Confusing error message when a getter would return zero values and the compiler refuses to generate it

[benediamond]

Original title: "unusual behavior with mapping within struct within library".

Description

When a library contains a struct type whose only member is a mapping, and a contract contains a public member of that struct type, then compilation fails with error Internal or recursive type is not allowed for public state variables.. But if you add a further field in the struct (of type uint256 for example), then compilation does not fail. This looks like a bug.

Environment

• Compiler version: 0.8.10
• Target EVM version (as per compiler settings): compiler default (London)
• Framework/IDE (e.g. Truffle or Remix): Remix
• EVM execution environment / backend / blockchain client:
• Operating system: MacOS

Steps to Reproduce

Lib.sol

pragma solidity 0.8.10;
library Lib {
    struct MyStruct {
        // uint256 myInt;
        mapping(uint256 => uint256) myMap;
    }
}

A.sol

pragma solidity 0.8.10;
import './Lib.sol';
contract A {
    Lib.MyStruct public myStruct;
}

TypeError: Internal or recursive type is not allowed for public state variables.
 --> contracts/A.sol:6:5:
  |
6 |     Lib.MyStruct public myStruct;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

BUT IF YOU UNCOMMENT the commented line, then it compiles.

[cameel]

This is not really a bug but a limitation of automatically generated getters:

The mapping and arrays (with the exception of byte arrays) in the struct are omitted because there is no good way to select individual struct members or provide a key for the mapping:

When all members of the struct are omitted, your getter would not return anything and the compiler issues an error. This is by design. I agree that the error message for this is pretty bad and should be changed though.

Related issue: #11369.