Incorrect diagnostics for complex union types

[HT154]

Repro:

class Expr
typealias Expression = Expr|String(startsWith("${"), endsWith("}"))

class A {
	username: String|Expression
}

x: A = new {
	username = "admin"
//             ^^^^^^^
//             Constraint violation.
//             Required: startsWith("${")
//             Found: "admin"
}

This goes away if Expression is aliased to just the constrained string or if the alias is "dereferenced" and copied directly into the type annotation of A.username

[HT154]

#104 has been reverted