[stdlib] Introduce _invariantCheck_5_1 for 5.1 and later assertions.

Inlinable and non-inlinable code can cause 5.1 code to intermix with
5.0 code on older OSes. Some (weak) invariants for 5.1 should only be
checked when the OS's code is 5.1 or later, which is the purpose of
_invariantCheck_5_1.

Applied to String.Index._isScalarAligned, which is a new bit
introduced in 5.1 from one of the reserved bits from 5.0. The bit is
set when the index is proven to be scalar aligned, and we want to
assert on this liberally in contexts where we expect it to be
so. However, older OSes might not set this bit when doing scalar
aligning, depending on exactly what got inlined where/when.

stdlib/public/core/Assert.swift

@@ -236,7 +236,7 @@ public func _overflowChecked<T>(
   let (result, error) = args
   if _isDebugAssertConfiguration() {
     if _slowPath(error) {
-      _fatalErrorMessage("Fatal error", "Overflow/underflow", 
+      _fatalErrorMessage("Fatal error", "Overflow/underflow",
         file: file, line: line, flags: _fatalErrorFlags())
     }
   } else {
@@ -297,6 +297,22 @@ internal func _internalInvariant(
 #endif
 }
 
+// Only perform the invariant check on Swift 5.1 and later
+@_alwaysEmitIntoClient // Swift 5.1
+@_transparent
+internal func _internalInvariant_5_1(
+  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
+  file: StaticString = #file, line: UInt = #line
+) {
+#if INTERNAL_CHECKS_ENABLED
+  // FIXME: The below won't run the assert on 5.1 stdlib if testing on older
+  // OSes, which means that testing may not test the assertion. We need a real
+  // solution to this.
+  guard #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) else { return }
+  _internalInvariant(condition(), message, file: file, line: line)
+#endif
+}
+
 @usableFromInline @_transparent
 internal func _internalInvariantFailure(
   _ message: StaticString = StaticString(),

stdlib/public/core/StringCharacterView.swift

@@ -201,7 +201,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(startingAt i: Index) -> Int {
-    _internalInvariant(i._isScalarAligned)
+    _internalInvariant_5_1(i._isScalarAligned)
 
     // Fast check if it's already been measured, otherwise check resiliently
     if let d = i.characterStride { return d }
@@ -213,7 +213,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(endingAt i: Index) -> Int {
-    _internalInvariant(i._isScalarAligned)
+    _internalInvariant_5_1(i._isScalarAligned)
 
     if i == startIndex { return 0 }
 

stdlib/public/core/StringIndex.swift

@@ -164,7 +164,7 @@ extension String.Index {
   internal func _invariantCheck() {
     _internalInvariant(_encodedOffset >= 0)
     if self._isScalarAligned {
-      _internalInvariant(transcodedOffset == 0)
+      _internalInvariant_5_1(transcodedOffset == 0)
     }
   }
   #endif // INTERNAL_CHECKS_ENABLED

stdlib/public/core/UnicodeHelpers.swift

@@ -172,22 +172,22 @@ extension _StringGuts {
 
     _internalInvariant(isOnUnicodeScalarBoundary(result),
       "Alignment bit is set for non-aligned index")
-    _internalInvariant(result._isScalarAligned)
+    _internalInvariant_5_1(result._isScalarAligned)
     return result
   }
 
   @inline(never) // slow-path
   @_alwaysEmitIntoClient // Swift 5.1
   internal func scalarAlignSlow(_ idx: Index) -> Index {
-    _internalInvariant(!idx._isScalarAligned)
+    _internalInvariant_5_1(!idx._isScalarAligned)
 
     if _slowPath(idx.transcodedOffset != 0 || idx._encodedOffset == 0) {
       // Transcoded index offsets are already scalar aligned
       return String.Index(_encodedOffset: idx._encodedOffset)._scalarAligned
     }
     if _slowPath(self.isForeign) {
       let foreignIdx = foreignScalarAlign(idx)
-      _internalInvariant(foreignIdx._isScalarAligned)
+      _internalInvariant_5_1(foreignIdx._isScalarAligned)
       return foreignIdx
     }