microsoft · Kingwl · Aug 27, 2018
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -14916,6 +14916,11 @@ namespace ts {
                 return getTypeWithFacts(type, facts);
             }
 
+            function areStringLiteralEnumComparable (source: Type, target: Type) {
+                const meaning = TypeFlags.StringLiteral | TypeFlags.EnumLiteral;
+                return !!(source.flags & meaning && target.flags & meaning && (<StringLiteralType>source).value === (<StringLiteralType>target).value);
+            }
+
             function narrowTypeBySwitchOnDiscriminant(type: Type, switchStatement: SwitchStatement, clauseStart: number, clauseEnd: number) {
                 // We only narrow if all case expressions specify values with unit types
                 const switchTypes = getSwitchClauseTypes(switchStatement);
@@ -14927,7 +14932,7 @@ namespace ts {
                 const discriminantType = getUnionType(clauseTypes);
                 const caseType =
                     discriminantType.flags & TypeFlags.Never ? neverType :
-                        replacePrimitivesWithLiterals(filterType(type, t => areTypesComparable(discriminantType, t)), discriminantType);
+                        replacePrimitivesWithLiterals(filterType(type, t => areTypesComparable(discriminantType, t) || areStringLiteralEnumComparable(discriminantType, t)), discriminantType);
                 if (!hasDefaultClause) {
                     return caseType;
                 }

diff --git a/tests/baselines/reference/typeGuardSwitchWithSameString.errors.txt b/tests/baselines/reference/typeGuardSwitchWithSameString.errors.txt
@@ -0,0 +1,60 @@
+tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts(33,15): error TS2339: Property 'foo' does not exist on type 'T'.
+  Property 'foo' does not exist on type 'IBar'.
+tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts(36,15): error TS2339: Property 'bar' does not exist on type 'T'.
+  Property 'bar' does not exist on type 'IFoo'.
+tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts(39,15): error TS2339: Property 'baz' does not exist on type 'T'.
+  Property 'baz' does not exist on type 'IFoo'.
+
+
+==== tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts (3 errors) ====
+    enum Foo {
+        baz = "baz"
+    }
+
+    enum Bar {
+        baz = "baz"
+    }
+
+    enum Baz {
+        baz = "ba" + "z"
+    }
+
+    interface IFoo {
+        type: Foo.baz
+        foo: string
+    }
+
+    interface IBar {
+        type: Bar.baz
+        bar: number
+    }
+
+    interface IBaz {
+        type: Baz.baz
+        baz: boolean
+    }
+
+    type T = IFoo | IBar | IBaz
+
+    function reduce(t: T) {
+        switch (t.type) {
+            case Foo.baz:
+                t.foo
+                  ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'T'.
+!!! error TS2339:   Property 'foo' does not exist on type 'IBar'.
+                break
+            case Bar.baz:
+                t.bar
+                  ~~~
+!!! error TS2339: Property 'bar' does not exist on type 'T'.
+!!! error TS2339:   Property 'bar' does not exist on type 'IFoo'.
+                break
+            case Baz.baz:
+                t.baz
+                  ~~~
+!!! error TS2339: Property 'baz' does not exist on type 'T'.
+!!! error TS2339:   Property 'baz' does not exist on type 'IFoo'.
+        }
+    }
+
diff --git a/tests/baselines/reference/typeGuardSwitchWithSameString.js b/tests/baselines/reference/typeGuardSwitchWithSameString.js
@@ -0,0 +1,69 @@
+//// [typeGuardSwitchWithSameString.ts]
+enum Foo {
+    baz = "baz"
+}
+
+enum Bar {
+    baz = "baz"
+}
+
+enum Baz {
+    baz = "ba" + "z"
+}
+
+interface IFoo {
+    type: Foo.baz
+    foo: string
+}
+
+interface IBar {
+    type: Bar.baz
+    bar: number
+}
+
+interface IBaz {
+    type: Baz.baz
+    baz: boolean
+}
+
+type T = IFoo | IBar | IBaz
+
+function reduce(t: T) {
+    switch (t.type) {
+        case Foo.baz:
+            t.foo
+            break
+        case Bar.baz:
+            t.bar
+            break
+        case Baz.baz:
+            t.baz
+    }
+}
+
+
+//// [typeGuardSwitchWithSameString.js]
+var Foo;
+(function (Foo) {
+    Foo["baz"] = "baz";
+})(Foo || (Foo = {}));
+var Bar;
+(function (Bar) {
+    Bar["baz"] = "baz";
+})(Bar || (Bar = {}));
+var Baz;
+(function (Baz) {
+    Baz["baz"] = "baz";
+})(Baz || (Baz = {}));
+function reduce(t) {
+    switch (t.type) {
+        case Foo.baz:
+            t.foo;
+            break;
+        case Bar.baz:
+            t.bar;
+            break;
+        case Baz.baz:
+            t.baz;
+    }
+}
diff --git a/tests/baselines/reference/typeGuardSwitchWithSameString.symbols b/tests/baselines/reference/typeGuardSwitchWithSameString.symbols
@@ -0,0 +1,102 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts ===
+enum Foo {
+>Foo : Symbol(Foo, Decl(typeGuardSwitchWithSameString.ts, 0, 0))
+
+    baz = "baz"
+>baz : Symbol(Foo.baz, Decl(typeGuardSwitchWithSameString.ts, 0, 10))
+}
+
+enum Bar {
+>Bar : Symbol(Bar, Decl(typeGuardSwitchWithSameString.ts, 2, 1))
+
+    baz = "baz"
+>baz : Symbol(Bar.baz, Decl(typeGuardSwitchWithSameString.ts, 4, 10))
+}
+
+enum Baz {
+>Baz : Symbol(Baz, Decl(typeGuardSwitchWithSameString.ts, 6, 1))
+
+    baz = "ba" + "z"
+>baz : Symbol(Baz.baz, Decl(typeGuardSwitchWithSameString.ts, 8, 10))
+}
+
+interface IFoo {
+>IFoo : Symbol(IFoo, Decl(typeGuardSwitchWithSameString.ts, 10, 1))
+
+    type: Foo.baz
+>type : Symbol(IFoo.type, Decl(typeGuardSwitchWithSameString.ts, 12, 16))
+>Foo : Symbol(Foo, Decl(typeGuardSwitchWithSameString.ts, 0, 0))
+>baz : Symbol(Foo.baz, Decl(typeGuardSwitchWithSameString.ts, 0, 10))
+
+    foo: string
+>foo : Symbol(IFoo.foo, Decl(typeGuardSwitchWithSameString.ts, 13, 17))
+}
+
+interface IBar {
+>IBar : Symbol(IBar, Decl(typeGuardSwitchWithSameString.ts, 15, 1))
+
+    type: Bar.baz
+>type : Symbol(IBar.type, Decl(typeGuardSwitchWithSameString.ts, 17, 16))
+>Bar : Symbol(Bar, Decl(typeGuardSwitchWithSameString.ts, 2, 1))
+>baz : Symbol(Bar.baz, Decl(typeGuardSwitchWithSameString.ts, 4, 10))
+
+    bar: number
+>bar : Symbol(IBar.bar, Decl(typeGuardSwitchWithSameString.ts, 18, 17))
+}
+
+interface IBaz {
+>IBaz : Symbol(IBaz, Decl(typeGuardSwitchWithSameString.ts, 20, 1))
+
+    type: Baz.baz
+>type : Symbol(IBaz.type, Decl(typeGuardSwitchWithSameString.ts, 22, 16))
+>Baz : Symbol(Baz, Decl(typeGuardSwitchWithSameString.ts, 6, 1))
+>baz : Symbol(Baz.baz, Decl(typeGuardSwitchWithSameString.ts, 8, 10))
+
+    baz: boolean
+>baz : Symbol(IBaz.baz, Decl(typeGuardSwitchWithSameString.ts, 23, 17))
+}
+
+type T = IFoo | IBar | IBaz
+>T : Symbol(T, Decl(typeGuardSwitchWithSameString.ts, 25, 1))
+>IFoo : Symbol(IFoo, Decl(typeGuardSwitchWithSameString.ts, 10, 1))
+>IBar : Symbol(IBar, Decl(typeGuardSwitchWithSameString.ts, 15, 1))
+>IBaz : Symbol(IBaz, Decl(typeGuardSwitchWithSameString.ts, 20, 1))
+
+function reduce(t: T) {
+>reduce : Symbol(reduce, Decl(typeGuardSwitchWithSameString.ts, 27, 27))
+>t : Symbol(t, Decl(typeGuardSwitchWithSameString.ts, 29, 16))
+>T : Symbol(T, Decl(typeGuardSwitchWithSameString.ts, 25, 1))
+
+    switch (t.type) {
+>t.type : Symbol(type, Decl(typeGuardSwitchWithSameString.ts, 12, 16), Decl(typeGuardSwitchWithSameString.ts, 17, 16), Decl(typeGuardSwitchWithSameString.ts, 22, 16))
+>t : Symbol(t, Decl(typeGuardSwitchWithSameString.ts, 29, 16))
+>type : Symbol(type, Decl(typeGuardSwitchWithSameString.ts, 12, 16), Decl(typeGuardSwitchWithSameString.ts, 17, 16), Decl(typeGuardSwitchWithSameString.ts, 22, 16))
+
+        case Foo.baz:
+>Foo.baz : Symbol(Foo.baz, Decl(typeGuardSwitchWithSameString.ts, 0, 10))
+>Foo : Symbol(Foo, Decl(typeGuardSwitchWithSameString.ts, 0, 0))
+>baz : Symbol(Foo.baz, Decl(typeGuardSwitchWithSameString.ts, 0, 10))
+
+            t.foo
+>t : Symbol(t, Decl(typeGuardSwitchWithSameString.ts, 29, 16))
+
+            break
+        case Bar.baz:
+>Bar.baz : Symbol(Bar.baz, Decl(typeGuardSwitchWithSameString.ts, 4, 10))
+>Bar : Symbol(Bar, Decl(typeGuardSwitchWithSameString.ts, 2, 1))
+>baz : Symbol(Bar.baz, Decl(typeGuardSwitchWithSameString.ts, 4, 10))
+
+            t.bar
+>t : Symbol(t, Decl(typeGuardSwitchWithSameString.ts, 29, 16))
+
+            break
+        case Baz.baz:
+>Baz.baz : Symbol(Baz.baz, Decl(typeGuardSwitchWithSameString.ts, 8, 10))
+>Baz : Symbol(Baz, Decl(typeGuardSwitchWithSameString.ts, 6, 1))
+>baz : Symbol(Baz.baz, Decl(typeGuardSwitchWithSameString.ts, 8, 10))
+
+            t.baz
+>t : Symbol(t, Decl(typeGuardSwitchWithSameString.ts, 29, 16))
+    }
+}
+
diff --git a/tests/baselines/reference/typeGuardSwitchWithSameString.types b/tests/baselines/reference/typeGuardSwitchWithSameString.types
@@ -0,0 +1,100 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts ===
+enum Foo {
+>Foo : Foo
+
+    baz = "baz"
+>baz : Foo
+>"baz" : "baz"
+}
+
+enum Bar {
+>Bar : Bar
+
+    baz = "baz"
+>baz : Bar
+>"baz" : "baz"
+}
+
+enum Baz {
+>Baz : Baz
+
+    baz = "ba" + "z"
+>baz : Baz
+>"ba" + "z" : string
+>"ba" : "ba"
+>"z" : "z"
+}
+
+interface IFoo {
+    type: Foo.baz
+>type : Foo
+>Foo : any
+
+    foo: string
+>foo : string
+}
+
+interface IBar {
+    type: Bar.baz
+>type : Bar
+>Bar : any
+
+    bar: number
+>bar : number
+}
+
+interface IBaz {
+    type: Baz.baz
+>type : Baz
+>Baz : any
+
+    baz: boolean
+>baz : boolean
+}
+
+type T = IFoo | IBar | IBaz
+>T : T
+
+function reduce(t: T) {
+>reduce : (t: T) => void
+>t : T
+
+    switch (t.type) {
+>t.type : Foo | Bar | Baz
+>t : T
+>type : Foo | Bar | Baz
+
+        case Foo.baz:
+>Foo.baz : Foo
+>Foo : typeof Foo
+>baz : Foo
+
+            t.foo
+>t.foo : any
+>t : T
+>foo : any
+
+            break
+        case Bar.baz:
+>Bar.baz : Bar
+>Bar : typeof Bar
+>baz : Bar
+
+            t.bar
+>t.bar : any
+>t : T
+>bar : any
+
+            break
+        case Baz.baz:
+>Baz.baz : Baz
+>Baz : typeof Baz
+>baz : Baz
+
+            t.baz
+>t.baz : any
+>t : T
+>baz : any
+    }
+}
+
diff --git a/tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts b/tests/cases/conformance/expressions/typeGuards/typeGuardSwitchWithSameString.ts
@@ -0,0 +1,41 @@
+enum Foo {
+    baz = "baz"
+}
+
+enum Bar {
+    baz = "baz"
+}
+
+enum Baz {
+    baz = "ba" + "z"
+}
+
+interface IFoo {
+    type: Foo.baz
+    foo: string
+}
+
+interface IBar {
+    type: Bar.baz
+    bar: number
+}
+
+interface IBaz {
+    type: Baz.baz
+    baz: boolean
+}
+
+type T = IFoo | IBar | IBaz
+
+function reduce(t: T) {
+    switch (t.type) {
+        case Foo.baz:
+            t.foo
+            break
+        case Bar.baz:
+            t.bar
+            break
+        case Baz.baz:
+            t.baz
+    }
+}