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'.
     }