Most validation parameter attribute types cause the AllowNull attribute to be ignored

Steps to reproduce

& { param([AllowNull()] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidateScript({$true})] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidateLength(0, 10)] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidateRange(1,2)] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidatePattern('.*')] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidateSet(1,2)] $Foo) $null -eq $Foo } $null
& { param([AllowNull()] [ValidateDrive("c:")] $Foo) $null -eq $Foo } $null

Expected behavior

True
True
True
True
True
True
True

To clarify, I expected the following behavior, which I think is a useful combination of AllowNull and other validation attributes:

If [AllowNull()] is (also) present:

• If $null is actually being passed, allow the value and ignore any other validation attributes.
• Otherwise, evaluate any other validation attributes as usual.

Actual behavior

True
param [...]  : Cannot validate argument on parameter 'Foo'. The argument is null, empty, or an element of the argument. [...]
[...]
param[...]  : Cannot validate argument on parameter 'Foo'. The argument is null, empty, or an element of the argument. [...]
[...]
param[...]  : Cannot validate argument on parameter 'Foo'. The argument is null, empty, or an element of the argument. [...]
[...]
param[...]  : Cannot validate argument on parameter 'Foo'. The argument is null, empty, or an element of the argument. [...] 
[...]
param[...] : Cannot validate argument on parameter 'Foo'. The argument is null, empty, or an element of the argument. [...]
[...]
param[...] : Cannot validate argument on parameter 'Foo'. The argument is null.

The AllowNull attribute is effectively ignored in the presence of the validation attributes in the snippet; switching the attribute order makes no difference.

Note how the last error message, triggered by the ValidateDrive attribute, differs.

The following validation attributes can be combined with AllowNull:

• ValidateCount
• AllowEmptyString
• AllowEmptyCollection

Environment data

PowerShell Core v6.0.0-beta.3 on macOS 10.12.5
PowerShell Core v6.0.0-beta.3 on Ubuntu 16.04.1 LTS
PowerShell Core v6.0.0-beta.3 on Microsoft Windows 10 Pro (64-bit; v10.0.14393)
Windows PowerShell v5.1.15063.413 on Microsoft Windows 10 Pro (64-bit; v10.0.15063)

[mklement0]

Yes, but only if $null is being passed; in other words:

If [AllowNull()] is (also) present:

• If $null is actually being passed, allow the value and ignore any other validation attributes.
• Otherwise, evaluate any other validation attributes as usual.

[mklement0]

@iSazonov:

Passing $null to param([AllowNull()] [ValidateScript({$true})] $Foo) satisfies both attributes, for instance, yet it fails as described.

By contrast, passing $null to param([AllowNull()] [ValidateCount(0,1)] $Foo) works.

To me it makes sense to allow these combinations, assuming the AllowNull attribute is explicitly specified.