Nested structs that are part of a mapping no longer compile with 0.5.0 (Test repo link included)

[roschler]

With the 5.0 version of solc, if a host struct nests another struct, and the host struct is the value in a mapping, the compiler emits a fatal compilation error using the UnimplementedFeature error code. This code did work in version v0.4.21.

UnimplementedFeatureError: Encoding type "struct TestNestedStructInMapping.structNested memory" not yet implemented.
Compilation failed. See above.
Truffle v5.0.0-beta.2 (core: 5.0.0-beta.2)
Solidity v0.5.0 (solc-js)
Node v8.11.1

Here is a link to a public repo with a minimal project that demonstrates the error:

https://github.com/roschler/nested-structs-tests.git

[roschler]

It does appear to be a breaking change. For a workaround see the reply to this Stack Exchange post:

https://ethereum.stackexchange.com/questions/63149/nested-structs-that-are-part-of-a-mapping-broken-in-solidity-0-5-0/63152#63152

[chriseth]

The code in question is

contract TestNestedStructInMapping {

    // The struct that is nested.
    struct structNested {
        uint dummy;
    }

    // The struct that holds the nested struct.
    struct structMain {
        structNested gamePaymentsSummary;
    }

    // The map that maps a game ID to a specific game.
    mapping(uint256 => structMain) public s_mapOfNestedStructs;
}

and it indeed seems to work in 0.4.25. Adding another variable to the struct, though, still not makes 0.4.25 complain, but the generated code is wrong - it only returns 32 bytes instead of 64.

I would propose to fix the internal compiler error by another check in the type checker - if possible.

[chriseth]

So to summarize: The reason this worked in 0.4.25 was by mere coincidence, looking at the code, nested structs in public getter functions were never meant to be supported.