Use control flow analyzer for switch case fallthrough checks

Author: ahejlsberg

src/compiler/binder.ts

@@ -1341,7 +1341,7 @@ namespace ts {
                 bind(clause);
                 fallthroughFlow = currentFlow;
                 if (!(currentFlow.flags & FlowFlags.Unreachable) && i !== clauses.length - 1 && options.noFallthroughCasesInSwitch) {
-                    errorOnFirstToken(clause, Diagnostics.Fallthrough_case_in_switch);
+                    clause.fallthroughFlowNode = currentFlow;
                 }
             }
             clauses.transformFlags = subtreeTransformFlags | TransformFlags.HasComputedFlags;

src/compiler/checker.ts

@@ -31079,6 +31079,9 @@ namespace ts {
                     }
                 }
                 forEach(clause.statements, checkSourceElement);
+                if (compilerOptions.noFallthroughCasesInSwitch && clause.fallthroughFlowNode && isReachableFlowNode(clause.fallthroughFlowNode)) {
+                    grammarErrorOnFirstToken(clause, Diagnostics.Fallthrough_case_in_switch);
+                }
             });
             if (node.caseBlock.locals) {
                 registerForUnusedIdentifiersCheck(node.caseBlock);

src/compiler/types.ts

@@ -2250,12 +2250,14 @@ namespace ts {
         parent: CaseBlock;
         expression: Expression;
         statements: NodeArray<Statement>;
+        /* @internal */ fallthroughFlowNode?: FlowNode;
     }
 
     export interface DefaultClause extends Node {
         kind: SyntaxKind.DefaultClause;
         parent: CaseBlock;
         statements: NodeArray<Statement>;
+        /* @internal */ fallthroughFlowNode?: FlowNode;
     }
 
     export type CaseOrDefaultClause = CaseClause | DefaultClause;