[SR-7554] Re-project cast subpatterns

A corner-case left over from an earlier fix that made exhaustiveness
checker aware of irrefutable coercions.  If the coercion occured as part
of a sub-pattern, that pattern would be projected with the wrong type
and would fail the intersection test.  Project the pattern with the type
of the scrutinee instead.

Author: CodaFi

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1650,13 +1650,24 @@ namespace {
         switch (IP->getCastKind()) {
         case CheckedCastKind::Coercion:
         case CheckedCastKind::BridgingCoercion: {
-          auto *subPattern = IP->getSubPattern();
-          if (subPattern)
-            return projectPattern(TC, subPattern, sawDowngradablePattern);
+          if (auto *subPattern = IP->getSubPattern()) {
+            // Project the cast target's subpattern.
+            Space castSubSpace = projectPattern(TC, subPattern,
+                                                sawDowngradablePattern);
+            // If we recieved a type space from a named pattern or a wildcard
+            // we have to re-project with the cast's target type to maintain
+            // consistency with the scrutinee's type.
+            if (castSubSpace.getKind() == SpaceKind::Type) {
+
+              return Space::forType(IP->getType(),
+                                    castSubSpace.getPrintingName());
+            }
+            return castSubSpace;
+          }
 
-          // With no subpattern coercions are irrefutable.  Project with the original
-          // type instead of the cast's target type to maintain consistency with the
-          // scrutinee's type.
+          // With no subpattern 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::forType(IP->getType(), Identifier());
         }
         case CheckedCastKind::Unresolved:

test/Sema/exhaustive_switch.swift

@@ -314,6 +314,7 @@ func switcheroo(a: XX, b: XX) -> Int {
 enum PatternCasts {
   case one(Any)
   case two
+  case three(String)
 }
 
 func checkPatternCasts() {
@@ -323,13 +324,22 @@ func checkPatternCasts() {
   case .one(let s as String): print(s)
   case .one: break
   case .two: break
+  case .three: break
   }
 
   // But should warn here.
   switch x {
   case .one(_): print(s)
   case .one: break // expected-warning {{case is already handled by previous patterns; consider removing it}}
   case .two: break
+  case .three: break
+  }
+
+  // And not here
+  switch x {
+  case .one: break
+  case .two: break
+  case .three(let s as String?): print(s as Any)
   }
 }