[SR-4005] Allow heterogenous comparisons in elementsEqual

When a user is supplying a predicate to compare the type equivalence
isn’t required

Author: Cbieniak

stdlib/public/core/SequenceAlgorithms.swift.gyb

@@ -358,12 +358,14 @@ ${equivalenceExplanation}
   public func elementsEqual<OtherSequence>(
     _ other: OtherSequence${"," if preds else ""}
 %   if preds:
-    by areEquivalent: (${GElement}, ${GElement}) throws -> Bool
+    by areEquivalent: (${GElement}, OtherSequence.${GElement}) throws -> Bool
 %   end
   ) ${rethrows_}-> Bool
     where
-    OtherSequence: Sequence,
+    OtherSequence: Sequence${" {" if preds else ","}
+%   if not preds:
     OtherSequence.${GElement} == ${GElement} {
+%   end
 
     var iter1 = self.makeIterator()
     var iter2 = other.makeIterator()

test/IDE/complete_from_stdlib.swift

@@ -172,7 +172,7 @@ func testArchetypeReplacement3 (_ a : [Int]) {
 // PRIVATE_NOMINAL_MEMBERS_7-DAG: Decl[InstanceMethod]/Super:         dropLast({#(n): Int#})[#ArraySlice<Int>#]
 // PRIVATE_NOMINAL_MEMBERS_7-DAG: Decl[InstanceMethod]/Super:         dropFirst({#(n): Int#})[#AnySequence<Int>#]
 // PRIVATE_NOMINAL_MEMBERS_7-DAG: Decl[InstanceMethod]/Super:         prefix({#(maxLength): Int#})[#AnySequence<Int>#]
-// PRIVATE_NOMINAL_MEMBERS_7-DAG: Decl[InstanceMethod]/Super:         elementsEqual({#(other): Sequence#}, {#by: (Int, Int) throws -> Bool##(Int, Int) throws -> Bool#})[' rethrows'][#Bool#]
+// PRIVATE_NOMINAL_MEMBERS_7-DAG: Decl[InstanceMethod]/Super:         elementsEqual({#(other): Sequence#}, {#by: (Int, IteratorProtocol.Element) throws -> Bool##(Int, IteratorProtocol.Element) throws -> Bool#})[' rethrows'][#Bool#]
 
 
 protocol P2 {

test/api-digester/source-stability.swift.expected

@@ -318,6 +318,7 @@ Func ReversedRandomAccessCollection.index(_:offsetBy:) has return type change fr
 Func ReversedRandomAccessCollection.index(_:offsetBy:limitedBy:) has return type change from ReversedRandomAccessIndex<Base>? to ReversedRandomAccessCollection.Index?
 Func ReversedRandomAccessCollection.index(after:) has return type change from ReversedRandomAccessIndex<Base> to ReversedRandomAccessCollection.Index
 Func ReversedRandomAccessCollection.index(before:) has return type change from ReversedRandomAccessIndex<Base> to ReversedRandomAccessCollection.Index
+Func Sequence.elementsEqual(_:by:) has 2nd parameter type change from (Self.Iterator.Element, Self.Iterator.Element) throws -> Bool to (Self.Iterator.Element, OtherSequence.Iterator.Element) throws -> Bool
 Func Set.formSymmetricDifference(_:) has parameter 0 type change from Set<Element> to Set<Set.Element>
 Func Set.index(after:) has return type change from SetIndex<Element> to Set<Element>.Index
 Func Set.index(of:) has return type change from SetIndex<Element>? to Set<Element>.Index?