Why is [pscustomobject] the same as [psobject], even though a distinct [System.Management.Automation.PSCustomObject] type exists?

Surprisingly, [pscustomobject] is the same as [psobject]: both these type accelerators point to type [System.Management.Automation.PSObject], even though there is a distinct [System.Management.Automation.PSCustomObject] type.

Mostly, this conflation goes unnoticed (and has come up before - see #2295), but:

• what is the rationale for it?

• it makes for surprising behavior on occasion - see below.

# Even though their names strongly suggest identity, they are different types.
> [pscustomobject] -eq [System.Management.Automation.PSCustomObject]
False

# Any object returned by a *command* (as opposed to an expression)
# returns $True for -is [psobject], and therefore also [pscustomobject]
> (Get-Item /) -is [pscustomobject]
True

# Casting anything other than a hashtable literal to [psobject] is a no-op, 
# and therefore also with [pscustomobject]
> ([pscustomobject] 666).GetType().Name
Int32

Environment data

PowerShell Core v6.0.0-beta.4

[AikenBM]

Personally, I’ve always found this behavior extremely confusing:

PS> (New-Object -TypeName System.Management.Automation.PSObject -Property @{Property = 'Value'}).GetType().FullName
System.Management.Automation.PSCustomObject

PS> (New-Object -TypeName System.Management.Automation.PSCustomObject -Property @{Property = 'Value'}).GetType().FullName
New-Object : A constructor was not found. Cannot find an appropriate constructor for type System.Management.Automation.PSCustomObject.

PS> ([PSCustomObject]@{Property = 'Value'}).GetType().FullName
System.Management.Automation.PSCustomObject

PS> ([PSObject]@{Property = 'Value'}).GetType().FullName
System.Collections.Hashtable

[SteveL-MSFT]

For now, I suggest using Documentation Needed as a way to review issues later to determine where we need to add documentation (and not just for PRs where a doc change is also needed).

Thanks for your continued depth of research into these advanced topics.