VIP: Disallow use of value as a variable name in a function input

[iamdefinitelyahuman]

Simple Summary

Vyper should not allow value as a function input variable name.

Motivation

Imagine the following contract Foo:

@public
@payable
def foo(bar: uint256 = 7, value: uint256 = 42):
    pass

and another contract interacting with the previous one:

import first_contract as Foo

@public
def call_to_foo(a: address):
    Foo(a).foo(value=31337)

The use of value as a kwarg typically indicates an amount of ether to send, but because of the input argument the intention becomes ambiguous. Is call_to_foo transferring 31337 wei, or sending an integer of 31337?

Specification

Use of value as a variable name in a function input should raise. We can suggest that users change it to _value.

Backwards Compatibility

It's a breaking change for some contracts, but with a well explained error message it's easily dealt with.

Copyright

Copyright and related rights waived via CC0

[fubuloubu]

Meeting notes: treat gas the same as value (since both are used in kwargs in function calls), disallow in function args only

[charles-cooper]

Meeting notes: treat gas the same as value (since both are used in kwargs in function calls), disallow in function args only

(gas is currently disallowed as a variable name so this would relax the restriction on gas to only disallow it in function args)

[iamdefinitelyahuman]

Closed in #2029