[stdlib] Fix _writeBackMutableSlice error messages (#2723)

Author: gribozavr

stdlib/public/core/WriteBackMutableSlice.swift

@@ -39,9 +39,9 @@ internal func _writeBackMutableSlice<
 
   _precondition(
     selfElementIndex == selfElementsEndIndex,
-    "Cannot replace a slice of a MutableCollection with a slice of a larger size")
+    "Cannot replace a slice of a MutableCollection with a slice of a smaller size")
   _precondition(
     newElementIndex == newElementsEndIndex,
-    "Cannot replace a slice of a MutableCollection with a slice of a smaller size")
+    "Cannot replace a slice of a MutableCollection with a slice of a larger size")
 }
 

validation-test/stdlib/CollectionType.swift.gyb

@@ -588,6 +588,26 @@ CollectionTypeTests.test("subscript(_: Range<Index>)/writeback") {
     [ 1, 2, 3, 4, 5, 0, -1, -2, -3, -4 ], collection)
 }
 
+CollectionTypeTests.test("subscript(_: Range<Index>)/defaultImplementation/sliceTooLarge")
+  .crashOutputMatches("Cannot replace a slice of a MutableCollection with a slice of a larger size")
+  .code {
+  var x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+  expectCrashLater()
+  x.withUnsafeMutableBufferPointer { buffer in
+    buffer[2..<4] = buffer[4..<8]
+  }
+}
+
+CollectionTypeTests.test("subscript(_: Range<Index>)/defaultImplementation/sliceTooSmall")
+  .crashOutputMatches("Cannot replace a slice of a MutableCollection with a slice of a smaller size")
+  .code {
+  var x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+  expectCrashLater()
+  x.withUnsafeMutableBufferPointer { buffer in
+    buffer[2..<6] = buffer[6..<8]
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // isEmpty
 //===----------------------------------------------------------------------===//