Fill the shorter target tuple with undefined when comparing signatures

Author: Andarist

src/compiler/checker.ts

@@ -19393,9 +19393,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
                     for (let i = 0; i < getTypeReferenceArity(sourceType); i++) {
                         elementTypes.push(
-                            t.target.elementFlags[i] & ElementFlags.Required
-                                ? typeArguments[i]
-                                : getElementTypeOfSliceOfTupleType(t, i)!
+                            i < typeArguments.length
+                                ? t.target.elementFlags[i] & ElementFlags.Required
+                                    ? typeArguments[i]
+                                    : getElementTypeOfSliceOfTupleType(t, i)!
+                                : undefinedType
                         );
                     }
 

tests/baselines/reference/genericRestParameters3.errors.txt

@@ -11,9 +11,10 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(23,1): error TS2322
         Type 'number' is not assignable to type 'string'.
 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]'.
-        Source has 1 element(s) but target requires 2.
+    Type '[number, boolean] | [string, undefined]' is not assignable to type '[y: number, z: boolean]'.
+      Type '[string, undefined]' is not assignable to type '[y: number, z: boolean]'.
+        Type at position 0 in source is not compatible with type at position 0 in target.
+          Type 'string' is not assignable to type 'number'.
 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'.
@@ -76,9 +77,10 @@ tests/cases/conformance/types/rest/genericRestParameters3.ts(59,5): error TS2345
     ~~
 !!! 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:         Source has 1 element(s) but target requires 2.
+!!! error TS2322:     Type '[number, boolean] | [string, undefined]' is not assignable to type '[y: number, z: boolean]'.
+!!! error TS2322:       Type '[string, undefined]' is not assignable to type '[y: number, z: boolean]'.
+!!! error TS2322:         Type at position 0 in source is not compatible with type at position 0 in target.
+!!! error TS2322:           Type 'string' is not assignable to type 'number'.
     f1 = f4;
 
     // Repro from #26110