X { get; }

    private event EventHandler Y;
}

And you get the ITypeSymbol for the type declaration from the symbolic model, it contains an IFieldSymbol representing the backing field for the property but none for the backing field of the event:

Why is an associated IFieldSymbol surfaced for an autoprop but not an auto event?
Is there any chance that an associated IFieldSymbol might not be surfaced for an autoprop in some circumstances if I got the containing ITypeSymbol from a TypeDeclarationSyntax?

@CyrusNajmabadi reproduced this and asked me to file since this doesn't seem right to him.

roslyn/src/Compilers/Core/Portable/Symbols/IFieldSymbol.cs

Lines 17 to 24 in 611578d

	/// <summary>
	/// If this field serves as a backing variable for an automatically generated
	/// property or a field-like event, returns that
	/// property/event. Otherwise returns null.
	/// Note, the set of possible associated symbols might be expanded in the future to
	/// reflect changes in the languages.
	/// </summary>
	ISymbol AssociatedSymbol { get; }

roslyn/src/Compilers/CSharp/Portable/Symbols/Source/SourceFieldLikeEventSymbol.cs

Lines 74 to 80 in 611578d

	// NOTE: if there's an initializer in source, we'd better create a backing field, regardless of
	// whether or not the initializer is legal.
	if (hasInitializer || !(this.IsExtern || this.IsAbstract))
	{
	_associatedField = MakeAssociatedField(declaratorSyntax);
	// Don't initialize this.type - we'll just use the type of the field (which is lazy and handles var)
	}

[CyrusNajmabadi]

Tagging @jcouv and @gafter

[JohanLarsson]

Also AssociatedField on SourceFieldLikeEventSymbol is internal and hence not exposed in IEventSymbol.

http://sourceroslyn.io/#Microsoft.CodeAnalysis.CSharp/Symbols/Source/SourceFieldLikeEventSymbol.cs,114

[jnm2]

15 hours in, PR is up at #40572.

[jnm2]

My original use case: https://gitter.im/dotnet/roslyn/archives/2019/06/07?at=5cfb12aa3dcdab40030b066c (keep in mind that my HasBackingField extension method was forced to use reflection)

Another use case: DotNetAnalyzers/ReflectionAnalyzers#206

[jnm2]

After discussion in #40572, closing in favor of #40103.