Remove invalid unwrap annotation

src/Elm.elm

@@ -558,10 +558,6 @@ unwrapper modName typename =
                 argVal : { name : String, typename : String, val : Compiler.Expression, index : Index.Index }
                 argVal =
                     Compiler.toVar index "val"
-
-                return : { name : String, typename : String, val : Compiler.Expression, index : Index.Index }
-                return =
-                    Compiler.toVar argVal.index "unwrapped"
             in
             { expression =
                 Exp.LambdaExpression
@@ -583,14 +579,18 @@ unwrapper modName typename =
                             )
                     }
             , annotation =
-                Ok
-                    { type_ =
-                        Annotation.FunctionTypeAnnotation
-                            (Compiler.nodify (Annotation.GenericType argVal.typename))
-                            (Compiler.nodify (Annotation.GenericType return.typename))
-                    , inferences = Dict.empty
-                    , aliases = Compiler.emptyAliases
-                    }
+                -- We can't generate a valid type annotation for an
+                -- unwrapper lambda: the arg type is the named custom
+                -- type (e.g. Wrapper), but the return type is its
+                -- unknown inner type. Generating `Wrapper -> a` would
+                -- be wrong because the body extracts a concrete value,
+                -- not a polymorphic one, so Elm would reject it with
+                -- a TYPE MISMATCH.
+                --
+                -- Returning `Err []` causes the declaration to be
+                -- generated without a type annotation, letting Elm
+                -- infer it from the custom type definition.
+                Err []
             , imports =
                 case modName of
                     [] ->

src/Internal/Compiler.elm

@@ -1573,9 +1573,36 @@ applyType :
     -> Result (List InferenceError) Inference
 applyType index annotation args =
     case annotation of
-        Err err ->
+        Err ((_ :: _) as err) ->
             Err err
 
+        Err [] ->
+            -- The function's type is intentionally unknown (e.g.
+            -- Elm.unwrapper can't derive an annotation for its lambda).
+            -- Synthesize a fresh generic return type so downstream
+            -- inference can still flow through the application.
+            --
+            -- Only synthesize when args is non-empty; `apply fn []`
+            -- with an unknown fn is still "unknown" (the result is
+            -- the fn itself, whose type we still don't know).
+            case ( Index.typecheck index, args ) of
+                ( True, _ :: _ ) ->
+                    case mergeArgInferences args [] Dict.empty of
+                        Ok mergedArgs ->
+                            Ok
+                                { type_ =
+                                    Annotation.GenericType
+                                        (Index.protectTypeName "result" index)
+                                , inferences = mergedArgs.inferences
+                                , aliases = emptyAliases
+                                }
+
+                        Err _ ->
+                            Err []
+
+                _ ->
+                    Err []
+
         Ok fnAnnotation ->
             if Index.typecheck index then
                 case mergeArgInferences args [] fnAnnotation.inferences of

tests/TypeChecking.elm

@@ -272,6 +272,66 @@ generatedCode =
                                 ( 1 + 2, x )
                             """
             ]
+        , test "Elm.unwrapper omits annotation to let Elm infer it" <|
+            -- unwrapper creates `\(Wrapper val) -> val` but can't
+            -- derive a valid type annotation because it doesn't know
+            -- the inner type of Wrapper. Any annotation we could
+            -- generate (like `Wrapper -> a`) would be rejected by Elm
+            -- because the extracted value has a concrete type, not a
+            -- polymorphic one. So we omit the annotation and let Elm
+            -- infer it from the custom type definition.
+            \_ ->
+                Elm.declaration "extract"
+                    (Elm.unwrapper [] "Wrapper")
+                    |> Elm.Expect.declarationAs
+                        """
+                        extract (Wrapper val) =
+                            val
+                        """
+        , test "Elm.unwrapper respects withType when caller provides an annotation" <|
+            \_ ->
+                Elm.declaration "extract"
+                    (Elm.unwrapper [] "Wrapper"
+                        |> Elm.withType (Type.function [ Type.named [] "Wrapper" ] Type.string)
+                    )
+                    |> Elm.Expect.declarationAs
+                        """
+                        extract : Wrapper -> String
+                        extract (Wrapper val) =
+                            val
+                        """
+        , test "Elm.unwrap result propagates through downstream inference" <|
+            -- Even though unwrapper itself can't produce a type
+            -- annotation, `apply` synthesizes a fresh generic return
+            -- type when the function's type is unknown, so outer
+            -- expressions can still unify and infer correctly.
+            \_ ->
+                Elm.declaration "foo"
+                    (Elm.Op.plus (Elm.int 1)
+                        (Elm.unwrap [] "Wrapper" (Elm.val "wrapped"))
+                    )
+                    |> Elm.Expect.declarationAs
+                        """
+                        foo : Int
+                        foo =
+                            1 + (\\(Wrapper val) -> val) wrapped
+                        """
+        , test "Elm.apply with zero args on an unknown function stays unknown" <|
+            -- `apply fn []` with an unknown fn type should not fabricate
+            -- a return type out of thin air. Without this guard, the
+            -- outer Op.plus would unify the fabricated generic with
+            -- `number` and emit `foo : Int`, even though the body is
+            -- `1 + (\(Wrapper val) -> val)` (adding an int to a lambda).
+            \_ ->
+                Elm.declaration "foo"
+                    (Elm.Op.plus (Elm.int 1)
+                        (Elm.apply (Elm.unwrapper [] "Wrapper") [])
+                    )
+                    |> Elm.Expect.declarationAs
+                        """
+                        foo =
+                            1 + (\\(Wrapper val) -> val)
+                        """
         , describe "aliasAs pattern type"
             [ test "aliasAs on record pattern uses the underlying record type" <|
                 \_ ->