How is that different from the workaround in this comment: Constraining associatedtype of non-generic typealias - #2 by Karl
typealias IsFloatColl<T> = T where T: Collection, T.Element: Strideable, T.Element.Stride: BinaryFloatingPoint
which already works today. Isn't a matter of syntax whether you introduce the T explicitly or implicitly desugar something like
// strawman short syntax
typealias IsFloatColl = Collection where Element: Strideable, Element.Stride: BinaryFloatingPoint
// (somehow) implicitly desugared to
typealias IsFloatColl<T> = T where T: Collection, T.Element: Strideable, T.Element.Stride: BinaryFloatingPoint
? Now it's a separate question of how the desugaring would work, but I don't see how it needs a new form of existentials.