Actually slice things in _StringGutsSlice (5.7 cherry pick)

stdlib/public/core/String.swift

@@ -946,10 +946,11 @@ extension _StringGutsSlice {
   }
 
   internal func _withNFCCodeUnits(_ f: (UInt8) throws -> Void) rethrows {
+    let substring = String(_guts)[range]
     // Fast path: If we're already NFC (or ASCII), then we don't need to do
     // anything at all.
     if _fastPath(_guts.isNFC) {
-      try String(_guts).utf8.forEach(f)
+      try substring.utf8.forEach(f)
       return
     }
 
@@ -962,7 +963,7 @@ extension _StringGutsSlice {
       // Because we have access to the fastUTF8, we can go through that instead
       // of accessing the UTF8 view on String.
       if isNFCQC {
-        try _guts.withFastUTF8 {
+        try withFastUTF8 {
           for byte in $0 {
             try f(byte)
           }
@@ -971,23 +972,23 @@ extension _StringGutsSlice {
         return
       }
     } else {
-      for scalar in String(_guts).unicodeScalars {
+      for scalar in substring.unicodeScalars {
         if !_isScalarNFCQC(scalar, &prevCCC) {
           isNFCQC = false
           break
         }
       }
 
       if isNFCQC {
-        for byte in String(_guts).utf8 {
+        for byte in substring.utf8 {
           try f(byte)
         }
 
         return
       }
     }
 
-    for scalar in String(_guts)._nfc {
+    for scalar in substring._nfc {
       try scalar.withUTF8CodeUnits {
         for byte in $0 {
           try f(byte)

test/stdlib/StringAPI.swift

@@ -157,6 +157,28 @@ func checkStringComparison(
   expectEqual(expected.isGE(), lhs >= rhs, stackTrace: stackTrace)
   expectEqual(expected.isGT(), lhs > rhs, stackTrace: stackTrace)
   checkComparable(expected, lhs, rhs, stackTrace: stackTrace.withCurrentLoc())
+
+  // Substring / Substring
+  // Matching slices of != Strings may still be ==, but not vice versa
+  if expected.isEQ() {
+    for i in 0 ..< Swift.min(lhs.count, rhs.count) {
+      let lhsSub = lhs.dropFirst(i)
+      let rhsSub = rhs.dropFirst(i)
+
+      expectEqual(expected.isEQ(), lhsSub == rhsSub, stackTrace: stackTrace)
+      expectEqual(expected.isNE(), lhsSub != rhsSub, stackTrace: stackTrace)
+      checkHashable(
+        expectedEqual: expected.isEQ(),
+        lhs, rhs, stackTrace: stackTrace.withCurrentLoc())
+
+      expectEqual(expected.isLT(), lhsSub < rhsSub, stackTrace: stackTrace)
+      expectEqual(expected.isLE(), lhsSub <= rhsSub, stackTrace: stackTrace)
+      expectEqual(expected.isGE(), lhsSub >= rhsSub, stackTrace: stackTrace)
+      expectEqual(expected.isGT(), lhsSub > rhsSub, stackTrace: stackTrace)
+      checkComparable(
+        expected, lhsSub, rhsSub, stackTrace: stackTrace.withCurrentLoc())
+    }
+  }
 
 #if _runtime(_ObjC)
   // NSString / NSString
@@ -471,5 +493,15 @@ StringTests.test("Regression/corelibs-foundation") {
   expectEqual(substring(of: s5, with: NSFakeRange(1,6)), "\ncats�")
 }
 
+StringTests.test("Regression/radar-87371813") {
+  let s1 = "what♕/".dropFirst(5)
+  let s2 = "/"[...]
+  let s3 = "/⚅".dropLast()
+  expectEqual(s1, s2)
+  expectEqual(s1, s3)
+  expectEqual(s1, s3)
+  expectEqual(s1.hashValue, s2.hashValue)
+  expectEqual(s2.hashValue, s3.hashValue)
+}
 
 runAllTests()