Don't generalize when assigment target is never (fix: #41707) (#59774)

src/compiler/checker.ts

@@ -22056,8 +22056,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             const [sourceType, targetType] = getTypeNamesForErrorDisplay(source, target);
             let generalizedSource = source;
             let generalizedSourceType = sourceType;
-
-            if (isLiteralType(source) && !typeCouldHaveTopLevelSingletonTypes(target)) {
+            
+            // Don't generalize on 'never' - we really want the original type
+            // to be displayed for use-cases like 'assertNever'.
+            if (!(target.flags & TypeFlags.Never) && isLiteralType(source) && !typeCouldHaveTopLevelSingletonTypes(target)) {
                 generalizedSource = getBaseTypeOfLiteralType(source);
                 Debug.assert(!isTypeAssignableTo(generalizedSource, target), "generalized source shouldn't be assignable");
                 generalizedSourceType = getTypeNameForErrorDisplay(generalizedSource);

tests/baselines/reference/controlFlowArrayErrors.errors.txt

@@ -6,7 +6,7 @@ controlFlowArrayErrors.ts(19,9): error TS7034: Variable 'x' implicitly has type
 controlFlowArrayErrors.ts(22,9): error TS7005: Variable 'x' implicitly has an 'any[]' type.
 controlFlowArrayErrors.ts(29,12): error TS2345: Argument of type 'boolean' is not assignable to parameter of type 'string | number'.
 controlFlowArrayErrors.ts(34,12): error TS2345: Argument of type 'boolean' is not assignable to parameter of type 'string | number'.
-controlFlowArrayErrors.ts(48,12): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+controlFlowArrayErrors.ts(48,12): error TS2345: Argument of type '99' is not assignable to parameter of type 'never'.
 controlFlowArrayErrors.ts(56,12): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
 controlFlowArrayErrors.ts(60,11): error TS7034: Variable 'x' implicitly has type 'any[]' in some locations where its type cannot be determined.
 controlFlowArrayErrors.ts(63,9): error TS7005: Variable 'x' implicitly has an 'any[]' type.
@@ -78,7 +78,7 @@ controlFlowArrayErrors.ts(63,9): error TS7005: Variable 'x' implicitly has an 'a
         x;           // boolean[] | (string | number)[]
         x.push(99);  // Error
                ~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '99' is not assignable to parameter of type 'never'.
     }
     
     function f7() {

tests/baselines/reference/dependentDestructuredVariables.errors.txt

@@ -1,6 +1,6 @@
 dependentDestructuredVariables.ts(334,5): error TS7022: 'value1' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.
 dependentDestructuredVariables.ts(334,5): error TS7031: Binding element 'value1' implicitly has an 'any' type.
-dependentDestructuredVariables.ts(431,15): error TS2322: Type 'number' is not assignable to type 'never'.
+dependentDestructuredVariables.ts(431,15): error TS2322: Type '1' is not assignable to type 'never'.
 
 
 ==== dependentDestructuredVariables.ts (3 errors) ====
@@ -440,7 +440,7 @@ dependentDestructuredVariables.ts(431,15): error TS2322: Type 'number' is not as
         if (y === undefined) {
             const shouldNotBeOk: never = x;  // Error
                   ~~~~~~~~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
         }
     }
     

tests/baselines/reference/errorOnFunctionReturnType.errors.txt

@@ -5,7 +5,7 @@ foo.js(16,60): error TS2355: A function whose declared type is neither 'undefine
 foo.js(21,20): error TS2355: A function whose declared type is neither 'undefined', 'void', nor 'any' must return a value.
 foo.js(31,10): error TS2534: A function returning 'never' cannot have a reachable end point.
 foo.js(35,12): error TS1065: The return type of an async function or method must be the global Promise<T> type.
-foo.js(37,5): error TS2322: Type 'string' is not assignable to type 'never'.
+foo.js(37,5): error TS2322: Type '"asd"' is not assignable to type 'never'.
 foo.js(40,56): error TS2534: A function returning 'never' cannot have a reachable end point.
 foo.js(45,18): error TS2534: A function returning 'never' cannot have a reachable end point.
 
@@ -64,7 +64,7 @@ foo.js(45,18): error TS2534: A function returning 'never' cannot have a reachabl
     async function testNever2() {
         return "asd";
         ~~~~~~
-!!! error TS2322: Type 'string' is not assignable to type 'never'.
+!!! error TS2322: Type '"asd"' is not assignable to type 'never'.
     }
     
     var testNever3 = /** @type {FunctionReturningNever} */ function() {

tests/baselines/reference/exhaustiveSwitchCheckCircularity.errors.txt

@@ -1,4 +1,4 @@
-exhaustiveSwitchCheckCircularity.ts(14,26): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+exhaustiveSwitchCheckCircularity.ts(14,26): error TS2345: Argument of type '"bbb"' is not assignable to parameter of type 'never'.
 
 
 ==== exhaustiveSwitchCheckCircularity.ts (1 errors) ====
@@ -17,7 +17,7 @@ exhaustiveSwitchCheckCircularity.ts(14,26): error TS2345: Argument of type 'stri
             }
             else if (isNever(foo)) {  // Error expected
                              ~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"bbb"' is not assignable to parameter of type 'never'.
                 break;
             }
         }

tests/baselines/reference/extractInferenceImprovement.errors.txt

@@ -1,4 +1,4 @@
-extractInferenceImprovement.ts(26,26): error TS2345: Argument of type 'typeof s' is not assignable to parameter of type 'never'.
+extractInferenceImprovement.ts(26,26): error TS2345: Argument of type 'unique symbol' is not assignable to parameter of type 'never'.
 extractInferenceImprovement.ts(28,1): error TS2322: Type 'string | number' is not assignable to type 'string'.
   Type 'number' is not assignable to type 'string'.
 extractInferenceImprovement.ts(28,26): error TS2345: Argument of type 'unique symbol' is not assignable to parameter of type '"first" | "second"'.
@@ -32,7 +32,7 @@ extractInferenceImprovement.ts(28,26): error TS2345: Argument of type 'unique sy
     // Should fail
     prop = getProperty2(obj, s);
                              ~
-!!! error TS2345: Argument of type 'typeof s' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type 'unique symbol' is not assignable to parameter of type 'never'.
     
     prop = getProperty3(obj, s);
     ~~~~

tests/baselines/reference/functionCallOnConstrainedTypeVariable.errors.txt

@@ -1,6 +1,6 @@
-functionCallOnConstrainedTypeVariable.ts(11,7): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
-functionCallOnConstrainedTypeVariable.ts(15,7): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
-functionCallOnConstrainedTypeVariable.ts(18,5): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+functionCallOnConstrainedTypeVariable.ts(11,7): error TS2345: Argument of type '"s"' is not assignable to parameter of type 'never'.
+functionCallOnConstrainedTypeVariable.ts(15,7): error TS2345: Argument of type '"s"' is not assignable to parameter of type 'never'.
+functionCallOnConstrainedTypeVariable.ts(18,5): error TS2345: Argument of type '""' is not assignable to parameter of type 'never'.
 functionCallOnConstrainedTypeVariable.ts(19,9): error TS2554: Expected 1 arguments, but got 4.
 
 
@@ -17,18 +17,18 @@ functionCallOnConstrainedTypeVariable.ts(19,9): error TS2554: Expected 1 argumen
     function call0(p: A | B) { 
       p.a("s"); // Error
           ~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"s"' is not assignable to parameter of type 'never'.
     }
     
     function callN<T extends A | B>(p: T) {
       p.a("s"); // Error
           ~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"s"' is not assignable to parameter of type 'never'.
       
       var a: T["a"] = p.a;
       a(""); // Error
         ~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '""' is not assignable to parameter of type 'never'.
       a("", "", "", ""); // Error
             ~~~~~~~~~~
 !!! error TS2554: Expected 1 arguments, but got 4.

tests/baselines/reference/keyofAndIndexedAccess2.errors.txt

@@ -1,9 +1,9 @@
 keyofAndIndexedAccess2.ts(4,5): error TS2322: Type 'string' is not assignable to type 'number'.
 keyofAndIndexedAccess2.ts(6,5): error TS2322: Type '2' is not assignable to type '0 | 1'.
 keyofAndIndexedAccess2.ts(7,5): error TS2322: Type '"x"' is not assignable to type '0 | 1'.
-keyofAndIndexedAccess2.ts(8,5): error TS2322: Type 'number' is not assignable to type 'never'.
-keyofAndIndexedAccess2.ts(9,5): error TS2322: Type 'number' is not assignable to type 'never'.
-keyofAndIndexedAccess2.ts(10,5): error TS2322: Type 'string' is not assignable to type 'never'.
+keyofAndIndexedAccess2.ts(8,5): error TS2322: Type '1' is not assignable to type 'never'.
+keyofAndIndexedAccess2.ts(9,5): error TS2322: Type '2' is not assignable to type 'never'.
+keyofAndIndexedAccess2.ts(10,5): error TS2322: Type '"x"' is not assignable to type 'never'.
 keyofAndIndexedAccess2.ts(14,5): error TS2739: Type '{ [key: string]: number; }' is missing the following properties from type '{ x: number; y: number; }': x, y
 keyofAndIndexedAccess2.ts(15,5): error TS2322: Type 'T' is not assignable to type '{ x: number; y: number; }'.
   Type '{ [key: string]: number; }' is missing the following properties from type '{ x: number; y: number; }': x, y
@@ -18,7 +18,7 @@ keyofAndIndexedAccess2.ts(31,5): error TS2322: Type '{ [key: string]: number; }'
 keyofAndIndexedAccess2.ts(38,5): error TS2322: Type '{ [x: string]: number; }' is not assignable to type '{ [P in K]: number; }'.
 keyofAndIndexedAccess2.ts(50,3): error TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Item'.
   No index signature with a parameter of type 'string' was found on type 'Item'.
-keyofAndIndexedAccess2.ts(51,3): error TS2322: Type 'number' is not assignable to type 'never'.
+keyofAndIndexedAccess2.ts(51,3): error TS2322: Type '123' is not assignable to type 'never'.
 keyofAndIndexedAccess2.ts(52,3): error TS2322: Type 'number' is not assignable to type 'T[keyof T]'.
   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'number'.
 keyofAndIndexedAccess2.ts(53,3): error TS2322: Type 'number' is not assignable to type 'T[K]'.
@@ -30,7 +30,7 @@ keyofAndIndexedAccess2.ts(67,3): error TS2322: Type 'number' is not assignable t
 keyofAndIndexedAccess2.ts(68,3): error TS2322: Type 'number' is not assignable to type 'T[K]'.
   'number' is assignable to the constraint of type 'T[K]', but 'T[K]' could be instantiated with a different subtype of constraint 'number'.
 keyofAndIndexedAccess2.ts(108,5): error TS2322: Type '123' is not assignable to type 'Type[K]'.
-  Type 'number' is not assignable to type 'never'.
+  Type '123' is not assignable to type 'never'.
 
 
 ==== keyofAndIndexedAccess2.ts (23 errors) ====
@@ -49,13 +49,13 @@ keyofAndIndexedAccess2.ts(108,5): error TS2322: Type '123' is not assignable to
 !!! error TS2322: Type '"x"' is not assignable to type '0 | 1'.
         obj[k2] = 1;    // Error
         ~~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
         obj[k2] = 2;    // Error
         ~~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '2' is not assignable to type 'never'.
         obj[k2] = 'x';  // Error
         ~~~~~~~
-!!! error TS2322: Type 'string' is not assignable to type 'never'.
+!!! error TS2322: Type '"x"' is not assignable to type 'never'.
     }
     
     function f2<T extends { [key: string]: number }>(a: { x: number, y: number }, b: { [key: string]: number }, c: T, k: keyof T) {
@@ -121,7 +121,7 @@ keyofAndIndexedAccess2.ts(108,5): error TS2322: Type '123' is not assignable to
 !!! error TS7053:   No index signature with a parameter of type 'string' was found on type 'Item'.
       obj[k2] = 123;  // Error
       ~~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '123' is not assignable to type 'never'.
       obj[k3] = 123;  // Error
       ~~~~~~~
 !!! error TS2322: Type 'number' is not assignable to type 'T[keyof T]'.
@@ -197,7 +197,7 @@ keyofAndIndexedAccess2.ts(108,5): error TS2322: Type '123' is not assignable to
         return 123;  // Error
         ~~~~~~
 !!! error TS2322: Type '123' is not assignable to type 'Type[K]'.
-!!! error TS2322:   Type 'number' is not assignable to type 'never'.
+!!! error TS2322:   Type '123' is not assignable to type 'never'.
     }
     
     // Repro from #30920

tests/baselines/reference/logicalAssignment6(target=es2015).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment6.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment6.ts(10,42): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment6.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                              ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment6(target=es2020).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment6.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment6.ts(10,42): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment6.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                              ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment6(target=es2021).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment6.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment6.ts(10,42): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment6.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                              ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment6(target=esnext).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment6.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment6.ts(10,42): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment6.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment6.ts(10,42): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                              ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment7(target=es2015).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment7.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment7.ts(10,40): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment7.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                            ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment7(target=es2020).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment7.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment7.ts(10,40): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment7.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                            ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment7(target=es2021).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment7.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment7.ts(10,40): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment7.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                            ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/logicalAssignment7(target=esnext).errors.txt

@@ -1,5 +1,5 @@
 logicalAssignment7.ts(10,5): error TS2532: Object is possibly 'undefined'.
-logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+logicalAssignment7.ts(10,40): error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
 
 
 ==== logicalAssignment7.ts (2 errors) ====
@@ -16,6 +16,6 @@ logicalAssignment7.ts(10,40): error TS2345: Argument of type 'number' is not ass
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                            ~~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '100' is not assignable to parameter of type 'never'.
     }
     

tests/baselines/reference/narrowingUnionToNeverAssigment.errors.txt

@@ -0,0 +1,19 @@
+narrowingUnionToNeverAssigment.ts(8,15): error TS2322: Type '"c" | "d"' is not assignable to type 'never'.
+  Type '"c"' is not assignable to type 'never'.
+
+
+==== narrowingUnionToNeverAssigment.ts (1 errors) ====
+    type Variants = "a" | "b" | "c" | "d";
+    
+    
+    function fx1(x: Variants) {
+        if (x === "a" || x === "b") {        
+        }
+        else {
+            const y: never = x;
+                  ~
+!!! error TS2322: Type '"c" | "d"' is not assignable to type 'never'.
+!!! error TS2322:   Type '"c"' is not assignable to type 'never'.
+        }
+    }
+    

tests/baselines/reference/narrowingUnionToNeverAssigment.js

@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/narrowingUnionToNeverAssigment.ts] ////
+
+//// [narrowingUnionToNeverAssigment.ts]
+type Variants = "a" | "b" | "c" | "d";
+
+
+function fx1(x: Variants) {
+    if (x === "a" || x === "b") {        
+    }
+    else {
+        const y: never = x;
+    }
+}
+
+
+//// [narrowingUnionToNeverAssigment.js]
+"use strict";
+function fx1(x) {
+    if (x === "a" || x === "b") {
+    }
+    else {
+        var y = x;
+    }
+}

tests/baselines/reference/narrowingUnionToNeverAssigment.symbols

@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/narrowingUnionToNeverAssigment.ts] ////
+
+=== narrowingUnionToNeverAssigment.ts ===
+type Variants = "a" | "b" | "c" | "d";
+>Variants : Symbol(Variants, Decl(narrowingUnionToNeverAssigment.ts, 0, 0))
+
+
+function fx1(x: Variants) {
+>fx1 : Symbol(fx1, Decl(narrowingUnionToNeverAssigment.ts, 0, 38))
+>x : Symbol(x, Decl(narrowingUnionToNeverAssigment.ts, 3, 13))
+>Variants : Symbol(Variants, Decl(narrowingUnionToNeverAssigment.ts, 0, 0))
+
+    if (x === "a" || x === "b") {        
+>x : Symbol(x, Decl(narrowingUnionToNeverAssigment.ts, 3, 13))
+>x : Symbol(x, Decl(narrowingUnionToNeverAssigment.ts, 3, 13))
+    }
+    else {
+        const y: never = x;
+>y : Symbol(y, Decl(narrowingUnionToNeverAssigment.ts, 7, 13))
+>x : Symbol(x, Decl(narrowingUnionToNeverAssigment.ts, 3, 13))
+    }
+}
+

tests/baselines/reference/narrowingUnionToNeverAssigment.types

@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/narrowingUnionToNeverAssigment.ts] ////
+
+=== narrowingUnionToNeverAssigment.ts ===
+type Variants = "a" | "b" | "c" | "d";
+>Variants : Variants
+>         : ^^^^^^^^
+
+
+function fx1(x: Variants) {
+>fx1 : (x: Variants) => void
+>    : ^ ^^        ^^^^^^^^^
+>x : Variants
+>  : ^^^^^^^^
+
+    if (x === "a" || x === "b") {        
+>x === "a" || x === "b" : boolean
+>                       : ^^^^^^^
+>x === "a" : boolean
+>          : ^^^^^^^
+>x : Variants
+>  : ^^^^^^^^
+>"a" : "a"
+>    : ^^^
+>x === "b" : boolean
+>          : ^^^^^^^
+>x : "b" | "c" | "d"
+>  : ^^^^^^^^^^^^^^^
+>"b" : "b"
+>    : ^^^
+    }
+    else {
+        const y: never = x;
+>y : never
+>  : ^^^^^
+>x : "c" | "d"
+>  : ^^^^^^^^^
+    }
+}
+

tests/baselines/reference/neverTypeErrors1.errors.txt

@@ -1,13 +1,13 @@
-neverTypeErrors1.ts(3,5): error TS2322: Type 'number' is not assignable to type 'never'.
-neverTypeErrors1.ts(4,5): error TS2322: Type 'string' is not assignable to type 'never'.
-neverTypeErrors1.ts(5,5): error TS2322: Type 'boolean' is not assignable to type 'never'.
+neverTypeErrors1.ts(3,5): error TS2322: Type '1' is not assignable to type 'never'.
+neverTypeErrors1.ts(4,5): error TS2322: Type '"abc"' is not assignable to type 'never'.
+neverTypeErrors1.ts(5,5): error TS2322: Type 'false' is not assignable to type 'never'.
 neverTypeErrors1.ts(6,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
 neverTypeErrors1.ts(7,5): error TS2322: Type 'null' is not assignable to type 'never'.
 neverTypeErrors1.ts(8,5): error TS2322: Type '{}' is not assignable to type 'never'.
 neverTypeErrors1.ts(9,5): error TS2349: This expression is not callable.
   Type 'never' has no call signatures.
 neverTypeErrors1.ts(13,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
-neverTypeErrors1.ts(17,5): error TS2322: Type 'number' is not assignable to type 'never'.
+neverTypeErrors1.ts(17,5): error TS2322: Type '1' is not assignable to type 'never'.
 neverTypeErrors1.ts(20,16): error TS2534: A function returning 'never' cannot have a reachable end point.
 neverTypeErrors1.ts(23,17): error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
 neverTypeErrors1.ts(24,17): error TS2407: The right-hand side of a 'for...in' statement must be of type 'any', an object type or a type parameter, but here has type 'never'.
@@ -18,13 +18,13 @@ neverTypeErrors1.ts(24,17): error TS2407: The right-hand side of a 'for...in' st
         let x: never;
         x = 1;
         ~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
         x = "abc";
         ~
-!!! error TS2322: Type 'string' is not assignable to type 'never'.
+!!! error TS2322: Type '"abc"' is not assignable to type 'never'.
         x = false;
         ~
-!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+!!! error TS2322: Type 'false' is not assignable to type 'never'.
         x = undefined;
         ~
 !!! error TS2322: Type 'undefined' is not assignable to type 'never'.
@@ -49,7 +49,7 @@ neverTypeErrors1.ts(24,17): error TS2407: The right-hand side of a 'for...in' st
     function f3(): never {
         return 1;
         ~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
     }
     
     function f4(): never {

tests/baselines/reference/neverTypeErrors2.errors.txt

@@ -1,13 +1,13 @@
-neverTypeErrors2.ts(3,5): error TS2322: Type 'number' is not assignable to type 'never'.
-neverTypeErrors2.ts(4,5): error TS2322: Type 'string' is not assignable to type 'never'.
-neverTypeErrors2.ts(5,5): error TS2322: Type 'boolean' is not assignable to type 'never'.
+neverTypeErrors2.ts(3,5): error TS2322: Type '1' is not assignable to type 'never'.
+neverTypeErrors2.ts(4,5): error TS2322: Type '"abc"' is not assignable to type 'never'.
+neverTypeErrors2.ts(5,5): error TS2322: Type 'false' is not assignable to type 'never'.
 neverTypeErrors2.ts(6,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
 neverTypeErrors2.ts(7,5): error TS2322: Type 'null' is not assignable to type 'never'.
 neverTypeErrors2.ts(8,5): error TS2322: Type '{}' is not assignable to type 'never'.
 neverTypeErrors2.ts(9,5): error TS2349: This expression is not callable.
   Type 'never' has no call signatures.
 neverTypeErrors2.ts(13,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
-neverTypeErrors2.ts(17,5): error TS2322: Type 'number' is not assignable to type 'never'.
+neverTypeErrors2.ts(17,5): error TS2322: Type '1' is not assignable to type 'never'.
 neverTypeErrors2.ts(20,16): error TS2534: A function returning 'never' cannot have a reachable end point.
 neverTypeErrors2.ts(23,17): error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
 neverTypeErrors2.ts(24,17): error TS2407: The right-hand side of a 'for...in' statement must be of type 'any', an object type or a type parameter, but here has type 'never'.
@@ -18,13 +18,13 @@ neverTypeErrors2.ts(24,17): error TS2407: The right-hand side of a 'for...in' st
         let x: never;
         x = 1;
         ~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
         x = "abc";
         ~
-!!! error TS2322: Type 'string' is not assignable to type 'never'.
+!!! error TS2322: Type '"abc"' is not assignable to type 'never'.
         x = false;
         ~
-!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+!!! error TS2322: Type 'false' is not assignable to type 'never'.
         x = undefined;
         ~
 !!! error TS2322: Type 'undefined' is not assignable to type 'never'.
@@ -49,7 +49,7 @@ neverTypeErrors2.ts(24,17): error TS2407: The right-hand side of a 'for...in' st
     function f3(): never {
         return 1;
         ~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'never'.
+!!! error TS2322: Type '1' is not assignable to type 'never'.
     }
     
     function f4(): never {

tests/baselines/reference/nonPrimitiveUnionIntersection.errors.txt

@@ -1,4 +1,4 @@
-nonPrimitiveUnionIntersection.ts(1,5): error TS2322: Type 'string' is not assignable to type 'never'.
+nonPrimitiveUnionIntersection.ts(1,5): error TS2322: Type '""' is not assignable to type 'never'.
 nonPrimitiveUnionIntersection.ts(3,5): error TS2322: Type 'number' is not assignable to type 'object'.
 nonPrimitiveUnionIntersection.ts(4,1): error TS2322: Type 'string' is not assignable to type 'never'.
 nonPrimitiveUnionIntersection.ts(8,38): error TS2353: Object literal may only specify known properties, and 'bar' does not exist in type 'object & { err: string; }'.
@@ -7,7 +7,7 @@ nonPrimitiveUnionIntersection.ts(8,38): error TS2353: Object literal may only sp
 ==== nonPrimitiveUnionIntersection.ts (4 errors) ====
     var a: object & string = ""; // error
         ~
-!!! error TS2322: Type 'string' is not assignable to type 'never'.
+!!! error TS2322: Type '""' is not assignable to type 'never'.
     var b: object | string = ""; // ok
     var c: object & {} = 123; // error
         ~

tests/baselines/reference/tsxUnionElementType1.errors.txt

@@ -1,4 +1,4 @@
-file.tsx(12,10): error TS2322: Type 'boolean' is not assignable to type 'never'.
+file.tsx(12,10): error TS2322: Type 'true' is not assignable to type 'never'.
 
 
 ==== file.tsx (1 errors) ====
@@ -15,4 +15,4 @@ file.tsx(12,10): error TS2322: Type 'boolean' is not assignable to type 'never'.
     var SFCComp = SFC1 || SFC2;
     <SFCComp x />
              ~
-!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+!!! error TS2322: Type 'true' is not assignable to type 'never'.

tests/baselines/reference/tsxUnionElementType4.errors.txt

@@ -1,4 +1,4 @@
-file.tsx(32,17): error TS2322: Type 'boolean' is not assignable to type 'never'.
+file.tsx(32,17): error TS2322: Type 'true' is not assignable to type 'never'.
 file.tsx(33,21): error TS2322: Type '{ x: number; }' is not assignable to type 'IntrinsicAttributes & IntrinsicClassAttributes<RC4> & { children?: ReactNode; }'.
   Property 'x' does not exist on type 'IntrinsicAttributes & IntrinsicClassAttributes<RC4> & { children?: ReactNode; }'.
 file.tsx(34,22): error TS2322: Type '{ prop: true; }' is not assignable to type 'IntrinsicAttributes & IntrinsicClassAttributes<RC3> & { children?: ReactNode; }'.
@@ -39,7 +39,7 @@ file.tsx(34,22): error TS2322: Type '{ prop: true; }' is not assignable to type
     // Error
     let a = <RCComp x />;
                     ~
-!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+!!! error TS2322: Type 'true' is not assignable to type 'never'.
 !!! related TS6500 file.tsx:3:36: The expected type comes from property 'x' which is declared here on type 'IntrinsicAttributes & IntrinsicClassAttributes<RC1 | RC2> & { x: number; } & { children?: ReactNode; } & { x: string; } & { children?: ReactNode; }'
     let b = <PartRCComp x={10} />
                         ~

tests/baselines/reference/unionTypeCallSignatures.errors.txt

@@ -12,8 +12,8 @@ unionTypeCallSignatures.ts(15,29): error TS2769: No overload matches this call.
   Overload 2 of 2, '(a: string): string | boolean', gave the following error.
     Argument of type 'boolean' is not assignable to parameter of type 'string'.
 unionTypeCallSignatures.ts(16,1): error TS2554: Expected 1 arguments, but got 0.
-unionTypeCallSignatures.ts(19,32): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
-unionTypeCallSignatures.ts(20,32): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+unionTypeCallSignatures.ts(19,32): error TS2345: Argument of type '10' is not assignable to parameter of type 'never'.
+unionTypeCallSignatures.ts(20,32): error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'never'.
 unionTypeCallSignatures.ts(21,1): error TS2554: Expected 1 arguments, but got 0.
 unionTypeCallSignatures.ts(24,1): error TS2554: Expected 1 arguments, but got 0.
 unionTypeCallSignatures.ts(26,36): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
@@ -78,10 +78,10 @@ unionTypeCallSignatures.ts(73,12): error TS2554: Expected 2 arguments, but got 1
     var unionOfDifferentParameterTypes: { (a: number): number; } | { (a: string): Date; };
     unionOfDifferentParameterTypes(10);// error - no call signatures
                                    ~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'never'.
     unionOfDifferentParameterTypes("hello");// error - no call signatures
                                    ~~~~~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'never'.
     unionOfDifferentParameterTypes();// error - no call signatures
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2554: Expected 1 arguments, but got 0.

tests/baselines/reference/unionTypeConstructSignatures.errors.txt

@@ -12,8 +12,8 @@ unionTypeConstructSignatures.ts(15,33): error TS2769: No overload matches this c
   Overload 2 of 2, '(a: string): string | boolean', gave the following error.
     Argument of type 'boolean' is not assignable to parameter of type 'string'.
 unionTypeConstructSignatures.ts(16,1): error TS2554: Expected 1 arguments, but got 0.
-unionTypeConstructSignatures.ts(19,36): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
-unionTypeConstructSignatures.ts(20,36): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+unionTypeConstructSignatures.ts(19,36): error TS2345: Argument of type '10' is not assignable to parameter of type 'never'.
+unionTypeConstructSignatures.ts(20,36): error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'never'.
 unionTypeConstructSignatures.ts(21,1): error TS2554: Expected 1 arguments, but got 0.
 unionTypeConstructSignatures.ts(24,1): error TS2554: Expected 1 arguments, but got 0.
 unionTypeConstructSignatures.ts(26,40): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
@@ -78,10 +78,10 @@ unionTypeConstructSignatures.ts(73,1): error TS2511: Cannot create an instance o
     var unionOfDifferentParameterTypes: { new (a: number): number; } | { new (a: string): Date; };
     new unionOfDifferentParameterTypes(10);// error - no call signatures
                                        ~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '10' is not assignable to parameter of type 'never'.
     new unionOfDifferentParameterTypes("hello");// error - no call signatures
                                        ~~~~~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"hello"' is not assignable to parameter of type 'never'.
     new unionOfDifferentParameterTypes();// error - no call signatures
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2554: Expected 1 arguments, but got 0.

tests/baselines/reference/unionTypeInference.errors.txt

@@ -1,5 +1,5 @@
 unionTypeInference.ts(13,24): error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.
-unionTypeInference.ts(31,15): error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+unionTypeInference.ts(31,15): error TS2345: Argument of type '42' is not assignable to parameter of type 'never'.
 
 
 ==== unionTypeInference.ts (2 errors) ====
@@ -37,7 +37,7 @@ unionTypeInference.ts(31,15): error TS2345: Argument of type 'number' is not ass
     const d2 = f4(s);
     const d3 = f4(42);  // Error
                   ~~
-!!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '42' is not assignable to parameter of type 'never'.
     
     export interface Foo<T> {
         then<U>(f: (x: T) => U | Foo<U>, g: U): Foo<U>;

tests/baselines/reference/unknownControlFlow.errors.txt

@@ -1,6 +1,6 @@
 unknownControlFlow.ts(18,9): error TS2322: Type 'unknown' is not assignable to type '{}'.
 unknownControlFlow.ts(283,5): error TS2536: Type 'keyof (T & {})' cannot be used to index type 'T'.
-unknownControlFlow.ts(290,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+unknownControlFlow.ts(290,11): error TS2345: Argument of type '"foo"' is not assignable to parameter of type 'never'.
 unknownControlFlow.ts(291,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
 unknownControlFlow.ts(293,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
 unknownControlFlow.ts(323,9): error TS2367: This comparison appears to be unintentional because the types 'T' and 'number' have no overlap.
@@ -303,7 +303,7 @@ unknownControlFlow.ts(341,9): error TS2367: This comparison appears to be uninte
     
     ff1(null, 'foo');  // Error
               ~~~~~
-!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+!!! error TS2345: Argument of type '"foo"' is not assignable to parameter of type 'never'.
     ff2(null, 'foo');  // Error
         ~~~~
 !!! error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.

tests/cases/compiler/narrowingUnionToNeverAssigment.ts

@@ -0,0 +1,12 @@
+// @strict: true
+
+type Variants = "a" | "b" | "c" | "d";
+
+
+function fx1(x: Variants) {
+    if (x === "a" || x === "b") {        
+    }
+    else {
+        const y: never = x;
+    }
+}