Fix aliasAs generating wrong type for the alias variable

src/Internal/Arg.elm

@@ -182,8 +182,20 @@ aliasAs aliasName (Arg toArgDetails) =
                 innerArgDetails =
                     toArgDetails index
 
-                ( aliasVal, _, _ ) =
-                    val innerArgDetails.index aliasName
+                -- The alias refers to the same value as the underlying
+                -- pattern, so it should have the same type. Use the
+                -- inner pattern's annotation (the record type, tuple
+                -- type, etc.) rather than creating a fresh type variable.
+                aliasVal : Compiler.Expression
+                aliasVal =
+                    Compiler.Expression <|
+                        \_ ->
+                            { expression =
+                                Exp.FunctionOrValue []
+                                    (Format.sanitize aliasName)
+                            , annotation = innerArgDetails.details.annotation
+                            , imports = []
+                            }
             in
             { details =
                 { imports = innerArgDetails.details.imports

tests/TypeChecking.elm

@@ -272,6 +272,24 @@ generatedCode =
                                 ( 1 + 2, x )
                             """
             ]
+        , describe "aliasAs pattern type"
+            [ test "aliasAs on record pattern uses the underlying record type" <|
+                \_ ->
+                    Elm.Declare.fn "describe"
+                        (Arg.record (\a b -> ( a, b ))
+                            |> Arg.field "name"
+                            |> Arg.field "age"
+                            |> Arg.aliasAs "person"
+                        )
+                        (\( ( name, _ ), person ) ->
+                            Elm.tuple name person
+                        )
+                        |> .declaration
+                        |> Elm.Expect.declarationAs
+                            """
+                            describe : { name : name, age : age } -> ( name, { name : name, age : age } )
+                            describe ({ name, age } as person) =
+                                ( name, person )
         , test "Elm.Let.fn declaration has a type annotation" <|
             \_ ->
                 Elm.declaration "useLetFn"