Pattern match warnings suggest internal constructors

Summary

The pattern-match exhaustiveness checker displays a list of non-matched constructors which are internal and shouldn't be displayed. I have checked the issues tagged with "pattern match warnings" in the issue tracker and haven't found a similar issue.

Steps to reproduce

Compile the following snippet with -Wall or -Wincomplete-patterns.

module Compiler where

import Data.Map

type Var = String

data Prim = Mult | Add

data Term = Var Var
          | BinaryPrim Term Prim Term

type Heap = Map Var Term

type State = (Heap, Term)

stepVar :: State -> Maybe State
stepVar (h, Var v) = if member v h 
                        then Just (h, h ! v)
                        else Nothing

This produces the following warning:

Compiler.hs:17:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘stepVar’:
        Patterns not matched:
            ((Data.Map.Internal.Bin _ _ _ _ _), (BinaryPrim _ _ _))
            (Data.Map.Internal.Tip, (BinaryPrim _ _ _))
   |
17 | stepVar (h, Var v) = if member v h 
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

Expected behavior

The internal implementation of Map in terms of Tip and Bin should not be made visible to the user. Importing the internal modules and pattern matching on Bin and Tip is almost always the wrong solution, and especially confusing for newcomers. The expected warning should look like this:

Compiler.hs:17:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘stepVar’:
        Patterns not matched:
            (_, (BinaryPrim _ _ _))
   |
17 | stepVar (h, Var v) = if member v h 
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

Environment

• GHC version used: 8.10.4