Ensure rest type for source parameter is readonly

Author: jakebailey

src/compiler/checker.ts

@@ -19810,7 +19810,7 @@ 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 sourceType = i === restIndex ? getRestTypeAtPosition(source, i, /*readonly*/ true) : tryGetTypeAtPosition(source, i);
             const targetType = i === restIndex ? getRestTypeAtPosition(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
@@ -34565,12 +34565,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return undefined;
     }
 
-    function getRestTypeAtPosition(source: Signature, pos: number): Type {
+    function getRestTypeAtPosition(source: Signature, pos: number, readonly = false): Type {
         const parameterCount = getParameterCount(source);
         const minArgumentCount = getMinArgumentCount(source);
         const restType = getEffectiveRestType(source);
         if (restType && pos >= parameterCount - 1) {
-            return pos === parameterCount - 1 ? restType : createArrayType(getIndexedAccessType(restType, numberType));
+            return pos === parameterCount - 1 ? restType : createArrayType(getIndexedAccessType(restType, numberType), readonly);
         }
         const types = [];
         const flags = [];
@@ -34589,7 +34589,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 names.push(name);
             }
         }
-        return createTupleType(types, flags, /*readonly*/ false, length(names) === length(types) ? names : undefined);
+        return createTupleType(types, flags, readonly, length(names) === length(types) ? names : undefined);
     }
 
     // Return the number of parameters in a signature. The rest parameter, if present, counts as one

tests/baselines/reference/contextualTupleTypeParameterReadonly.errors.txt

@@ -6,19 +6,19 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(18,1): error TS2345
     Source has 0 element(s) but target requires 2.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(23,1): error TS2322: Type '(x: string, y: string) => void' is not assignable to type '(x: string, ...args: [string] | [number, boolean]) => void'.
   Types of parameters 'y' and 'args' are incompatible.
-    Type '[string] | [number, boolean]' is not assignable to type '[y: string]'.
-      Type '[number, boolean]' is not assignable to type '[y: string]'.
+    Type '[string] | [number, boolean]' is not assignable to type 'readonly [y: string]'.
+      Type '[number, boolean]' is not assignable to type 'readonly [y: string]'.
         Source has 2 element(s) but target allows only 1.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(24,1): error TS2322: Type '(x: string, y: number, z: boolean) => void' is not assignable to type '(x: string, ...args: [string] | [number, boolean]) => void'.
   Types of parameters 'y' and 'args' are incompatible.
-    Type '[string] | [number, boolean]' is not assignable to type '[y: number, z: boolean]'.
-      Type '[string]' is not assignable to type '[y: number, z: boolean]'.
+    Type '[string] | [number, boolean]' is not assignable to type 'readonly [y: number, z: boolean]'.
+      Type '[string]' is not assignable to type 'readonly [y: number, z: boolean]'.
         Source has 1 element(s) but target requires 2.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(35,1): error TS2554: Expected 1 arguments, but got 0.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(36,21): error TS2345: Argument of type 'number' is not assignable to parameter of type '(...args: CoolArray<any>) => void'.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(37,21): error TS2345: Argument of type '<T extends any[]>(cb: (...args: T) => void) => void' is not assignable to parameter of type '(...args: CoolArray<any>) => void'.
   Types of parameters 'cb' and 'args' are incompatible.
-    Property '0' is missing in type 'CoolArray<any>' but required in type '[cb: (...args: any[]) => void]'.
+    Property '0' is missing in type 'CoolArray<any>' but required in type 'readonly [cb: (...args: any[]) => void]'.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(44,32): error TS2345: Argument of type '[10, 20]' is not assignable to parameter of type 'CoolArray<number>'.
   Property 'hello' is missing in type '[10, 20]' but required in type 'CoolArray<number>'.
 tests/cases/conformance/types/rest/genericRestParameters3.ts(49,1): error TS2345: Argument of type '[]' is not assignable to parameter of type 'CoolArray<never>'.
@@ -69,15 +69,15 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(59,5): error TS2345
     ~~
 !!! error TS2322: Type '(x: string, y: string) => void' is not assignable to type '(x: string, ...args: [string] | [number, boolean]) => void'.
 !!! error TS2322:   Types of parameters 'y' and 'args' are incompatible.
-!!! error TS2322:     Type '[string] | [number, boolean]' is not assignable to type '[y: string]'.
-!!! error TS2322:       Type '[number, boolean]' is not assignable to type '[y: string]'.
+!!! error TS2322:     Type '[string] | [number, boolean]' is not assignable to type 'readonly [y: string]'.
+!!! error TS2322:       Type '[number, boolean]' is not assignable to type 'readonly [y: string]'.
 !!! error TS2322:         Source has 2 element(s) but target allows only 1.
     f1 = f3;  // Error
     ~~
 !!! error TS2322: Type '(x: string, y: number, z: boolean) => void' is not assignable to type '(x: string, ...args: [string] | [number, boolean]) => void'.
 !!! error TS2322:   Types of parameters 'y' and 'args' are incompatible.
-!!! error TS2322:     Type '[string] | [number, boolean]' is not assignable to type '[y: number, z: boolean]'.
-!!! error TS2322:       Type '[string]' is not assignable to type '[y: number, z: boolean]'.
+!!! error TS2322:     Type '[string] | [number, boolean]' is not assignable to type 'readonly [y: number, z: boolean]'.
+!!! error TS2322:       Type '[string]' is not assignable to type 'readonly [y: number, z: boolean]'.
 !!! error TS2322:         Source has 1 element(s) but target requires 2.
     f1 = f4;
 
@@ -100,7 +100,7 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(59,5): error TS2345
                         ~~~
 !!! error TS2345: Argument of type '<T extends any[]>(cb: (...args: T) => void) => void' is not assignable to parameter of type '(...args: CoolArray<any>) => void'.
 !!! error TS2345:   Types of parameters 'cb' and 'args' are incompatible.
-!!! error TS2345:     Property '0' is missing in type 'CoolArray<any>' but required in type '[cb: (...args: any[]) => void]'.
+!!! error TS2345:     Property '0' is missing in type 'CoolArray<any>' but required in type 'readonly [cb: (...args: any[]) => void]'.
 
     function bar<T extends any[]>(...args: T): T {
         return args;

tests/baselines/reference/genericRestParameters3.errors.txt

@@ -1,7 +1,7 @@
 tests/cases/conformance/types/rest/restTuplesFromContextualTypes.ts(56,7): error TS2345: Argument of type '(a: number, b: T[0], ...x: T[number][]) => void' is not assignable to parameter of type '(x: number, ...args: T) => void'.
   Types of parameters 'b' and 'args' are incompatible.
-    Type 'T' is not assignable to type '[b: T[0], ...x: T[number][]]'.
-      Type 'any[]' is not assignable to type '[b: T[0], ...x: T[number][]]'.
+    Type 'T' is not assignable to type 'readonly [b: T[0], ...x: T[number][]]'.
+      Type 'any[]' is not assignable to type 'readonly [b: T[0], ...x: T[number][]]'.
         Source provides no match for required element at position 0 in target.
 
 
@@ -65,8 +65,8 @@ tests/cases/conformance/types/rest/restTuplesFromContextualTypes.ts(56,7): error
           ~~~~~~~~~~~~~~~~~~
 !!! error TS2345: Argument of type '(a: number, b: T[0], ...x: T[number][]) => void' is not assignable to parameter of type '(x: number, ...args: T) => void'.
 !!! error TS2345:   Types of parameters 'b' and 'args' are incompatible.
-!!! error TS2345:     Type 'T' is not assignable to type '[b: T[0], ...x: T[number][]]'.
-!!! error TS2345:       Type 'any[]' is not assignable to type '[b: T[0], ...x: T[number][]]'.
+!!! error TS2345:     Type 'T' is not assignable to type 'readonly [b: T[0], ...x: T[number][]]'.
+!!! error TS2345:       Type 'any[]' is not assignable to type 'readonly [b: T[0], ...x: T[number][]]'.
 !!! error TS2345:         Source provides no match for required element at position 0 in target.
     }