explicit lambda return type behavior with covariant delegates

[alrz]

I think as long as the delegate is covariant this should work since an indirect assignment of the same expression is valid.

// ok
Func<object> a1 = () => "";
        
// unexpected error
Func<object> b1 = string () => "";

[CyrusNajmabadi]

Definitely an interesting case! Would have to see how the spec works here. but i soemwhat agree. This feels like it should fall out from lambda return types and variance. i wonder if there's some rule preventing this, or if ti's an accidental compiler bug.

[jaredpar]

@cston is this "by design"? Not sure it's going to meet the bar for fixing right now but curious as to the intent.

[cston]

Variance is not supported for an explicit lambda return type in the anonymous function conversion: see Explicit return type.

Variance conversions are not allowed from lambda return type to delegate return type (matching similar behavior for parameter types).

That matches the conversion requirement for lambdas with explicitly-typed parameters: see 10.7 Anonymous function conversions.

• If F has an explicitly typed parameter list, each parameter in D has the same modifiers as the corresponding parameter in F and an identity conversion exists between the corresponding parameter in F.

Parameter example: see sharplab.io

Action<string> a1 = (object o) => { }; // error CS1661: ... parameter types do not match

Action<object> a2 = (object o) => { };