Support for fallthrough into cases with pattern variables.

lib/Sema/MiscDiagnostics.cpp

@@ -2355,6 +2355,29 @@ public:
           });
         }
     }
+    
+    // A fallthrough dest case's bound variable means the source case's
+    // var of the same name is read.
+    if (auto *fallthroughStmt = dyn_cast<FallthroughStmt>(S)) {
+      if (auto *sourceCase = fallthroughStmt->getFallthroughSource()) {
+        SmallVector<VarDecl *, 4> sourceVars;
+        auto sourcePattern = sourceCase->getCaseLabelItems()[0].getPattern();
+        sourcePattern->collectVariables(sourceVars);
+        
+        auto destCase = fallthroughStmt->getFallthroughDest();
+        auto destPattern = destCase->getCaseLabelItems()[0].getPattern();
+        destPattern->forEachVariable([&](VarDecl *V) {
+          if (!V->hasName())
+            return;
+          for (auto *var : sourceVars) {
+            if (var->hasName() && var->getName() == V->getName()) {
+              VarDecls[var] |= RK_Read;
+              break;
+            }
+          }
+        });
+      }
+    }
       
     return { true, S };
   }