Compiler Error CS1750

A value of type cannot be used as a default parameter because there are no standard conversions to type

Example

The following sample generates CS1750:

public struct S
{
    public override string ToString() { return "S::ToString"; }
}
public class A
{
    public static S Goo(S p = 42) { return p; }
}

There is no standard conversion between int and the newly declared struct S. Using an int compile-time constant to initialize an instance of struct S results in CS1750. Adding a user-defined conversion operator (for example, public static implicit operator S(int n) => ...) will not correct this error because that does not add a standard conversion.