Fix backwards count of Indic graphemes

fix other bug add extra test

Author: Azoy

stdlib/public/core/StringGraphemeBreaking.swift

@@ -390,7 +390,8 @@ extension _StringGuts {
       // If we're currently in an indic sequence (or if our lhs is a linking
       // consonant), then this check and everything underneath ensures that
       // we continue being in one and may check if this extend is a Virama.
-      if state.isInIndicSequence || scalar1._isLinkingConsonant {
+      if state.isInIndicSequence ||
+         (!isBackwards && scalar1._isLinkingConsonant) {
         if y == .extend {
           let extendNormData = Unicode._NormData(scalar2, fastUpperbound: 0x300)
 
@@ -440,7 +441,8 @@ extension _StringGuts {
     // GB999
     default:
       // GB9c
-      if state.isInIndicSequence, state.hasSeenVirama, scalar2._isLinkingConsonant {
+      if !isBackwards, state.isInIndicSequence, state.hasSeenVirama,
+         scalar2._isLinkingConsonant {
         state.hasSeenVirama = false
         return false
       }

validation-test/stdlib/String.swift

@@ -2332,6 +2332,17 @@ StringTests.test("GraphemeBreaking.Indic Sequences") {
   let test10 = "\u{003F}\u{094D}\u{0924}" // 2
   expectEqual(2, test10.count)
   expectBidirectionalCount(2, test10)
+
+#if _runtime(_ObjC)
+  let test11Foreign = NSString(string: "\u{930}\u{93e}\u{91c}\u{94d}") // 2
+  let test11 = test11Foreign as String
+  expectEqual(2, test11.count)
+  expectBidirectionalCount(2, test11)
+#endif
+
+  let test12 = "a\u{0915}\u{093C}\u{200D}\u{094D}\u{0924}a" // 3
+  expectEqual(3, test12.count)
+  expectBidirectionalCount(3, test12)
 }
 
 runAllTests()