I agree, it seems to be heavily influenced by C#. I am primarily a C# dev and reading through the code and docs I seem to have no mental friction, as opposed to ObjC which seemed to make me bang my head on my desk.

I think any attempt to precisely pin down where a particular language feature has come from or where a language draws inspiration from is pretty futile.

Many language constructs were invented simultaneously under different names in different languages.

One of the Swift developers is a former F# dev:

https://twitter.com/jopamer

The language seems more closer to F# than C# to me. It's all good though!

Type parameters constrained by sets of method signatures dates to 1977: http://en.m.wikipedia.org/wiki/CLU_(programming_language). See also, Theta and some of Todd Millstien's work in the 1990s.