swift234

Restore FixedPointDiagnostics.swift.gyb, it is a useful test

1. We have complex logic in stdlib/public/core/FixedPoint.swift.gyb that emits
or hides certain initializers on integer types based on the bit width
combination.  We check that those APIs are indeed present or absent in cases we
expect.

All of [U]Int{8,16,32,64,} initializers, labelled and unlabelled.

(swift) UInt16(-10 as Int32)
<REPL Input>:1:1: error: integer overflows when converted from 'Int32' to 'UInt16'
UInt16(-10 as Int32)

(swift) UInt16(truncatingBitPattern: -10 as Int32)
// r3 : UInt16 = 65526

(swift) UInt16(truncatingBitPattern: -10 as Int16)
<REPL Input>:1:1: error: cannot invoke initializer for type 'UInt16' with an argument list of type '(truncatingBitPattern: Int16)'
UInt16(truncatingBitPattern: -10 as Int16)
<REPL Input>:1:1: note: overloads for 'UInt16' exist with these partially matching parameter lists: (Int16), (bitPattern: Int16)

A slight change in the type combination dramatically changes the available
overload set.  You can’t check this with one or two tests and be confident that
every source/destination combination is correct.

2. We check that the APIs above are present or absent regardless of the target
platform so that the code is portable (for example, because Int64 -> Int is a
truncating operation on at least one platform, the corresponding “truncating”
initializer is available for portability on 64-bit platforms, even though it is
not actually truncating there.)

3. We rely on ambiguity in the overload set of “+“ and “-“ for certain
combinations of arguments to disallow mixed-sign arithmetic.  Again, these
ambiguities rely on a tricky combination of protocols, associated types, the
way those associated types are defined by our integer types, and overload
resolution rules in the type checker.

4. The test also checks migration facilities for Word and UWord.

Swift SVN r30655