@xedin, @jckarter, think this would make a good Starter Bug?

I think so; this should be straightforward to check for and emit a custom "tuples not implemented yet" message for.

Can this be assigned to me? I would like to fix this issue 🙂

Never mind, JIRA was acting weird, I have now assigned it to myself!

I believe I need to emit a different diagnostic in

void PreCheckExpression::resolveKeyPathExpr(KeyPathExpr *KPE) {}

So:

if (auto TE = dyn_cast<TupleExpr>(expr)) { // emit custom diagnostic }

Just wondering if I am in the right path 🙂

It looks like the current message is coming from diagnoseKeyPathComponents in CSDiag.cpp, which makes sense because PreCheckExpression happens before we figure out the types of things.

@belkadan I see!

if (currentType) TC.diagnose(componentNameLoc, diag::could_not_find_type_member, currentType, componentName);

So, currentType is declared as:

Type currentType = rootType;

So, perhaps I can do:

if (isa<TupleType>(currentType)) { // emit custom diagnostics }

@jckarter's the real expert, but that approach seems reasonable to me!

@belkadan okay 🙂

My Mac is still building the compiler, so I haven't tested anything yet, so just waiting for it to finish building.

In the meantime, I think it's slightly better to do it before:

for (auto &component : KPE->getComponents()) {}

So before that for-loop, we can do:

if (state == ResolvingType && isa<TupleType>(currentType)) {
   // emit diagnostics
   return true
}

It could also probably be done even earlier, so just before this is declared:

SmallString<32> keyPathScratch;

because there's no point of continuing forward and we could return early from diagnoseKeyPathComponents if we know the rootType is TupleType.

Aha, but what if the second component is a tuple?

@belkadan Hmm, you mean if we had a nested tuple?

let things: (first: Int, second: (third: Int, fourth: Int))

I am under the assumption that rootType will be a tuple as things is a tuple. If that is true, then there is no need to inspect the components as the whole thing is a tuple?

I was thinking more about something like this:

struct Pointish {
  var value: (x: Int, y: Int)
}
let keyPath = \Pointish.value.x

Heh, in fact Alex's original example is one of those.

@belkadan: Here's the pull request: #20495

PR approved and merged.