Compiler does not require System.Attribute for attribute annotation types

[eiriktsarpalis]

I just came across an interesting behaviour, in which the F# compiler only requires a type to have a parameterless constructor in order to be accepted as an attribute annotation.

Repro steps

The following code compiles without issue:

type Foo () = class end
[<Foo>] type Bar = class end

Reproducible in both F# 4.1 and 4.0.

Expected behavior

Compilation should fail. Compare with C#

public class Foo
{
     public Foo() { }
}

[Foo]
public class Bar { }

which fails with the error

error CS0616: 'Foo' is not an attribute class

This issue actually caused runtime issues for me, because I accidentally inserted a wrong type in attribute annotations.

[dsyme]

Thanks @eiriktsarpalis, yes, we should fix that

[cartermp]

This is completed