SR-11687: Scanner.scanDouble returns Optional(0.0) instead of nil on failure

- Fix Scanner._scan(): Make the local result an Optional and dont
  initalise it until a valid number has been found.

Author: spevans

Foundation/Scanner.swift

@@ -381,10 +381,11 @@ extension String {
     private func _scan<T: BinaryFloatingPoint>(buffer buf: inout _NSStringBuffer, locale: Locale?, neg: Bool, to: (T) -> Void, base: UInt,
                                                numericValue: ((_: unichar) -> Int?)) -> Bool {
         let ds = (locale ?? Locale.current).decimalSeparator?.first ?? Character(".")
-        var localResult: T = T(0)
+        var localResult: T! = nil
         var neg = neg
 
         while let numeral = numericValue(buf.currentCharacter) {
+            localResult = localResult ?? T(0)
             // if (localResult >= T.greatestFiniteMagnitude / T(10)) && ((localResult > T.greatestFiniteMagnitude / T(10)) || T(numericValue(buf.currentCharacter) - (neg ? 1 : 0)) >= T.greatestFiniteMagnitude - localResult * T(10))  is evidently too complex; so break it down to more "edible chunks"
             let limit1 = localResult >= T.greatestFiniteMagnitude / T(base)
             let limit2 = localResult > T.greatestFiniteMagnitude / T(base)
@@ -407,12 +408,17 @@ extension String {
             var factor = 1 / T(base)
             buf.advance()
             while let numeral = numericValue(buf.currentCharacter) {
+                localResult = localResult ?? T(0)
                 localResult = localResult + T(numeral) * factor
                 factor = factor / T(base)
                 buf.advance()
             }
         }
 
+        guard localResult != nil else {
+            return false
+        }
+
         // If this is used to parse a number in Hexadecimal, this will never be true as the 'e' or 'E' will be caught by the previous loop.
         if buf.currentCharacter == unichar(unicodeScalarLiteral: "e") || buf.currentCharacter == unichar(unicodeScalarLiteral: "E") {
             var exponent = Double(0)
@@ -435,7 +441,7 @@ extension String {
                 }
             }
         }
-        
+
         to(neg ? T(-1) * localResult : localResult)
         return true
     }

TestFoundation/TestScanner.swift

@@ -75,6 +75,19 @@ class TestScanner : XCTestCase {
             expectEqual($0.scanDouble(), atof("3.14"), "Roundtrip: 1")
             expectEqual($0.scanDouble(), atof("-100"), "Roundtrip: 2")
         }
+
+        withScanner(for: "0.5 bla 0. .1 1e2 e+3 e4") {
+            expectEqual($0.scanDouble(), 0.5, "Parse '0.5' as Double")
+            expectEqual($0.scanDouble(), nil, "Dont parse 'bla' as a Double")     // "bla" doesnt parse as Double
+            expectEqual($0.scanString("bla"), "bla", "Consume the 'bla'")
+            expectEqual($0.scanDouble(), 0, "Parse '0.' as a Double")
+            expectEqual($0.scanDouble(), 0.1, "Parse '.1' as a Double")
+            expectEqual($0.scanDouble(), 100, "Parse '1e2' as a Double")
+            expectEqual($0.scanDouble(), nil, "Dont parse 'e+3' as a Double")     // "e+3" doesnt parse as Double
+            expectEqual($0.scanString("e+3"), "e+3", "Consume the 'e+3'")
+            expectEqual($0.scanDouble(), nil, "Dont parse 'e4' as a Double'")     // "e3" doesnt parse as Double
+            expectEqual($0.scanString("e4"), "e4", "Consume the 'e4'")
+        }
     }
 
     func testHexRepresentation() {