No implicit returns following exhaustive switch statements

src/compiler/binder.ts

@@ -650,6 +650,11 @@ namespace ts {
             return isNarrowableReference(expr);
         }
 
+        function isNarrowingSwitchStatement(switchStatement: SwitchStatement) {
+            const expr = switchStatement.expression;
+            return expr.kind === SyntaxKind.PropertyAccessExpression && isNarrowableReference((<PropertyAccessExpression>expr).expression);
+        }
+
         function createBranchLabel(): FlowLabel {
             return {
                 flags: FlowFlags.BranchLabel,
@@ -699,8 +704,7 @@ namespace ts {
         }
 
         function createFlowSwitchClause(antecedent: FlowNode, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number): FlowNode {
-            const expr = switchStatement.expression;
-            if (expr.kind !== SyntaxKind.PropertyAccessExpression || !isNarrowableReference((<PropertyAccessExpression>expr).expression)) {
+            if (!isNarrowingSwitchStatement(switchStatement)) {
                 return antecedent;
             }
             setFlowNodeReferenced(antecedent);
@@ -939,6 +943,9 @@ namespace ts {
             bind(node.caseBlock);
             addAntecedent(postSwitchLabel, currentFlow);
             const hasDefault = forEach(node.caseBlock.clauses, c => c.kind === SyntaxKind.DefaultClause);
+            // We mark a switch statement as possibly exhaustive if it has no default clause and if all
+            // case clauses have unreachable end points (e.g. they all return).
+            node.possiblyExhaustive = !hasDefault && !postSwitchLabel.antecedents;
             if (!hasDefault) {
                 addAntecedent(postSwitchLabel, createFlowSwitchClause(preSwitchCaseFlow, node, 0, 0));
             }

src/compiler/checker.ts

@@ -11808,10 +11808,42 @@ namespace ts {
             return aggregatedTypes;
         }
 
+        function isExhaustiveSwitchStatement(node: SwitchStatement): boolean {
+            const expr = node.expression;
+            if (!node.possiblyExhaustive || expr.kind !== SyntaxKind.PropertyAccessExpression) {
+                return false;
+            }
+            const type = checkExpression((<PropertyAccessExpression>expr).expression);
+            if (!(type.flags & TypeFlags.Union)) {
+                return false;
+            }
+            const propName = (<PropertyAccessExpression>expr).name.text;
+            const propType = getTypeOfPropertyOfType(type, propName);
+            if (!propType || !isStringLiteralUnionType(propType)) {
+                return false;
+            }
+            const switchTypes = getSwitchClauseTypes(node);
+            if (!switchTypes.length) {
+                return false;
+            }
+            return eachTypeContainedIn(propType, switchTypes);
+        }
+
+        function functionHasImplicitReturn(func: FunctionLikeDeclaration) {
+            if (!(func.flags & NodeFlags.HasImplicitReturn)) {
+                return false;
+            }
+            const lastStatement = lastOrUndefined((<Block>func.body).statements);
+            if (lastStatement && lastStatement.kind === SyntaxKind.SwitchStatement && isExhaustiveSwitchStatement(<SwitchStatement>lastStatement)) {
+                return false;
+            }
+            return true;
+        }
+
         function checkAndAggregateReturnExpressionTypes(func: FunctionLikeDeclaration, contextualMapper: TypeMapper): Type[] {
             const isAsync = isAsyncFunctionLike(func);
             const aggregatedTypes: Type[] = [];
-            let hasReturnWithNoExpression = !!(func.flags & NodeFlags.HasImplicitReturn);
+            let hasReturnWithNoExpression = functionHasImplicitReturn(func);
             let hasReturnOfTypeNever = false;
             forEachReturnStatement(<Block>func.body, returnStatement => {
                 const expr = returnStatement.expression;
@@ -11868,7 +11900,7 @@ namespace ts {
 
             // If all we have is a function signature, or an arrow function with an expression body, then there is nothing to check.
             // also if HasImplicitReturn flag is not set this means that all codepaths in function body end with return or throw
-            if (nodeIsMissing(func.body) || func.body.kind !== SyntaxKind.Block || !(func.flags & NodeFlags.HasImplicitReturn)) {
+            if (nodeIsMissing(func.body) || func.body.kind !== SyntaxKind.Block || !functionHasImplicitReturn(func)) {
                 return;
             }
 

src/compiler/types.ts

@@ -1188,6 +1188,7 @@ namespace ts {
     export interface SwitchStatement extends Statement {
         expression: Expression;
         caseBlock: CaseBlock;
+        possiblyExhaustive?: boolean;
     }
 
     // @kind(SyntaxKind.CaseBlock)