Unable to index into union type from two arrays

[afscrome]

Bicep version
0.3.255 (VS Code)

Describe the bug
In the below code locations always gets set to some form of array, however trying to index into regions results in the error

Cannot index over expression of type "'northeurope'[] | array". Arrays or objects are required.bicep(BCP076)

To Reproduce

param isProdLike bool

var testLocations = [
  'northeurope'
]
var prodLocations = [
  'northeurope'
  'westeurope'
]
var locations = isProdLike ? prodLocations : testLocations
var primaryLocation = regions[0]

Additional context

The problem seems to be because locations is being considered as type 'northeurope'[] | array rather than just array. A workaround for this behaviour is to explicitly testLocations to be an array type, but I wouldn't expect this cast to be needed

// Error
var locations = isProdLike ? prodLocations :  testLocations
// No error
var locations = isProdLike ? prodLocations :  array(testLocations)

[majastrz]

Yeah, it's a bug in our type system around handling of union types. Something like northeurope[] | array should collapse into array type.

[majastrz]

Related to #449 and #1735. I think we need a mechanism to simplify redundant union types in the type system. For this specific issue, we need to simplify the type of the item in an array type. But there are other cases. For example, string | 'a' | 'b' is equivalent to string. Similarly string | string is the same as the string type.

[majastrz]

In addition to not simplifying redundant union types, we're also not handling union types correctly when type checking array access expressions. Indexing over a type like 'northeurope'[] | array via an integer should produce an item type of 'northeurope' | any or just any once we have the logic to implify redundant union types.

A similar problem exists with the logic that type checks property access expressions.