Error called at Type/Solve.hs:206:15 -- thread blocked indefinitely in an MVar operation -- caused by two or more examples of recursion with wrong number of args

[SiriusStarr]

Quick Summary: While trying to compile invalid code (but the sort one might reasonably generate on accident, since I did while eliminating a parameter from a function), compiler crashes with the following error (boilerplate parts excluded):

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

    ? [rank = 1]

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

>   thread blocked indefinitely in an MVar operation

SSCCE

module Main exposing (func)


func : Int -> Int
func a =
    let
        f1 : number -> Int -> Int
        f1 b c =
            f1 a b c

        f2 : number -> Int -> Int
        f2 d e =
            f2 a d e
    in
    0

• Elm: 0.19.1
• Browser: N/A
• Operating System: Linux

Additional Details

I was unable to further reduce the above SSCCE (other than removing the type annotation on func). The code itself is invalid, as it involves a recursive call of a function with the wrong number of arguments. I've tested the following changes to the code, noting whether or not the error persists (i.e. whether the compiler crashes or reports a reasonable issue with the code):

• a cannot be moved to either of the other positions in f1 or f2, e.g. if the recursive call is f1 b a c or f2 d e a, the error does not occur.
• The number of parameters of f1 and f2 cannot be reduced, i.e. they must have at least 2 parameters (the erroneous call having at least 3). More can be added with the error still occurring.
• The type signatures must be present on f1 and f2.
• The type signature must be a type variable or Elm-"typeclass"-equivalent, i.e. the error occurs with number, appendable, or var, but not with Int, Bool, etc.
• a, b, c, etc. cannot be constants, e.g. the error does not occur with f1 0 b c or f2 a d 5.
• The error must occur in two functions, as in this example, i.e. the error does not occur if f1 or f2 is removed entirely. Other let bindings can be added, however, in any position, whether they are valid or erroneous.

[github-actions]

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