go beyond never

Author: Andarist

src/compiler/checker.ts

@@ -36065,8 +36065,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             const declaration = parameter.valueDeclaration as ParameterDeclaration;
             if (!getEffectiveTypeAnnotationNode(declaration)) {
                 let type = tryGetTypeAtPosition(context, i);
-                if (type && type.flags & TypeFlags.Never && declaration.initializer) {
-                    type = widenTypeInferredFromInitializer(declaration, checkDeclarationInitializer(declaration, CheckMode.Normal));
+                if (type && declaration.initializer) {
+                    const initializerType = widenTypeInferredFromInitializer(declaration, checkDeclarationInitializer(declaration, CheckMode.Normal));
+                    if (!isTypeAssignableTo(initializerType, type) && isTypeAssignableTo(type, initializerType)) {
+                        type = initializerType;
+                    }
                 }
                 assignParameterType(parameter, type);
             }

tests/baselines/reference/contextuallyTypedNeverParameterWithInitializer.symbols

@@ -1,8 +1,8 @@
-contextuallyTypedParametersWithInitializers.ts(24,24): error TS7006: Parameter 'x' implicitly has an 'any' type.
-contextuallyTypedParametersWithInitializers.ts(40,5): error TS7006: Parameter 'x' implicitly has an 'any' type.
+contextuallyTypedParametersWithInitializers1.ts(24,24): error TS7006: Parameter 'x' implicitly has an 'any' type.
+contextuallyTypedParametersWithInitializers1.ts(40,5): error TS7006: Parameter 'x' implicitly has an 'any' type.
 
 
-==== contextuallyTypedParametersWithInitializers.ts (2 errors) ====
+==== contextuallyTypedParametersWithInitializers1.ts (2 errors) ====
     declare function id1<T>(input: T): T;
     declare function id2<T extends (x: any) => any>(input: T): T;
     declare function id3<T extends (x: { foo: any }) => any>(input: T): T;

tests/baselines/reference/contextuallyTypedParametersWithInitializers.errors.txt renamed to tests/baselines/reference/contextuallyTypedParametersWithInitializers1.errors.txt

@@ -1,6 +1,6 @@
-//// [tests/cases/compiler/contextuallyTypedParametersWithInitializers.ts] ////
+//// [tests/cases/compiler/contextuallyTypedParametersWithInitializers1.ts] ////
 
-//// [contextuallyTypedParametersWithInitializers.ts]
+//// [contextuallyTypedParametersWithInitializers1.ts]
 declare function id1<T>(input: T): T;
 declare function id2<T extends (x: any) => any>(input: T): T;
 declare function id3<T extends (x: { foo: any }) => any>(input: T): T;
@@ -86,7 +86,7 @@ const fz1 = (debug = true) => false;
 const fz2: Function = (debug = true) => false;
 
 
-//// [contextuallyTypedParametersWithInitializers.js]
+//// [contextuallyTypedParametersWithInitializers1.js]
 "use strict";
 Object.defineProperty(exports, "__esModule", { value: true });
 exports.executeSomething = void 0;
@@ -234,5 +234,5 @@ var fz2 = function (debug) {
 };
 
 
-//// [contextuallyTypedParametersWithInitializers.d.ts]
+//// [contextuallyTypedParametersWithInitializers1.d.ts]
 export declare function executeSomething(): Promise<string>;