Handle when associated name can be undefined

Fixes #40251

src/compiler/checker.ts

@@ -10081,7 +10081,7 @@ namespace ts {
                 const associatedNames = restType.target.labeledElementDeclarations;
                 const restParams = map(elementTypes, (t, i) => {
                     // Lookup the label from the individual tuple passed in before falling back to the signature `rest` parameter name
-                    const tupleLabelName = !!associatedNames && getTupleElementLabel(associatedNames[i]);
+                    const tupleLabelName = !!associatedNames?.[i] && getTupleElementLabel(associatedNames[i]);
                     const name = tupleLabelName || getParameterNameAtPosition(sig, restIndex + i, restType);
                     const flags = restType.target.elementFlags[i];
                     const checkFlags = flags & ElementFlags.Variable ? CheckFlags.RestParameter :
@@ -28672,7 +28672,7 @@ namespace ts {
             if (isTupleType(restType)) {
                 const associatedNames = (<TupleType>(<TypeReference>restType).target).labeledElementDeclarations;
                 const index = pos - paramCount;
-                return associatedNames && getTupleElementLabel(associatedNames[index]) || restParameter.escapedName + "_" + index as __String;
+                return associatedNames?.[index] && getTupleElementLabel(associatedNames[index]) || restParameter.escapedName + "_" + index as __String;
             }
             return restParameter.escapedName;
         }

tests/baselines/reference/parameterNamesInRestParameter.errors.txt

@@ -0,0 +1,13 @@
+/index.js(2,3): error TS2684: The 'this' context of type '<T>(...items: T[]) => T[]' is not assignable to method's 'this' of type '(this: () => void) => unknown[]'.
+  Types of parameters 'items' and 'args_0' are incompatible.
+    Type 'any' is not assignable to type 'never'.
+
+
+==== /index.js (1 errors) ====
+    function F() { }
+    !(Array.of.call(F) instanceof F);
+      ~~~~~~~~
+!!! error TS2684: The 'this' context of type '<T>(...items: T[]) => T[]' is not assignable to method's 'this' of type '(this: () => void) => unknown[]'.
+!!! error TS2684:   Types of parameters 'items' and 'args_0' are incompatible.
+!!! error TS2684:     Type 'any' is not assignable to type 'never'.
+

tests/baselines/reference/parameterNamesInRestParameter.js

@@ -0,0 +1,9 @@
+//// [index.js]
+function F() { }
+!(Array.of.call(F) instanceof F);
+
+
+//// [index.js]
+"use strict";
+function F() { }
+!(Array.of.call(F) instanceof F);

tests/baselines/reference/parameterNamesInRestParameter.symbols

@@ -0,0 +1,13 @@
+=== /index.js ===
+function F() { }
+>F : Symbol(F, Decl(index.js, 0, 0))
+
+!(Array.of.call(F) instanceof F);
+>Array.of.call : Symbol(CallableFunction.call, Decl(lib.es5.d.ts, --, --))
+>Array.of : Symbol(ArrayConstructor.of, Decl(lib.es2015.core.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+>of : Symbol(ArrayConstructor.of, Decl(lib.es2015.core.d.ts, --, --))
+>call : Symbol(CallableFunction.call, Decl(lib.es5.d.ts, --, --))
+>F : Symbol(F, Decl(index.js, 0, 0))
+>F : Symbol(F, Decl(index.js, 0, 0))
+

tests/baselines/reference/parameterNamesInRestParameter.types

@@ -0,0 +1,17 @@
+=== /index.js ===
+function F() { }
+>F : () => void
+
+!(Array.of.call(F) instanceof F);
+>!(Array.of.call(F) instanceof F) : boolean
+>(Array.of.call(F) instanceof F) : boolean
+>Array.of.call(F) instanceof F : boolean
+>Array.of.call(F) : unknown[]
+>Array.of.call : <T, A extends any[], R>(this: (this: T, ...args: A) => R, thisArg: T, ...args: A) => R
+>Array.of : <T>(...items: T[]) => T[]
+>Array : ArrayConstructor
+>of : <T>(...items: T[]) => T[]
+>call : <T, A extends any[], R>(this: (this: T, ...args: A) => R, thisArg: T, ...args: A) => R
+>F : () => void
+>F : () => void
+

tests/cases/compiler/parameterNamesInRestParameter.ts

@@ -0,0 +1,9 @@
+// @lib: es6
+// @allowJs: true
+// @checkJs: true
+// @outDir: out
+// @strict: true
+
+// @filename: /index.js
+function F() { }
+!(Array.of.call(F) instanceof F);