Pointed out on the forum: 

https://forum.dlang.org/post/lssjxaqshkvytbeeyoic@forum.dlang.org

```
void main() {
  auto a = new BigInt[](100);
  a[] = BigInt(1);  // works fine
  a[] += BigInt(1); // Error: `@nogc` function `core.internal.array.operations.arrayOp!
                    // (BigInt[], BigInt, "+=").arrayOp` cannot call non-@nogc function
                    // `std.bigint.BigInt.opOpAssign!("+", BigInt).opOpAssign`
}
```

arrayOp is a template, so function attributes should be inferred.