Ignore non-useful casts during switch exhaustiveness checking.

Author: gregomni

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1346,11 +1346,19 @@ namespace {
         auto *IP = cast<IsPattern>(item);
         switch (IP->getCastKind()) {
         case CheckedCastKind::Coercion:
-        case CheckedCastKind::BridgingCoercion:
+        case CheckedCastKind::BridgingCoercion: {
+          // If the pattern and subpattern types are identical than this is a
+          // non-useful cast that we've already warned about, but it also means
+          // this pattern itself is a no-op and we should examine the subpattern.
+          auto *subPattern = IP->getSubPattern();
+          if (subPattern && IP->getType()->isEqual(subPattern->getType()))
+            return projectPattern(TC, subPattern, sawDowngradablePattern);
+
           // These coercions are irrefutable.  Project with the original type
           // instead of the cast's target type to maintain consistency with the
           // scrutinee's type.
           return Space(IP->getType(), Identifier());
+        }
         case CheckedCastKind::Unresolved:
         case CheckedCastKind::ValueCast:
         case CheckedCastKind::ArrayDowncast:

test/Sema/exhaustive_switch.swift

@@ -786,3 +786,19 @@ func checkLiteralTuples() {
   default: break
   }
 }
+
+func sr6975() {
+  enum E {
+    case a, b
+  }
+
+  let e = E.b
+  switch e {
+  case .a as E: // expected-warning {{'as' test is always true}}
+    print("a")
+  case .b: // Valid!
+    print("b")
+  case .a: // expected-warning {{case is already handled by previous patterns; consider removing it}}
+    print("second a")
+  }
+}