Issue Description:

It is currently allowed by the compiler to define variables having opaque result types, but creating objects conforming these protocols generates a compiler error.

Example:

protocol Foo {
    associatedtype FooType
}


protocol Bar {
    var barvar: some Foo { get } //No complains here
}


struct RealFoo: Foo {
    typealias FooType = Any
}


struct Baz: Bar { // The compiler assert that Baz does not conform Bar
    var barvar: some Foo { RealFoo() } 
}

Nt sure if the bug is in the fact that protocols should not allow the definition of opaque pointers to avoid associated types or in the fact that the compiler is wrongfully erroring a legitimate conformance... either ways there is something wrong here.

In the first case, the compiler should not allow the definition of var barvar: some Foo { get } in the protocol.

In the second case, Baz should be considered conformant to Bar as it fullfils all the requirements.