Why does explicit interface implementation cause CS0453, where implicit does not?

[ashmind]

Example code:

#nullable enable

using System.Collections.Generic;

public interface I1 {
    IEnumerable<T?> M<T>();
}
public class C1 : I1 {
    public IEnumerable<T?> M<T>() => new T[0];
}

public interface I2 {
    IEnumerable<T?> M<T>();
}
public class C2 : I2 {
    // CS0453: The type 'T' must be a non-nullable value type in order 
    // to use it as parameter 'T' in the generic type or method 'Nullable<T>'
    // (and few other errors, but this is the main one I think)
    IEnumerable<T?> I2.M<T>() => new T[0];
}

https://sharplab.io/#v2:D4YgdgrgNlCGBGUCmACJYHILAChcAEAGFfARgBYBuXAgZhQEswAXJAJwDNYBjVASVIoA3rhRiSpWgB4AKgH4AfCgCyshQAoAlNRwBfOiQBMKAMKCAXCgHDR4/PTLT5S1TI2aUAXiVgkAdxQZAG1CAF0dfTwce0YWdi5eK2MRHHEJJ0UVNS0Ig3xjE2NLPmTbMUdZTJKAOld3Lx9/QJDw3F0gA===

Why does C2 cause CS0453, while C1 does not?

[333fred]

This is one of the cases you need a where T : default: https://sharplab.io/#v2:D4YgdgrgNlCGBGUCmACJYHILAChcAEAGFfARgBYBuXAgZhQEswAXJAJwDNYBjVASVIoA3rhRiSpWgB4AKgH4AfCgCyshQAoAlNRwBfOiQBMKAMKCAXCgHDR4/PTLT5S1TI2aUAXiVgkAdxQZAG1CAF0dfTwce0YWdi5eK2MRHHEJJ0UVNS0Ig3xjE2NLPmTbMUdZTJKAOld3FD8AC3ZUGRRLABMkLmhmbxRfAOCwiKA=

[ashmind]

Thanks! It wasn't obvious at all, but now I get it.

For anyone finding this discussion: the reason why it needs default is because explicit implementations (and overriding members) could do T? without where T: struct even before C# 9. They inherited constraints from the parent, so it just meant Nullable<T> and an expectation for the parent to provide where T: struct.

Adding where T: default is a pseudo-constraint that makes it clear to the compiler it's the C# 9 T? and not historical Nullable<T>.

I'll raise a suggestion in Roslyn to improve error experience here.

[ashmind]

Raised a Roslyn suggestion: dotnet/roslyn#52545