Disallow parameter (argument) redefinition

[certik]

def f(x: i32) -> i32:
    x = 2
    return x * x

print(f(3)) # => 4

Currently this is allowed:

$ lpython a.py 
4

But we should give a compile time error message "input parameter x cannot be assigned to"

[certik]

For a comparison, in Fortran this:

integer function f(x) result(y)
integer, intent(in) :: x
x = 2
y = x*x
end function

is not allowed:

$ gfortran a.f90 
a.f90:3:0:

    3 | x = 2
      | 
Error: Dummy argument 'x' with INTENT(IN) in variable definition context (assignment) at (1)
$ lfortran a.f90 
semantic error: Cannot assign to an intent(in) variable `x`
 --> a.f90:3:1
  |
3 | x = 2
  | ^ 

[Shaikh-Ubaid]

It seems that the example

from lpython import i32

def f(x: i32) -> i32:
    x = 2
    return x * x

print(f(3)) # => 4

works in cpython.

$ python examples/expr2.py
4

Also, I think assignment to function parameter is allowed in C/CPP, for example:

$ cat examples/expr2.cpp 
#include <iostream>

int f(int x) {
    x = 2;
    return x * x;
}

int main() {
    std::cout << f(3) << std::endl;
}
$ g++ examples/expr2.cpp -o expr2cpp
$ ./expr2cpp 
4

Thus, it seems assignment to function parameter could be allowed in lpython.

[Shaikh-Ubaid]

I shared a concern here #1803 (comment).