Enforce forward collection preconditions in LazyFilterCollection

Author: Max Moiseev

stdlib/public/core/Filter.swift

@@ -241,32 +241,6 @@ extension LazyFilterCollection : LazyCollectionProtocol, Collection {
     i = index
   }
 
-  @_inlineable // FIXME(sil-serialize-all)
-  // FIXME(conditional-conformances): this is a copy of a default
-  // implementation. Can be removed once teh default one allows negative
-  // results to be returned.
-  public func distance(from start: Index, to end: Index) -> Int {
-    var _start: Index
-    let _end: Index
-    let step: Int
-    if start > end {
-      _start = end
-      _end = start
-      step = -1
-    }
-    else {
-      _start = start
-      _end = end
-      step = 1
-    }
-    var count = 0
-    while _start != _end {
-      count += step
-      formIndex(after: &_start)
-    }
-    return count
-  }
-
   @inline(__always)
   @_inlineable // FIXME(sil-serialize-all)
   @_versioned // FIXME(sil-serialize-all)
@@ -276,10 +250,29 @@ extension LazyFilterCollection : LazyCollectionProtocol, Collection {
       } while i != _base.endIndex && !_predicate(_base[i])
   }
 
+  @inline(__always)
+  @_inlineable // FIXME(sil-serialize-all)
+  @_versioned // FIXME(sil-serialize-all)
+  internal func _ensureBidirectional(step: Int) {
+    // FIXME: This seems to be the best way of checking whether _base is
+    // forward only without adding an extra protocol requirement.
+    // index(_:offsetBy:limitedBy:) is chosen becuase it is supposed to return
+    // nil when the resulting index lands outside the collection boundaries,
+    // and therefore likely does not trap in these cases.
+    if step < 0 {
+      _ = _base.index(
+        _base.endIndex, offsetBy: step, limitedBy: _base.startIndex)
+    }
+  }
+
   @_inlineable // FIXME(sil-serialize-all)
   public func index(_ i: Index, offsetBy n: Int) -> Index {
     var i = i
     let step = n.signum()
+    // The following line makes sure that index(_:offsetBy:) is invoked on the
+    // _base at least once, to trigger a _precondition in forward only
+    // collections.
+    _ensureBidirectional(step: step)
     for _ in 0 ..< abs(numericCast(n)) {
       _advanceIndex(&i, step: step)
     }
@@ -297,6 +290,10 @@ extension LazyFilterCollection : LazyCollectionProtocol, Collection {
   ) -> Index? {
     var i = i
     let step = n.signum()
+    // The following line makes sure that index(_:offsetBy:limitedBy:) is
+    // invoked on the _base at least once, to trigger a _precondition in
+    // forward only collections.
+    _ensureBidirectional(step: step)
     for _ in 0 ..< abs(numericCast(n)) {
       if i == limit {
         return nil