How to type-cast a function pointer in Solidity?

Question

I have a function that takes a parameter of type function (uint256) view returns (uint256).

I want to pass it a pointer to a function of type function (uint256) pure returns (uint256).

Obviously there shouldn't be a problem with passing this pure function into a parameter that expects a view function, because the parameter and returns types are the same and pure is even more limiting than view. However, I get this TypeError:

TypeError: Invalid type for argument in function call. Invalid implicit conversion from function (uint256) pure returns (uint256) to function (uint256) view returns (uint256) requested.

Things I've tried:

Assigning the function to a local variable gives a similar TypeError:

function(uint256) view returns(uint256) a = func;

TypeError: Type function (uint256) pure returns (uint256) is not implicitly convertible to expected type function (uint256) view returns (uint256)

The usual type-casting syntax of Solidity gives a ParserError at the function keyword

function(uint256) view returns(uint256) a = function(uint256) view returns(uint256)  (func);

ParserError: Expected primary expression

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I've noticed that the TypeError's keep telling me that the function pointer is not implicitly convertible. Is there a way to explicitly convert it?

Answer 1

Update: In recent Solidity versions, pure function pointers are now implicitly convertible to view function pointers.

I did not find a solution to this problem, so I submitted an issue on Github: https://github.com/ethereum/solidity/issues/4349

I did find two ways to work around the problem:

1: Downgrade all my pure functions to view.

2: Use inline assembly to assign to a local variable, thereby circumventing the Solidity type system:

function(uint256)view returns(uint256) a;
assembly { a := func }

Answer 2

I cannot understand your point.

In solidity they do not exist function pointers, because the calling mechanism is to pass the control to the smart contract which owns the function and to use ABI table to jump there to the function first instruction.

So there is not a pointer to a function you can use like you do in C!

In Solidity the address of any function IS the ethereum addess of its smart contract

I.e. in a qualitative way, the address (function foo()) is the address of the smart contract implementing foo()

May be I did not understand properly your question!

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EDIT AFTER COMMENTS

Typecast cannot be applied to function type, as far as I know only implicit conversion can be done and this IS the demonstration that the function pointer as lvalue DO NOT EXISTS.

From solidity docs:

"A value of external function type can be explicitly converted to address resulting in the address of the contract of the function.

A function type A is implicitly convertible to a function type B if and only if their parameter types are identical, their return types are identical, their internal/external property is identical and the state mutability of A is not more restrictive than the state mutability of B. In particular:

pure functions can be converted to view and non-payable functions view functions can be converted to non-payable functions payable functions can be converted to non-payable functions

No other conversions between function types are possible."

Anyway if we are here to learn, so I hope to learn something new from you soon.

Check also function as parameter