doc: introduce overload resolution

[pirapira]

The documentation talks about overload resolution in several locations, but all of these assume that overload resolution is already known. Solidity features wiki page contained a section about this concept. Perhaps we should add something similar to the documentation in this repository.

The section removed from the wiki page:

Overloading Functions

This seems to be missing from the official documentation.

PT Contracts can have multiple functions of the same name as long as the parameters differ in number or type. If such an overloaded function is referenced, it has to be called immediately to resolve the ambiguity using the types of the arguments. It is an error if not exactly one of the possible functions can be called with the given arguments.

contract Base {
function f(uint a) {}
}
contract Derived is Base {
function f(uint8 b) {}
function g() {
// f(250); would create a type error since 250 can be implicitly
// converted both to a uint8 and to a uint type
f(2000); // calls f from Base
}
}

Of course overloading a function does not need inheritance, i.e. f(uint a) could as well have been defined directly in Derived.

Overloaded functions are also present in the external interface. It is an error if two externally visible functions differ by their Solidity types but not by their external types, e.g. f(Derived _d) and f(address _a) both end up accepting an address type for the ABI although they are considered different inside Solidity.

[axic]

Also should make clear that return parameters are not taken into account for overload resolution.

[axic]

It seems that we don’t even have a section for functions (just view, pure and fallback). I think it should be in this order:

• functions (including payable),
• view,
• pure,
• constructor (mention that it can be payable, but not view/pure),
• fallback (mention that it can be payable, but not view/pure),
• overload resolution