is duplicated by:

• SR-1664 Improve diagnostic for inability to infer return types from multi line closures
• SR-1773 ..< is ambiguous together with map for certain closures

relates to:

• SR-1570 'Generic parameter 'Result' could not be inferred' compiler error for wrapper functions like autoreleasepool that return the result of their closure argument.

Issue Description:

When the map method is called with closure for its transform argument, and the closure return type specification is omitted, everything works fine if the type of the returned expression is implicit – for example,

let x1 = [1,2].map { _ in
    return "abc"
}

But if the same expression is assigned to a variable, and the variable is returned:

let x2 = [1,2].map { _ in
    let x = "abc"
    return x
}

then the compiler gives an error. In Xcode 7.2.1, the error is

Cannot invoke 'map' with an argument list of type '(@noescape (Int) throws -> _)'
Expected an argument list of type '(@noescape (Self.Generator.Element) throws -> T)'

It seems that in both calls, the transform closure is returning a String, and the compiler knows it is a String, so the error in the second call looks like a compiler bug.

(Making the return type explicit – map { _ -> String in ... } – works around the bug.)