fix: use Number instead of unary plus for BigInt

Fixes bug in fdf5578

src/contract/Contract.ts

@@ -312,7 +312,7 @@ class Contract<M extends ContractMethodsBase> {
 
     if (fn == null) throw new MissingFunctionNameError();
     if (fn === 'init' && callStatic !== true) throw new InvalidMethodInvocationError('"init" can be called only via dryRun');
-    if (fn !== 'init' && opt.amount != null && +opt.amount > 0 && !fnAci.payable) {
+    if (fn !== 'init' && opt.amount != null && Number(opt.amount) > 0 && !fnAci.payable) {
       throw new NotPayableFunctionError(opt.amount, fn);
     }
 

src/tx/builder/field-types/u-int.ts

@@ -5,7 +5,7 @@ import { toBytes } from '../../../utils/bytes';
 
 export default {
   serialize(value: Int): Buffer {
-    if (+value < 0) throw new ArgumentError('value', 'greater or equal to 0', value);
+    if (Number(value) < 0) throw new ArgumentError('value', 'greater or equal to 0', value);
     return toBytes(value, true);
   },
 

test/integration/contract-aci.ts

@@ -412,6 +412,15 @@ describe('Contract instance', () => {
     balanceAfter.should.be.equal(`${+contractBalance + 100}`);
   });
 
+  it('pays to payable function using BigInt', async () => {
+    assertNotNull(testContract.$options.address);
+    const contractBalance = await aeSdk.getBalance(testContract.$options.address);
+    // bigint is not assignable to amount
+    await testContract.stringFn('test', { amount: 100n as unknown as string, callStatic: false });
+    const balanceAfter = await aeSdk.getBalance(testContract.$options.address);
+    balanceAfter.should.be.equal(`${+contractBalance + 100}`);
+  });
+
   it('calls on specific account', async () => {
     const onAccount = aeSdk.accounts[aeSdk.addresses()[1]];
     const { result } = await testContract.intFn(123, { onAccount });