Cannot use existing at scope: tenant() for built-in policies

[alex-frankel]

Bicep version
Bicep CLI version 0.3.539 (c8b397d)

Describe the bug
I cannot get a reference to a built-in policy definition

To Reproduce

resource policydef 'Microsoft.Authorization/policyDefinitions@2020-09-01' existing = {
  scope: tenant()
  name: '10ee2ea2-fb4d-45b8-a7e9-a2e770044cd9'
}

Gives me the error:

Scope "tenant" is not valid for this resource type. Permitted scopes: "managementGroup", "subscription".

goal is to pass policydef.id to a policy assignment

[alex-frankel]

Related to:
#2504 (comment)

[stephaniezyen]

#1398 Duplicate