Add typeof-for-switch

Initial draft that works for union types

First draft of PR ready code with tests

Revert changed line for testing

Add exhaustiveness checking and move narrowByTypeOfWitnesses

Try caching mechanism

Comment out exhaustiveness checking to find perf regression

Re-enable exhaustiveness checking for typeof switches

Check if changes to narrowByTypeOfWitnesses fix perf alone.

Improve switch narrowing:

+ Take into account repeated clauses in the switch.
+ Handle unions of constrained type parameters.

Add more tests

Comments

Revert back to if-like behaviour

Remove redundant checks and simplify exhaustiveness checks

Change comment for narrowBySwitchOnTypeOf

Reduce implied type with getAssignmentReducedType

Remove any annotations

src/compiler/binder.ts

@@ -737,6 +737,8 @@ namespace ts {
                     return isNarrowingBinaryExpression(<BinaryExpression>expr);
                 case SyntaxKind.PrefixUnaryExpression:
                     return (<PrefixUnaryExpression>expr).operator === SyntaxKind.ExclamationToken && isNarrowingExpression((<PrefixUnaryExpression>expr).operand);
+                case SyntaxKind.TypeOfExpression:
+                    return isNarrowingExpression((<TypeOfExpression>expr).expression);
             }
             return false;
         }

src/compiler/checker.ts

@@ -12840,6 +12840,21 @@ namespace ts {
             return links.switchTypes;
         }
 
+        function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement): (string | undefined)[] {
+            const witnesses: (string | undefined)[] = [];
+            for (const clause of switchStatement.caseBlock.clauses) {
+                if (clause.kind === SyntaxKind.CaseClause) {
+                    if (clause.expression.kind === SyntaxKind.StringLiteral) {
+                        witnesses.push((clause.expression as StringLiteral).text);
+                        continue;
+                    }
+                    return emptyArray;
+                }
+                witnesses.push(/*explicitDefaultStatement*/ undefined);
+            }
+            return witnesses;
+        }
+
         function eachTypeContainedIn(source: Type, types: Type[]) {
             return source.flags & TypeFlags.Union ? !forEach((<UnionType>source).types, t => !contains(types, t)) : contains(types, source);
         }
@@ -13253,6 +13268,9 @@ namespace ts {
                 else if (isMatchingReferenceDiscriminant(expr, type)) {
                     type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                 }
+                else if (expr.kind === SyntaxKind.TypeOfExpression && isMatchingReference(reference, (expr as TypeOfExpression).expression)) {
+                    type = narrowBySwitchOnTypeOf(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
+                }
                 return createFlowType(type, isIncomplete(flowType));
             }
 
@@ -13549,6 +13567,57 @@ namespace ts {
                 return caseType.flags & TypeFlags.Never ? defaultType : getUnionType([caseType, defaultType]);
             }
 
+            function narrowBySwitchOnTypeOf(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number): Type {
+                const switchWitnesses = getSwitchClauseTypeOfWitnesses(switchStatement);
+                if (!switchWitnesses.length) {
+                    return type;
+                }
+                const clauseWitnesses = switchWitnesses.slice(clauseStart, clauseEnd);
+                //  Equal start and end denotes implicit fallthrough; undefined marks explicit default clause
+                const hasDefaultClause = clauseStart === clauseEnd || contains(clauseWitnesses, /*explicitDefaultStatement*/ undefined);
+                const switchFacts = getFactsFromTypeofSwitch(clauseStart, clauseEnd, switchWitnesses, hasDefaultClause);
+                // The implied type is the raw type suggested by a
+                // value being caught in this clause.
+                // - If there is a default the implied type is not used.
+                // - Otherwise, take the union of the types in the
+                // clause. We narrow the union using facts to remove
+                // types that appear multiple types and are
+                // unreachable.
+                // Example:
+                //
+                // switch (typeof x) {
+                //     case 'number':
+                //     case 'string': break;
+                //     default: break;
+                //     case 'number':
+                //     case 'boolean': break
+                // }
+                //
+                // The implied type of the first clause number | string.
+                // The implied type of the second clause is string (but this doesn't get used).
+                // The implied type of the third clause is boolean (number has already be caught).
+                if (!(hasDefaultClause || (type.flags & TypeFlags.Union))) {
+                    let impliedType = getTypeWithFacts(getUnionType(clauseWitnesses.map(text => typeofTypesByName.get(text) || neverType)), switchFacts);
+                    if (impliedType.flags & TypeFlags.Union) {
+                        impliedType = getAssignmentReducedType(impliedType as UnionType, getBaseConstraintOfType(type) || type);
+                    }
+                    if (!(impliedType.flags & TypeFlags.Never)) {
+                        if (isTypeSubtypeOf(impliedType, type)) {
+                            return impliedType;
+                        }
+                        if (type.flags & TypeFlags.Instantiable) {
+                            const constraint = getBaseConstraintOfType(type) || anyType;
+                            if (isTypeSubtypeOf(impliedType, constraint)) {
+                                return getIntersectionType([type, impliedType]);
+                            }
+                        }
+                    }
+                }
+                return hasDefaultClause ?
+                    filterType(type, t => (getTypeFacts(t) & switchFacts) === switchFacts) :
+                    getTypeWithFacts(type, switchFacts);
+            }
+
             function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 const left = getReferenceCandidate(expr.left);
                 if (!isMatchingReference(reference, left)) {
@@ -18944,10 +19013,60 @@ namespace ts {
                 : Diagnostics.Type_of_yield_operand_in_an_async_generator_must_either_be_a_valid_promise_or_must_not_contain_a_callable_then_member);
         }
 
+        /**
+         * Collect the TypeFacts learned from a typeof switch with
+         * total clauses `witnesses`, and the active clause ranging
+         * from `start` to `end`. Parameter `hasDefault` denotes
+         * whether the active clause contains a default clause.
+         */
+        function getFactsFromTypeofSwitch(start: number, end: number, witnesses: (string | undefined)[], hasDefault: boolean): TypeFacts {
+            let facts: TypeFacts = TypeFacts.None;
+            // When in the default we only collect inequality facts
+            // because default is 'in theory' a set of infinite
+            // equalities.
+            if (hasDefault) {
+                // Value is not equal to any types after the active clause.
+                for (let i = end; i < witnesses.length; i++) {
+                    facts |= typeofNEFacts.get(witnesses[i]) || TypeFacts.TypeofNEHostObject;
+                }
+                // Remove inequalities for types that appear in the
+                // active clause because they appear before other
+                // types collected so far.
+                for (let i = start; i < end; i++) {
+                    facts &= ~(typeofNEFacts.get(witnesses[i]) || 0);
+                }
+                // Add inequalities for types before the active clause unconditionally.
+                for (let i = 0; i < start; i++) {
+                    facts |= typeofNEFacts.get(witnesses[i]) || TypeFacts.TypeofNEHostObject;
+                }
+            }
+            // When in an active clause without default the set of
+            // equalities is finite.
+            else {
+                // Add equalities for all types in the active clause.
+                for (let i = start; i < end; i++) {
+                    facts |= typeofEQFacts.get(witnesses[i]) || TypeFacts.TypeofEQHostObject;
+                }
+                // Remove equalities for types that appear before the
+                // active clause.
+                for (let i = 0; i < start; i++) {
+                    facts &= ~(typeofEQFacts.get(witnesses[i]) || 0);
+                }
+            }
+            return facts;
+        }
+
         function isExhaustiveSwitchStatement(node: SwitchStatement): boolean {
             if (!node.possiblyExhaustive) {
                 return false;
             }
+            if (node.expression.kind === SyntaxKind.TypeOfExpression) {
+                const operandType = getTypeOfExpression((node.expression as TypeOfExpression).expression);
+                // Type is not equal to every type in the switch.
+                const notEqualFacts = getFactsFromTypeofSwitch(0, 0, getSwitchClauseTypeOfWitnesses(node), /*hasDefault*/ true);
+                const type = getBaseConstraintOfType(operandType) || operandType;
+                return !!(filterType(type, t => (getTypeFacts(t) & notEqualFacts) === notEqualFacts).flags & TypeFlags.Never);
+            }
             const type = getTypeOfExpression(node.expression);
             if (!isLiteralType(type)) {
                 return false;