Merge pull request #10296 from Microsoft/fixDiscriminantWithPrimtive

Fix discriminated unions with primtive types

src/compiler/checker.ts

@@ -245,7 +245,8 @@ namespace ts {
             NEUndefinedOrNull = 1 << 19,  // x != undefined / x != null
             Truthy = 1 << 20,             // x
             Falsy = 1 << 21,              // !x
-            All = (1 << 22) - 1,
+            Discriminatable = 1 << 22,    // May have discriminant property
+            All = (1 << 23) - 1,
             // The following members encode facts about particular kinds of types for use in the getTypeFacts function.
             // The presence of a particular fact means that the given test is true for some (and possibly all) values
             // of that kind of type.
@@ -275,9 +276,9 @@ namespace ts {
             TrueFacts = BaseBooleanFacts | Truthy,
             SymbolStrictFacts = TypeofEQSymbol | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
             SymbolFacts = SymbolStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
-            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
+            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
             ObjectFacts = ObjectStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
-            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
+            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
             FunctionFacts = FunctionStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
             UndefinedFacts = TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | EQUndefined | EQUndefinedOrNull | NENull | Falsy,
             NullFacts = TypeofEQObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | TypeofNEHostObject | EQNull | EQUndefinedOrNull | NEUndefined | Falsy,
@@ -5351,12 +5352,12 @@ namespace ts {
             }
         }
 
-        // We deduplicate the constituent types based on object identity. If the subtypeReduction flag is
-        // specified we also reduce the constituent type set to only include types that aren't subtypes of
-        // other types. Subtype reduction is expensive for large union types and is possible only when union
+        // We sort and deduplicate the constituent types based on object identity. If the subtypeReduction
+        // flag is specified we also reduce the constituent type set to only include types that aren't subtypes
+        // of other types. Subtype reduction is expensive for large union types and is possible only when union
         // types are known not to circularly reference themselves (as is the case with union types created by
         // expression constructs such as array literals and the || and ?: operators). Named types can
-        // circularly reference themselves and therefore cannot be deduplicated during their declaration.
+        // circularly reference themselves and therefore cannot be subtype reduced during their declaration.
         // For example, "type Item = string | (() => Item" is a named type that circularly references itself.
         function getUnionType(types: Type[], subtypeReduction?: boolean, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
             if (types.length === 0) {
@@ -5378,15 +5379,23 @@ namespace ts {
                     typeSet.containsUndefined ? typeSet.containsNonWideningType ? undefinedType : undefinedWideningType :
                         neverType;
             }
-            else if (typeSet.length === 1) {
-                return typeSet[0];
+            return getUnionTypeFromSortedList(typeSet, aliasSymbol, aliasTypeArguments);
+        }
+
+        // This function assumes the constituent type list is sorted and deduplicated.
+        function getUnionTypeFromSortedList(types: Type[], aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
+            if (types.length === 0) {
+                return neverType;
             }
-            const id = getTypeListId(typeSet);
+            if (types.length === 1) {
+                return types[0];
+            }
+            const id = getTypeListId(types);
             let type = unionTypes[id];
             if (!type) {
-                const propagatedFlags = getPropagatingFlagsOfTypes(typeSet, /*excludeKinds*/ TypeFlags.Nullable);
+                const propagatedFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
                 type = unionTypes[id] = <UnionType>createObjectType(TypeFlags.Union | propagatedFlags);
-                type.types = typeSet;
+                type.types = types;
                 type.aliasSymbol = aliasSymbol;
                 type.aliasTypeArguments = aliasTypeArguments;
             }
@@ -7848,18 +7857,25 @@ namespace ts {
         }
 
         function isDiscriminantProperty(type: Type, name: string) {
-            if (type) {
-                const nonNullType = getNonNullableType(type);
-                if (nonNullType.flags & TypeFlags.Union) {
-                    const prop = getPropertyOfType(nonNullType, name);
-                    if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
-                        if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
-                            (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
-                                isUnitUnionType(getTypeOfSymbol(prop));
-                        }
-                        return (<TransientSymbol>prop).isDiscriminantProperty;
+            if (type && type.flags & TypeFlags.Union) {
+                let prop = getPropertyOfType(type, name);
+                if (!prop) {
+                    // The type may be a union that includes nullable or primitive types. If filtering
+                    // those out produces a different type, get the property from that type instead.
+                    // Effectively, we're checking if this *could* be a discriminant property once nullable
+                    // and primitive types are removed by other type guards.
+                    const filteredType = getTypeWithFacts(type, TypeFacts.Discriminatable);
+                    if (filteredType !== type && filteredType.flags & TypeFlags.Union) {
+                        prop = getPropertyOfType(filteredType, name);
                     }
                 }
+                if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
+                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
+                        (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
+                            isUnitUnionType(getTypeOfSymbol(prop));
+                    }
+                    return (<TransientSymbol>prop).isDiscriminantProperty;
+                }
             }
             return false;
         }
@@ -7907,10 +7923,10 @@ namespace ts {
         // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
         // we remove type string.
         function getAssignmentReducedType(declaredType: UnionType, assignedType: Type) {
-            if (declaredType !== assignedType && declaredType.flags & TypeFlags.Union) {
-                const reducedTypes = filter(declaredType.types, t => typeMaybeAssignableTo(assignedType, t));
-                if (reducedTypes.length) {
-                    return reducedTypes.length === 1 ? reducedTypes[0] : getUnionType(reducedTypes);
+            if (declaredType !== assignedType) {
+                const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
+                if (reducedType !== neverType) {
+                    return reducedType;
                 }
             }
             return declaredType;
@@ -7924,6 +7940,14 @@ namespace ts {
             return result;
         }
 
+        function isFunctionObjectType(type: ObjectType): boolean {
+            // We do a quick check for a "bind" property before performing the more expensive subtype
+            // check. This gives us a quicker out in the common case where an object type is not a function.
+            const resolved = resolveStructuredTypeMembers(type);
+            return !!(resolved.callSignatures.length || resolved.constructSignatures.length ||
+                hasProperty(resolved.members, "bind") && isTypeSubtypeOf(type, globalFunctionType));
+        }
+
         function getTypeFacts(type: Type): TypeFacts {
             const flags = type.flags;
             if (flags & TypeFlags.String) {
@@ -7952,8 +7976,7 @@ namespace ts {
                     type === falseType ? TypeFacts.FalseFacts : TypeFacts.TrueFacts;
             }
             if (flags & TypeFlags.ObjectType) {
-                const resolved = resolveStructuredTypeMembers(type);
-                return resolved.callSignatures.length || resolved.constructSignatures.length || isTypeSubtypeOf(type, globalFunctionType) ?
+                return isFunctionObjectType(<ObjectType>type) ?
                     strictNullChecks ? TypeFacts.FunctionStrictFacts : TypeFacts.FunctionFacts :
                     strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
             }
@@ -7977,26 +8000,7 @@ namespace ts {
         }
 
         function getTypeWithFacts(type: Type, include: TypeFacts) {
-            if (!(type.flags & TypeFlags.Union)) {
-                return getTypeFacts(type) & include ? type : neverType;
-            }
-            let firstType: Type;
-            let types: Type[];
-            for (const t of (type as UnionType).types) {
-                if (getTypeFacts(t) & include) {
-                    if (!firstType) {
-                        firstType = t;
-                    }
-                    else {
-                        if (!types) {
-                            types = [firstType];
-                        }
-                        types.push(t);
-                    }
-                }
-            }
-            return types ? getUnionType(types) :
-                firstType ? firstType : neverType;
+            return filterType(type, t => (getTypeFacts(t) & include) !== 0);
         }
 
         function getTypeWithDefault(type: Type, defaultExpression: Expression) {
@@ -8172,9 +8176,12 @@ namespace ts {
         }
 
         function filterType(type: Type, f: (t: Type) => boolean): Type {
-            return type.flags & TypeFlags.Union ?
-                getUnionType(filter((<UnionType>type).types, f)) :
-                f(type) ? type : neverType;
+            if (type.flags & TypeFlags.Union) {
+                const types = (<UnionType>type).types;
+                const filtered = filter(types, f);
+                return filtered === types ? type : getUnionTypeFromSortedList(filtered);
+            }
+            return f(type) ? type : neverType;
         }
 
         function isIncomplete(flowType: FlowType) {
@@ -8512,7 +8519,7 @@ namespace ts {
                 }
                 if (assumeTrue && !(type.flags & TypeFlags.Union)) {
                     // We narrow a non-union type to an exact primitive type if the non-union type
-                    // is a supertype of that primtive type. For example, type 'any' can be narrowed
+                    // is a supertype of that primitive type. For example, type 'any' can be narrowed
                     // to one of the primitive types.
                     const targetType = getProperty(typeofTypesByName, literal.text);
                     if (targetType && isTypeSubtypeOf(targetType, type)) {
@@ -8601,9 +8608,9 @@ namespace ts {
                 // If the current type is a union type, remove all constituents that couldn't be instances of
                 // the candidate type. If one or more constituents remain, return a union of those.
                 if (type.flags & TypeFlags.Union) {
-                    const assignableConstituents = filter((<UnionType>type).types, t => isTypeInstanceOf(t, candidate));
-                    if (assignableConstituents.length) {
-                        return getUnionType(assignableConstituents);
+                    const assignableType = filterType(type, t => isTypeInstanceOf(t, candidate));
+                    if (assignableType !== neverType) {
+                        return assignableType;
                     }
                 }
                 // If the candidate type is a subtype of the target type, narrow to the candidate type.

src/compiler/core.ts

@@ -147,17 +147,29 @@ namespace ts {
         return count;
     }
 
+    /**
+     * Filters an array by a predicate function. Returns the same array instance if the predicate is
+     * true for all elements, otherwise returns a new array instance containing the filtered subset.
+     */
     export function filter<T>(array: T[], f: (x: T) => boolean): T[] {
-        let result: T[];
         if (array) {
-            result = [];
-            for (const item of array) {
-                if (f(item)) {
-                    result.push(item);
+            const len = array.length;
+            let i = 0;
+            while (i < len && f(array[i])) i++;
+            if (i < len) {
+                const result = array.slice(0, i);
+                i++;
+                while (i < len) {
+                    const item = array[i];
+                    if (f(item)) {
+                        result.push(item);
+                    }
+                    i++;
                 }
+                return result;
             }
         }
-        return result;
+        return array;
     }
 
     export function filterMutate<T>(array: T[], f: (x: T) => boolean): void {

tests/baselines/reference/discriminantsAndPrimitives.js

@@ -0,0 +1,84 @@
+//// [discriminantsAndPrimitives.ts]
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+    kind: "foo";
+    name: string;
+}
+
+interface Bar {
+    kind: "bar";
+    length: string;
+}
+
+function f1(x: Foo | Bar | string) {
+    if (typeof x !== 'string') {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+    if (typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+    if (x && typeof x !== "string") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+    if (x && typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+//// [discriminantsAndPrimitives.js]
+// Repro from #10257 plus other tests
+function f1(x) {
+    if (typeof x !== 'string') {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f2(x) {
+    if (typeof x === "object") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f3(x) {
+    if (x && typeof x !== "string") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f4(x) {
+    if (x && typeof x === "object") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}

tests/baselines/reference/discriminantsAndPrimitives.symbols

@@ -0,0 +1,117 @@
+=== tests/cases/compiler/discriminantsAndPrimitives.ts ===
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+
+    kind: "foo";
+>kind : Symbol(Foo.kind, Decl(discriminantsAndPrimitives.ts, 3, 15))
+
+    name: string;
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+}
+
+interface Bar {
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    kind: "bar";
+>kind : Symbol(Bar.kind, Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+    length: string;
+>length : Symbol(Bar.length, Decl(discriminantsAndPrimitives.ts, 9, 16))
+}
+
+function f1(x: Foo | Bar | string) {
+>f1 : Symbol(f1, Decl(discriminantsAndPrimitives.ts, 11, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (typeof x !== 'string') {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+>f2 : Symbol(f2, Decl(discriminantsAndPrimitives.ts, 20, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (typeof x === "object") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+>f3 : Symbol(f3, Decl(discriminantsAndPrimitives.ts, 29, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (x && typeof x !== "string") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+>f4 : Symbol(f4, Decl(discriminantsAndPrimitives.ts, 38, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (x && typeof x === "object") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}

tests/baselines/reference/discriminantsAndPrimitives.types

@@ -0,0 +1,141 @@
+=== tests/cases/compiler/discriminantsAndPrimitives.ts ===
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+>Foo : Foo
+
+    kind: "foo";
+>kind : "foo"
+
+    name: string;
+>name : string
+}
+
+interface Bar {
+>Bar : Bar
+
+    kind: "bar";
+>kind : "bar"
+
+    length: string;
+>length : string
+}
+
+function f1(x: Foo | Bar | string) {
+>f1 : (x: string | Foo | Bar) => void
+>x : string | Foo | Bar
+>Foo : Foo
+>Bar : Bar
+
+    if (typeof x !== 'string') {
+>typeof x !== 'string' : boolean
+>typeof x : string
+>x : string | Foo | Bar
+>'string' : "string"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+>f2 : (x: string | Foo | Bar | undefined) => void
+>x : string | Foo | Bar | undefined
+>Foo : Foo
+>Bar : Bar
+
+    if (typeof x === "object") {
+>typeof x === "object" : boolean
+>typeof x : string
+>x : string | Foo | Bar | undefined
+>"object" : "object"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+>f3 : (x: string | Foo | Bar | null) => void
+>x : string | Foo | Bar | null
+>Foo : Foo
+>Bar : Bar
+>null : null
+
+    if (x && typeof x !== "string") {
+>x && typeof x !== "string" : boolean | "" | null
+>x : string | Foo | Bar | null
+>typeof x !== "string" : boolean
+>typeof x : string
+>x : string | Foo | Bar
+>"string" : "string"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+>f4 : (x: string | number | Foo | Bar | null) => void
+>x : string | number | Foo | Bar | null
+>Foo : Foo
+>Bar : Bar
+>null : null
+
+    if (x && typeof x === "object") {
+>x && typeof x === "object" : boolean | "" | 0 | null
+>x : string | number | Foo | Bar | null
+>typeof x === "object" : boolean
+>typeof x : string
+>x : string | number | Foo | Bar
+>"object" : "object"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}

tests/cases/compiler/discriminantsAndPrimitives.ts

@@ -0,0 +1,49 @@
+// @strictNullChecks: true
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+    kind: "foo";
+    name: string;
+}
+
+interface Bar {
+    kind: "bar";
+    length: string;
+}
+
+function f1(x: Foo | Bar | string) {
+    if (typeof x !== 'string') {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+    if (typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+    if (x && typeof x !== "string") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+    if (x && typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}