Prevent export of default data constructor for NonEmptyMap.

This change adds a test to check that the default data constructor for
`NonEmptyMap` is not exported.

The default data constructor for 'NonEmptyMap' is not exported, by
design, as the internal data structure has an invariant that must be
preserved across all operations.

Exporting the default constructor would make it possible for functions
in other modules to break the invariant, opening the door to subtle
regressions.

lib/core/cardano-wallet-core.cabal

@@ -257,6 +257,7 @@ test-suite unit
     , scrypt
     , servant
     , servant-server
+    , should-not-typecheck
     , stm
     , openapi3 >= 3.0.0.1
     , servant-openapi3
@@ -332,6 +333,7 @@ test-suite unit
       Cardano.WalletSpec
       Data.Function.UtilsSpec
       Data.Map.NonEmpty.StrictSpec
+      Data.Map.NonEmpty.StrictSpec.TypeErrorSpec
       Data.QuantitySpec
       Data.Time.TextSpec
       Data.Time.UtilsSpec

lib/core/src/Data/Map/NonEmpty/Strict/Internal.hs

@@ -19,6 +19,22 @@
 module Data.Map.NonEmpty.Strict.Internal
     (
     -- * Map type
+
+      -- Important:
+      --
+      -- The default data constructor for 'NonEmptyMap' is not exported, by
+      -- design, as the internal data structure has an invariant that must be
+      -- preserved across all operations.
+      --
+      -- Exporting the default constructor would make it possible for functions
+      -- outside this module to break the invariant, opening the door to subtle
+      -- regressions.
+      --
+      -- See the definition of 'NonEmptyMap' for more details of the invariant.
+      --
+      -- To construct a 'NonEmptyMap', use one of the provided constructors,
+      -- all of which are tested to check that they respect the invariant.
+      --
       NonEmptyMap
 
     -- * Construction

lib/core/test/unit/Data/Map/NonEmpty/StrictSpec/TypeErrorSpec.hs

@@ -0,0 +1,40 @@
+{-# OPTIONS_GHC -fdefer-type-errors #-}
+{-# OPTIONS_GHC -fno-warn-deferred-out-of-scope-variables #-}
+{-# OPTIONS_GHC -fno-warn-deferred-type-errors #-}
+{-# OPTIONS_GHC -fno-warn-dodgy-imports #-}
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
+
+module Data.Map.NonEmpty.StrictSpec.TypeErrorSpec
+    ( spec
+    ) where
+
+import Prelude
+
+import Data.Map.NonEmpty.Strict.Internal
+    ( NonEmptyMap (..) )
+import Test.Hspec
+    ( Expectation, Spec, describe, it )
+import Test.ShouldNotTypecheck
+    ( shouldNotTypecheck )
+
+spec :: Spec
+spec = describe "Strict non-empty map type error tests" $
+
+    it "Default data constructor is not exported" $
+        testDefaultDataConstructorNotExported
+
+-- Test that the default data constructor for 'NonEmptyMap' is not exported.
+--
+-- The default data constructor for 'NonEmptyMap' is not exported, by design,
+-- as the internal data structure has an invariant that must be preserved
+-- across all operations.
+--
+-- Exporting the default data constructor would make it possible for functions
+-- in external modules to break the invariant, opening the door to subtle
+-- regressions.
+--
+-- See the definition of 'NonEmptyMap' for more details of the invariant.
+--
+testDefaultDataConstructorNotExported :: Expectation
+testDefaultDataConstructorNotExported =
+    shouldNotTypecheck $ NonEmptyMap ((), ()) mempty