Request: Make generic types with explicit layout a compiler error

[leppie]

The following compiles, but results in a TypeLoadException:

[StructLayout(LayoutKind.Explicit)]
struct Union<T1, T2>
{
    [FieldOffset(0)]
    readonly T1 t1;
    [FieldOffset(0)]
    readonly T2 t2;
}

Could not load type 'Union`2' ... because generic types cannot have explicit layout.

Given it will always fail at runtime, should this not be considered to be a compiler error instead?

[leppie]

While this is not illegal (C#), I think there should be at least a warning, if detectable. I am not sure if this is forbidden in the CLR or just a limitation that could be done in some other implementation.

[jskeet]

Presumably this wouldn't be the case if T1 and T2 were both constrained to be reference types?

[leppie]

@jskeet This happens when constrained too.

This also happens with a class with explicit layout.

[leppie]

This also fails in the following scenario, even though the size can be reliably determined:

class Pointer<T> // or struct
{
    public T Value;
}

[StructLayout(LayoutKind.Explicit)]
struct Union<T1, T2>
{
    [FieldOffset(0)]
    readonly Pointer<T1> t1;
    [FieldOffset(0)]
    readonly Pointer<T2> t2;
}

Maybe a runtime issue?