is duplicated by:

• SR-3134 force-unwrapping fails mysteriously when interleaved with + operators
• SR-2636 + Operator fails to infer type for more than 2 force unwrapped values
• SR-3483 error: ambiguous reference to member '+'

Issue Description:

When trying to concatenate more than two strings in swift3 using the "!" symbol on each string, swift outputs an error. This used to work in swift 2.2.

When compiling this:

var a:String? = "a"
var b:String? = "b"
var c:String? = "c"
var d:String? = a! + b! + c!

I get the following error:

error: cannot convert value of type 'String' to specified type 'String?'
var d:String? = a! + b! + c!
                ~~~~~~~~^~~~

Here is the same example in the IBM sandbox

See the answer here on Stack Overflow for more details