Make any[] the effective rest type when any is spread

Author: ahejlsberg

src/compiler/checker.ts

@@ -20837,8 +20837,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const restIndex = sourceRestType || targetRestType ? paramCount - 1 : -1;
 
         for (let i = 0; i < paramCount; i++) {
-            const sourceType = i === restIndex ? getRestTypeAtPosition(source, i) : tryGetTypeAtPosition(source, i);
-            const targetType = i === restIndex ? getRestTypeAtPosition(target, i) : tryGetTypeAtPosition(target, i);
+            const sourceType = i === restIndex ? getRestOrAnyTypeAtPosition(source, i) : tryGetTypeAtPosition(source, i);
+            const targetType = i === restIndex ? getRestOrAnyTypeAtPosition(target, i) : tryGetTypeAtPosition(target, i);
             if (sourceType && targetType) {
                 // In order to ensure that any generic type Foo<T> is at least co-variant with respect to T no matter
                 // how Foo uses T, we need to relate parameters bi-variantly (given that parameters are input positions,
@@ -36464,6 +36464,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return createTupleType(types, flags, readonly, names);
     }
 
+    // Return the rest type at the given position, transforming `any[]` into just `any`. We do this because
+    // in signatures we want `any[]` in a rest position to be compatible with anything, but `any[]` isn't
+    // assignable to tuple types with required elements.
+    function getRestOrAnyTypeAtPosition(source: Signature, pos: number): Type {
+        const restType = getRestTypeAtPosition(source, pos);
+        const elementType = restType && getElementTypeOfArrayType(restType);
+        return elementType && isTypeAny(elementType) ? anyType : restType;
+    }
+
     // Return the number of parameters in a signature. The rest parameter, if present, counts as one
     // parameter. For example, the parameter count of (x: number, y: number, ...z: string[]) is 3 and
     // the parameter count of (x: number, ...args: [number, ...string[], boolean])) is also 3. In the
@@ -36527,7 +36536,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         if (signatureHasRestParameter(signature)) {
             const restType = getTypeOfSymbol(signature.parameters[signature.parameters.length - 1]);
             if (!isTupleType(restType)) {
-                return restType;
+                return isTypeAny(restType) ? anyArrayType : restType;
             }
             if (restType.target.hasRestElement) {
                 return sliceTupleType(restType, restType.target.fixedLength);