Should VariantExpression allow expressions as keys?

[stasm]

Given a term defined as follows:

-thing = {
    *[one] Thing
    [other] Things
}

Should we allow this?

own = You have { $count ->
    *[one] {-thing[$count]}
    [other] {-thing[$count]}
}

Right now we require the proper key to be specified in the VariantExpression, like so:

own = You have { $count ->
    *[one] {-thing[one]}
    [other] {-thing[other]}
}

Note that because we allow all valid variant keys, this is currently also legal:

own = You have { $count ->
    *[one] {-thing[1]}
    [other] {-thing[other]}
}

[stasm]

@Pike pointed out that we can't allow all inline expressions as keys because of the ambiguity of the -term[key] syntax. Right now key is the name of the requested variant, but if all expressions are allowed, it would parse as a MessageReference.

[stasm]

With the removal of VariantLists and VariantExpressions in #204 and #214, this issue can be closed. However, the problem remains valid for named arguments in call expressions. I filed #230 to continue the discussion about them.