Getters returning structs can not be defined in interfaces

[axic]

interface I {
    struct S {
        uint a;
    }

    function s() external returns (S memory);
}

contract C is I {
    S public override s;
}

The function s can not be defined in a way this works. The problem lies in the impossibility to define a working location.

S storage is not allowed in interfaces, but that could be one option:

Error: Data location must be "memory" or "calldata" for return parameter in function, but "storage" was given.
 --> struct-interface.sol:6:36:
  |
6 |     function s() external returns (S storage);
  |                                    ^^^^^^^^^

S memory is not allowed by overriding, but that could be equally correct:

Error: Overriding public state variable return types differ.
  --> struct-interface.sol:10:5:
   |
10 |     S public override s;
   |     ^^^^^^^^^^^^^^^^^^^
Note: Overridden public state variable is here:
 --> struct-interface.sol:6:5:
  |
6 |     function s() external returns (S memory);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[axic]

Oops, forgot that struct getters return the members, and not the struct itself. That is super unintuitive.

[leonardoalt]

Oops, forgot that struct getters return the members, and not the struct itself. That is super unintuitive.

Don't forget that if the struct has arrays those are ignored.

[axic]

Error: Overriding public state variable return types differ.
  --> struct-interface.sol:10:5:
   |
10 |     S public override s;
   |     ^^^^^^^^^^^^^^^^^^^

Could also consider displaying the return types in this error.

[ekpyron]

Oops, forgot that struct getters return the members, and not the struct itself. That is super unintuitive.

Can we change that at some point :-)?

[cameel]

Even weirder that this goes only one level deep. You can have a getter returning a struct if it's nested inside another struct.

For example this compiles just fine and both the function and the getter return S:

interface I {
    struct S {
        uint a;
    }
    struct WrappedS {
        S s;
    }

    function s() external returns (S memory);
}

contract C is I {
    WrappedS public override s;
}

Anyway, you should update issue the description to say what we want to do about it. Do we want to make getters return structs instead of tuples (breaking change I guess?). Or just make the message more explicit?

[abhijitgawai]

@axic How to write Interface for such view function. What should be data type

[github-actions]

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[github-actions]

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