Just as with concrete types, if we find that the same suffix has two
different superclass symbols in the property map, we need to introduce
same-type requirements between their generic parameters.
The added wrinkle is that the classes might be different; in this case,
one must be a superclass of the other, and we repeatedly substitute
the generic arguments until we get the generic arguments of the
superclass before we unify.
Also, introduce the layout requirement implied by a superclass requirement
when lowering requirements, instead of when building the property map.
Right now this is functionally equivalent, but if we ever start to
test properties by checking for joinability of T with T.[p] instead of
looking at the property map entry of T, this new formulation is the
right one.
The property map stores the concrete type or superclass bound for all
terms whose suffix is equal to some key, so eg if you have
protocol P {
associatedtype T where T == U?
associatedtype U
}
Then you have this rewrite system
[P].T => [P:T]
[P].U => [P:U]
[P:T].[concrete: Optional<τ_0_0> with <[P:U]>] => [P:T]
Which creates this property map
[P:T] => { [concrete: Optional<τ_0_0> with <[P:U]>] }
Now if I start with a generic signature like
<T, U where U : P>
This produces the following rewrite system:
τ_0_1.[U] => τ_0_1
τ_0_1.T => τ_0_1.[P:T]
τ_0_1.U => τ_0_1.[P:U]
Consider the computation of the canonical type of τ_0_1.T. This term
reduces to τ_0_1.[P:T]. The suffix [P:T] has a concrete type symbol in
the property map, [concrete: Optional<τ_0_0> with <[P:U]>].
However it would not be correct to canonicalize τ_0_1.[P:T] to
Optional<τ_0_0>.subst { τ_0_0 => getTypeForTerm([P:T]) }; this
produces the type Optional<τ_0_0.T>, and not Optional<τ_0_1.T> as
expected.
The reason is that the substitution τ_0_0 => getTypeForTerm([P:T])
is "relative" to the protocol Self type of P, since domain([P:T]) = {P}.
Indeed, the right solution here is to note that τ_0_1.[P:T] has a suffix
equal to the key of the property map entry, [P:T]. If we strip off the
suffix, we get τ_0_1. If we then prepend τ_0_1 to the substitution term,
we get the term τ_0_1.[P:U], whose canonical type is τ_0_1.U.
Now, applying the substitution τ_0_0 => τ_0_1.U to Optional<τ_0_0>
produces the desired result, Optional<τ_0_1.U>.
Note that this is the same "prepend a prefix to each substitution of
a concrete type symbol" operation that is performed in checkForOverlap()
and PropertyBag::copyPropertiesFrom(), however we can simplify things
slightly by open-coding it instead of calling the utility method
prependPrefixToConcreteSubstitutions(), since the latter creates a
new symbol, which we don't actually need.
