[SDK] Fix IndexSet.startIndex for empty sets that used to have values

We can't rely on `_range(at:)` producing valid results for sets that
have no ranges.

Fixes SR-4947.

Author: lilyball

stdlib/public/SDK/Foundation/IndexSet.swift

@@ -237,9 +237,14 @@ public struct IndexSet : ReferenceConvertible, Equatable, BidirectionalCollectio
     }
 
     public var startIndex: Index {
-        // If this winds up being NSNotFound, that's ok because then endIndex is also NSNotFound, and empty collections have startIndex == endIndex
-        let extent = _range(at: 0)
-        return Index(value: extent.lowerBound, extent: extent, rangeIndex: 0, rangeCount: _rangeCount)
+        let rangeCount = _rangeCount
+        if rangeCount > 0 {
+            // If this winds up being NSNotFound, that's ok because then endIndex is also NSNotFound, and empty collections have startIndex == endIndex
+            let extent = _range(at: 0)
+            return Index(value: extent.lowerBound, extent: extent, rangeIndex: 0, rangeCount: _rangeCount)
+        } else {
+            return Index(value: 0, extent: 0..<0, rangeIndex: -1, rangeCount: rangeCount)
+        }
     }
 
     public var endIndex: Index {

test/stdlib/TestIndexSet.swift

@@ -326,7 +326,7 @@ class TestIndexSet : TestIndexSetSuper {
     }
 
     func testEmptyIteration() {
-        let empty = IndexSet()
+        var empty = IndexSet()
         let start = empty.startIndex
         let end = empty.endIndex
 
@@ -345,6 +345,21 @@ class TestIndexSet : TestIndexSetSuper {
         }
 
         expectEqual(count, 0)
+
+        empty.insert(5)
+        empty.remove(5)
+        
+        count = 0
+        for _ in empty {
+            count += 1
+        }
+        expectEqual(count, 0)
+
+        count = 0
+        for _ in empty.rangeView {
+            count += 1
+        }
+        expectEqual(count, 0)
     }
 
     func testSubsequences() {