Compiler bug: "? [rank = 1]" in Type/Solve.hs

[ianmackenzie]

I ran into a compiler bug when attempting to 'genericize' some code; basically I have a function that accepts a List Point2d and I wanted to convert it into a function that accepts a List vertex, by supplying a vertex -> Point2d function. I worked the code down to as small as an SSCCE as I could, switching to primitive types in the process.

SSCCE

If I attempt to compile

module CompilerTest exposing (foo)


foo : (a -> String) -> List a -> Maybe String
foo toString items =
    case items of
        firstItem :: remainingItems ->
            fooHelp toString (toString firstItem) remainingItems

        _ ->
            Nothing


fooHelp : (a -> String) -> String -> List a -> Maybe String
fooHelp toString firstString currentItems =
    case currentItems of
        firstItem :: remainingItems ->
            fooHelp toString firstItem remainingItems

        [] ->
            Nothing

I get

Compiling ...elm: You ran into a compiler bug. Here are some details for the developers:

    ? [rank = 1]

Please create an <http://sscce.org/> and then report it
at <https://github.com/elm/compiler/issues>


CallStack (from HasCallStack):
  error, called at compiler/src/Type/Solve.hs:206:15 in main:Type.Solve

-- ERROR -----------------------------------------------------------------------

I ran into something that bypassed the normal error reporting process! I
extracted whatever information I could from the internal error:

>   thread blocked indefinitely in an MVar operation

The bug in the code is that the recursive fooHelp call is missing a toString call; it should be

fooHelp toString (toString firstItem) remainingItems

like it is in foo, and if it is changed to that the code compiles fine.

• Elm: 0.19.1
• Browser: N/A
• Operating System: Tested on Windows and Linux (WSL 2)

[github-actions]

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