Add flag to omit default case

Author: jack-williams

src/compiler/checker.ts

@@ -16908,7 +16908,9 @@ namespace ts {
 
         // Get the types from all cases in a switch on `typeof`. An
         // `undefined` element denotes an explicit `default` clause.
-        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement): (string | undefined)[] {
+        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement, retainDefault: false): string[];
+        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement, retainDefault: boolean): (string | undefined)[];
+        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement, retainDefault: boolean): (string | undefined)[] {
             const witnesses: (string | undefined)[] = [];
             for (const clause of switchStatement.caseBlock.clauses) {
                 if (clause.kind === SyntaxKind.CaseClause) {
@@ -16918,7 +16920,7 @@ namespace ts {
                     }
                     return emptyArray;
                 }
-                witnesses.push(/*explicitDefaultStatement*/ undefined);
+                if (retainDefault) witnesses.push(/*explicitDefaultStatement*/ undefined);
             }
             return witnesses;
         }
@@ -17934,7 +17936,7 @@ namespace ts {
             }
 
             function narrowBySwitchOnTypeOf(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number): Type {
-                const switchWitnesses = getSwitchClauseTypeOfWitnesses(switchStatement);
+                const switchWitnesses = getSwitchClauseTypeOfWitnesses(switchStatement, /*retainDefault*/ true);
                 if (!switchWitnesses.length) {
                     return type;
                 }
@@ -24014,8 +24016,7 @@ namespace ts {
         function computeExhaustiveSwitchStatement(node: SwitchStatement): boolean {
             if (node.expression.kind === SyntaxKind.TypeOfExpression) {
                 const operandType = getTypeOfExpression((node.expression as TypeOfExpression).expression);
-                // This cast is safe because the switch is possibly exhaustive and does not contain a default case, so there can be no undefined.
-                const witnesses = <string[]>getSwitchClauseTypeOfWitnesses(node);
+                const witnesses = getSwitchClauseTypeOfWitnesses(node, /*retainDefault*/ false);
                 // notEqualFacts states that the type of the switched value is not equal to every type in the switch.
                 const notEqualFacts = getFactsFromTypeofSwitch(0, 0, witnesses, /*hasDefault*/ true);
                 const type = getBaseConstraintOfType(operandType) || operandType;