Poor diagnostic wrong number of Type Arguments...

[YairHalberstadt]

... with a type that exists in both a generic and non generic version

Version Used:

master

Steps to Reproduce:

Compile the following code

using System;
public class C : IComparable<int, int> {}

https://sharplab.io/#v2:EYLgtghgzgLgpgJwD4AEAMACFBGA3AWACgUBmLAJgwGEMQMBJKgezAAcIEJgAbOAHgCWAOxgAaDMJgA+DAG8AvkA

Expected Behavior:

error CS0305: Using the generic type 'IComparable<T>' requires 1 type arguments

Actual Behavior:

error CS0308: The non-generic type 'IComparable' cannot be used with type arguments

Whilst the semantics are debatable, this error message is (almost) just plain false :-)

[gafter]

The diagnostic is precisely correct. See https://docs.microsoft.com/en-us/dotnet/api/system.icomparable?view=netframework-4.7.2 : the non-generic type IComparable indeed cannot be used with type arguments.

[sharwell]

I agree with @YairHalberstadt here (CS0305 is more likely related to the user's intent than CS0308). A similar situation occurs with ValueTuple<int, int, int, int, int, int, int, int, int>. Rather than assume the user meant to reference ValueTuple, it seems like the generic type with the closest number of type parameters should be used as the reference. For the original case above, this would be IComparable<T>. For the case I added, it would be ValueTuple<T1, T2, T3, T4, T5, T6, T7, TRest>.