microsoft · jack-williams · Mar 4, 2018
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -19260,6 +19260,22 @@ namespace ts {
                     }
                     if (!isTypeEqualityComparableTo(leftType, rightType) && !isTypeEqualityComparableTo(rightType, leftType)) {
                         reportOperatorError();
+                        const leftDeclaredType = getDeclaredTypeOfReference(left) || leftType;
+                        const rightDeclaredType = getDeclaredTypeOfReference(right) || rightType;
+                        if (isTypeEqualityComparableTo(leftDeclaredType, rightDeclaredType) || isTypeEqualityComparableTo(rightDeclaredType, leftDeclaredType)) {
+                            if (leftDeclaredType === leftType) {
+                                error(errorNode || operatorToken,
+                                    Diagnostics.Earlier_code_in_this_block_narrowed_type_0_to_1_making_this_operation_invalid,
+                                    typeToString(rightDeclaredType),
+                                    typeToString(rightType));
+                            }
+                            else {
+                                error(errorNode || operatorToken,
+                                    Diagnostics.Earlier_code_in_this_block_narrowed_type_0_to_1_making_this_operation_invalid,
+                                    typeToString(leftDeclaredType),
+                                    typeToString(leftType));
+                            }
+                        }
                     }
                     return booleanType;
                 case SyntaxKind.InstanceOfKeyword:

diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
@@ -2321,6 +2321,10 @@
         "category": "Error",
         "code": 2724
     },
+    "Earlier code in this block narrowed type '{0}' to '{1}' making this operation invalid.": {
+        "category": "Error",
+        "code": 2725
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

diff --git a/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.errors.txt b/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.errors.txt
@@ -0,0 +1,97 @@
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(13,9): error TS2365: Operator '===' cannot be applied to types 'Keys.Shift' and 'Keys.Tab'.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(13,9): error TS2725: Earlier code in this block narrowed type 'Keys' to 'Keys.Shift' making this operation invalid.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(28,12): error TS2365: Operator '===' cannot be applied to types 'string' and 'number'.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(28,12): error TS2725: Earlier code in this block narrowed type 'string | number' to 'string' making this operation invalid.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(47,12): error TS2365: Operator '===' cannot be applied to types 'number' and 'string'.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(47,12): error TS2725: Earlier code in this block narrowed type 'string | number' to 'string' making this operation invalid.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(55,9): error TS2365: Operator '===' cannot be applied to types 'false' and 'true'.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(55,9): error TS2725: Earlier code in this block narrowed type 'boolean' to 'false' making this operation invalid.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(60,9): error TS2365: Operator '===' cannot be applied to types 'false' and 'true'.
+tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts(60,9): error TS2725: Earlier code in this block narrowed type 'boolean' to 'false' making this operation invalid.
+
+
+==== tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts (10 errors) ====
+    const enum Keys {
+        Tab = 10,
+        Shift = 13
+    }
+
+    function enumType() {
+        let key: Keys;
+
+        if (key === Keys.Tab) {
+            return;
+        }
+
+        if (key === Keys.Tab || key === Keys.Shift) {
+            ~~~~~~~~~~~~~~~~
+!!! error TS2365: Operator '===' cannot be applied to types 'Keys.Shift' and 'Keys.Tab'.
+            ~~~~~~~~~~~~~~~~
+!!! error TS2725: Earlier code in this block narrowed type 'Keys' to 'Keys.Shift' making this operation invalid.
+            return;
+        }
+    }
+
+
+    function unionType() {
+        let x!: string | number;
+        let y!: string | number;
+        if (typeof x === 'number') {
+            return;
+        }
+        if (typeof y === 'string') {
+            return;
+        }
+        return x === y;
+               ~~~~~~~
+!!! error TS2365: Operator '===' cannot be applied to types 'string' and 'number'.
+               ~~~~~~~
+!!! error TS2725: Earlier code in this block narrowed type 'string | number' to 'string' making this operation invalid.
+    }
+
+    interface Left {
+        kind: 'left'
+        data: number;
+    }
+
+    interface Right {
+        kind: 'right'
+        data: string;
+    }
+
+    function discriminatedType() {
+        let x!: Left;
+        let y!: Left | Right;
+        if (y.kind === 'left') {
+            return;
+        }
+        return x.data === y.data;
+               ~~~~~~~~~~~~~~~~~
+!!! error TS2365: Operator '===' cannot be applied to types 'number' and 'string'.
+               ~~~~~~~~~~~~~~~~~
+!!! error TS2725: Earlier code in this block narrowed type 'string | number' to 'string' making this operation invalid.
+    }
+
+
+    function booleanType(bar: boolean) {
+        if (bar === true) {
+            return "true";
+        }
+        if (bar === true) {
+            ~~~~~~~~~~~~
+!!! error TS2365: Operator '===' cannot be applied to types 'false' and 'true'.
+            ~~~~~~~~~~~~
+!!! error TS2725: Earlier code in this block narrowed type 'boolean' to 'false' making this operation invalid.
+            return "false";
+        }
+
+        const f: boolean = false;
+        if (f === true) {
+            ~~~~~~~~~~
+!!! error TS2365: Operator '===' cannot be applied to types 'false' and 'true'.
+            ~~~~~~~~~~
+!!! error TS2725: Earlier code in this block narrowed type 'boolean' to 'false' making this operation invalid.
+            return "false";
+        }
+    }
+
diff --git a/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.js b/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.js
@@ -0,0 +1,107 @@
+//// [operatorErrorsDetectedByNarrowingMessage.ts]
+const enum Keys {
+    Tab = 10,
+    Shift = 13
+}
+
+function enumType() {
+    let key: Keys;
+
+    if (key === Keys.Tab) {
+        return;
+    }
+
+    if (key === Keys.Tab || key === Keys.Shift) {
+        return;
+    }
+}
+
+
+function unionType() {
+    let x!: string | number;
+    let y!: string | number;
+    if (typeof x === 'number') {
+        return;
+    }
+    if (typeof y === 'string') {
+        return;
+    }
+    return x === y;
+}
+
+interface Left {
+    kind: 'left'
+    data: number;
+}
+
+interface Right {
+    kind: 'right'
+    data: string;
+}
+
+function discriminatedType() {
+    let x!: Left;
+    let y!: Left | Right;
+    if (y.kind === 'left') {
+        return;
+    }
+    return x.data === y.data;
+}
+
+
+function booleanType(bar: boolean) {
+    if (bar === true) {
+        return "true";
+    }
+    if (bar === true) {
+        return "false";
+    }
+
+    const f: boolean = false;
+    if (f === true) {
+        return "false";
+    }
+}
+
+
+//// [operatorErrorsDetectedByNarrowingMessage.js]
+function enumType() {
+    var key;
+    if (key === 10 /* Tab */) {
+        return;
+    }
+    if (key === 10 /* Tab */ || key === 13 /* Shift */) {
+        return;
+    }
+}
+function unionType() {
+    var x;
+    var y;
+    if (typeof x === 'number') {
+        return;
+    }
+    if (typeof y === 'string') {
+        return;
+    }
+    return x === y;
+}
+function discriminatedType() {
+    var x;
+    var y;
+    if (y.kind === 'left') {
+        return;
+    }
+    return x.data === y.data;
+}
+function booleanType(bar) {
+    if (bar === true) {
+        return "true";
+    }
+    if (bar === true) {
+        return "false";
+    }
+    var f = false;
+    if (f === true) {
+        return "false";
+    }
+}
diff --git a/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.symbols b/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.symbols
@@ -0,0 +1,140 @@
+=== tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts ===
+const enum Keys {
+>Keys : Symbol(Keys, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 0))
+
+    Tab = 10,
+>Tab : Symbol(Keys.Tab, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 17))
+
+    Shift = 13
+>Shift : Symbol(Keys.Shift, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 1, 13))
+}
+
+function enumType() {
+>enumType : Symbol(enumType, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 3, 1))
+
+    let key: Keys;
+>key : Symbol(key, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 6, 7))
+>Keys : Symbol(Keys, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 0))
+
+    if (key === Keys.Tab) {
+>key : Symbol(key, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 6, 7))
+>Keys.Tab : Symbol(Keys.Tab, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 17))
+>Keys : Symbol(Keys, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 0))
+>Tab : Symbol(Keys.Tab, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 17))
+
+        return;
+    }
+
+    if (key === Keys.Tab || key === Keys.Shift) {
+>key : Symbol(key, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 6, 7))
+>Keys.Tab : Symbol(Keys.Tab, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 17))
+>Keys : Symbol(Keys, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 0))
+>Tab : Symbol(Keys.Tab, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 17))
+>key : Symbol(key, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 6, 7))
+>Keys.Shift : Symbol(Keys.Shift, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 1, 13))
+>Keys : Symbol(Keys, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 0, 0))
+>Shift : Symbol(Keys.Shift, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 1, 13))
+
+        return;
+    }
+}
+
+
+function unionType() {
+>unionType : Symbol(unionType, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 15, 1))
+
+    let x!: string | number;
+>x : Symbol(x, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 19, 7))
+
+    let y!: string | number;
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 20, 7))
+
+    if (typeof x === 'number') {
+>x : Symbol(x, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 19, 7))
+
+        return;
+    }
+    if (typeof y === 'string') {
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 20, 7))
+
+        return;
+    }
+    return x === y;
+>x : Symbol(x, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 19, 7))
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 20, 7))
+}
+
+interface Left {
+>Left : Symbol(Left, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 28, 1))
+
+    kind: 'left'
+>kind : Symbol(Left.kind, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 30, 16))
+
+    data: number;
+>data : Symbol(Left.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 31, 16))
+}
+
+interface Right {
+>Right : Symbol(Right, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 33, 1))
+
+    kind: 'right'
+>kind : Symbol(Right.kind, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 35, 17))
+
+    data: string;
+>data : Symbol(Right.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 36, 17))
+}
+
+function discriminatedType() {
+>discriminatedType : Symbol(discriminatedType, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 38, 1))
+
+    let x!: Left;
+>x : Symbol(x, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 41, 7))
+>Left : Symbol(Left, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 28, 1))
+
+    let y!: Left | Right;
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 42, 7))
+>Left : Symbol(Left, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 28, 1))
+>Right : Symbol(Right, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 33, 1))
+
+    if (y.kind === 'left') {
+>y.kind : Symbol(kind, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 30, 16), Decl(operatorErrorsDetectedByNarrowingMessage.ts, 35, 17))
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 42, 7))
+>kind : Symbol(kind, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 30, 16), Decl(operatorErrorsDetectedByNarrowingMessage.ts, 35, 17))
+
+        return;
+    }
+    return x.data === y.data;
+>x.data : Symbol(Left.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 31, 16))
+>x : Symbol(x, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 41, 7))
+>data : Symbol(Left.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 31, 16))
+>y.data : Symbol(Right.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 36, 17))
+>y : Symbol(y, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 42, 7))
+>data : Symbol(Right.data, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 36, 17))
+}
+
+
+function booleanType(bar: boolean) {
+>booleanType : Symbol(booleanType, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 47, 1))
+>bar : Symbol(bar, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 50, 21))
+
+    if (bar === true) {
+>bar : Symbol(bar, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 50, 21))
+
+        return "true";
+    }
+    if (bar === true) {
+>bar : Symbol(bar, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 50, 21))
+
+        return "false";
+    }
+
+    const f: boolean = false;
+>f : Symbol(f, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 58, 9))
+
+    if (f === true) {
+>f : Symbol(f, Decl(operatorErrorsDetectedByNarrowingMessage.ts, 58, 9))
+
+        return "false";
+    }
+}
+
diff --git a/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.types b/tests/baselines/reference/operatorErrorsDetectedByNarrowingMessage.types
@@ -0,0 +1,166 @@
+=== tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts ===
+const enum Keys {
+>Keys : Keys
+
+    Tab = 10,
+>Tab : Keys.Tab
+>10 : 10
+
+    Shift = 13
+>Shift : Keys.Shift
+>13 : 13
+}
+
+function enumType() {
+>enumType : () => void
+
+    let key: Keys;
+>key : Keys
+>Keys : Keys
+
+    if (key === Keys.Tab) {
+>key === Keys.Tab : boolean
+>key : Keys
+>Keys.Tab : Keys.Tab
+>Keys : typeof Keys
+>Tab : Keys.Tab
+
+        return;
+    }
+
+    if (key === Keys.Tab || key === Keys.Shift) {
+>key === Keys.Tab || key === Keys.Shift : boolean
+>key === Keys.Tab : boolean
+>key : Keys.Shift
+>Keys.Tab : Keys.Tab
+>Keys : typeof Keys
+>Tab : Keys.Tab
+>key === Keys.Shift : boolean
+>key : Keys.Shift
+>Keys.Shift : Keys.Shift
+>Keys : typeof Keys
+>Shift : Keys.Shift
+
+        return;
+    }
+}
+
+
+function unionType() {
+>unionType : () => boolean
+
+    let x!: string | number;
+>x : string | number
+
+    let y!: string | number;
+>y : string | number
+
+    if (typeof x === 'number') {
+>typeof x === 'number' : boolean
+>typeof x : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>x : string | number
+>'number' : "number"
+
+        return;
+    }
+    if (typeof y === 'string') {
+>typeof y === 'string' : boolean
+>typeof y : "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>y : string | number
+>'string' : "string"
+
+        return;
+    }
+    return x === y;
+>x === y : boolean
+>x : string
+>y : number
+}
+
+interface Left {
+>Left : Left
+
+    kind: 'left'
+>kind : "left"
+
+    data: number;
+>data : number
+}
+
+interface Right {
+>Right : Right
+
+    kind: 'right'
+>kind : "right"
+
+    data: string;
+>data : string
+}
+
+function discriminatedType() {
+>discriminatedType : () => boolean
+
+    let x!: Left;
+>x : Left
+>Left : Left
+
+    let y!: Left | Right;
+>y : Left | Right
+>Left : Left
+>Right : Right
+
+    if (y.kind === 'left') {
+>y.kind === 'left' : boolean
+>y.kind : "left" | "right"
+>y : Left | Right
+>kind : "left" | "right"
+>'left' : "left"
+
+        return;
+    }
+    return x.data === y.data;
+>x.data === y.data : boolean
+>x.data : number
+>x : Left
+>data : number
+>y.data : string
+>y : Right
+>data : string
+}
+
+
+function booleanType(bar: boolean) {
+>booleanType : (bar: boolean) => "true" | "false"
+>bar : boolean
+
+    if (bar === true) {
+>bar === true : boolean
+>bar : boolean
+>true : true
+
+        return "true";
+>"true" : "true"
+    }
+    if (bar === true) {
+>bar === true : boolean
+>bar : false
+>true : true
+
+        return "false";
+>"false" : "false"
+    }
+
+    const f: boolean = false;
+>f : boolean
+>false : false
+
+    if (f === true) {
+>f === true : boolean
+>f : false
+>true : true
+
+        return "false";
+>"false" : "false"
+    }
+}
+
diff --git a/tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts b/tests/cases/compiler/operatorErrorsDetectedByNarrowingMessage.ts
@@ -0,0 +1,63 @@
+const enum Keys {
+    Tab = 10,
+    Shift = 13
+}
+
+function enumType() {
+    let key: Keys;
+
+    if (key === Keys.Tab) {
+        return;
+    }
+
+    if (key === Keys.Tab || key === Keys.Shift) {
+        return;
+    }
+}
+
+
+function unionType() {
+    let x!: string | number;
+    let y!: string | number;
+    if (typeof x === 'number') {
+        return;
+    }
+    if (typeof y === 'string') {
+        return;
+    }
+    return x === y;
+}
+
+interface Left {
+    kind: 'left'
+    data: number;
+}
+
+interface Right {
+    kind: 'right'
+    data: string;
+}
+
+function discriminatedType() {
+    let x!: Left;
+    let y!: Left | Right;
+    if (y.kind === 'left') {
+        return;
+    }
+    return x.data === y.data;
+}
+
+
+function booleanType(bar: boolean) {
+    if (bar === true) {
+        return "true";
+    }
+    if (bar === true) {
+        return "false";
+    }
+
+    const f: boolean = false;
+    if (f === true) {
+        return "false";
+    }
+}