Allow spreading arrays after required parameters

This allows:

1. Spreading arrays into all-optional parameters whose types match.
2. Spreading arrays into parameter lists that are too short and whose
body presumably uses `arguments`.

Author: sandersn

src/compiler/checker.ts

@@ -9598,6 +9598,10 @@ namespace ts {
             return signature.hasRestParameter && parameterIndex >= signature.parameters.length - 1;
         }
 
+        function areAllParametersOptionalAfter(signature: Signature, parameterIndex: number) {
+            return parameterIndex >= signature.minArgumentCount;
+        }
+
         function isSupertypeOfEach(candidate: Type, types: Type[]): boolean {
             for (const t of types) {
                 if (candidate !== t && !isTypeSubtypeOf(t, candidate)) return false;
@@ -14649,7 +14653,7 @@ namespace ts {
             // If spread arguments are present, check that they correspond to a rest parameter. If so, no
             // further checking is necessary.
             if (spreadArgIndex >= 0) {
-                return isRestParameterIndex(signature, spreadArgIndex);
+                return isRestParameterIndex(signature, spreadArgIndex) || areAllParametersOptionalAfter(signature, spreadArgIndex);
             }
 
             // Too many arguments implies incorrect arity.