Reject __struct__ field on struct/2

Author: José Valim

lib/elixir/lib/kernel.ex

@@ -1494,55 +1494,6 @@ defmodule Kernel do
     :erlang.setelement(index + 1, tuple, value)
   end
 
-  @doc """
-  Creates and updates structs.
-
-  The struct argument may be an atom (which defines `defstruct`)
-  or a struct itself. The second argument is any Enumerable that
-  emits two-item tuples (key-value) during enumeration.
-
-  If one of the keys in the Enumerable does not exist in the struct,
-  they are automatically discarded.
-
-  This function is useful for dynamically creating and updating
-  structs.
-
-  ## Example
-
-      defmodule User do
-        defstruct name: "jose"
-      end
-
-      struct(User)
-      #=> %User{name: "jose"}
-
-      opts = [name: "eric"]
-      user = struct(User, opts)
-      #=> %User{name: "eric"}
-
-      struct(user, unknown: "value")
-      #=> %User{name: "eric"}
-
-  """
-  def struct(struct, kv \\ [])
-
-  def struct(struct, []) when is_atom(struct) do
-    apply(struct, :__struct__, [])
-  end
-
-  def struct(struct, kv) when is_atom(struct) do
-    struct(apply(struct, :__struct__, []), kv)
-  end
-
-  def struct(%{ __struct__: _ } = struct, kv) do
-    Enum.reduce(kv, struct, fn { k, v }, acc ->
-      case :maps.is_key(k, acc) do
-        true  -> :maps.put(k, v, acc)
-        false -> acc
-      end
-    end)
-  end
-
   ## Implemented in Elixir
 
   @doc """
@@ -1928,6 +1879,55 @@ defmodule Kernel do
     Inspect.Algebra.pretty(Inspect.Algebra.to_doc(arg, opts), limit)
   end
 
+  @doc """
+  Creates and updates structs.
+
+  The struct argument may be an atom (which defines `defstruct`)
+  or a struct itself. The second argument is any Enumerable that
+  emits two-item tuples (key-value) during enumeration.
+
+  If one of the keys in the Enumerable does not exist in the struct,
+  they are automatically discarded.
+
+  This function is useful for dynamically creating and updating
+  structs.
+
+  ## Example
+
+      defmodule User do
+        defstruct name: "jose"
+      end
+
+      struct(User)
+      #=> %User{name: "jose"}
+
+      opts = [name: "eric"]
+      user = struct(User, opts)
+      #=> %User{name: "eric"}
+
+      struct(user, unknown: "value")
+      #=> %User{name: "eric"}
+
+  """
+  def struct(struct, kv \\ [])
+
+  def struct(struct, []) when is_atom(struct) do
+    apply(struct, :__struct__, [])
+  end
+
+  def struct(struct, kv) when is_atom(struct) do
+    struct(apply(struct, :__struct__, []), kv)
+  end
+
+  def struct(%{ __struct__: _ } = struct, kv) do
+    Enum.reduce(kv, struct, fn { k, v }, acc ->
+      case :maps.is_key(k, acc) and k != :__struct__ do
+        true  -> :maps.put(k, v, acc)
+        false -> acc
+      end
+    end)
+  end
+
   @doc """
   Converts the argument to a string according to the String.Chars protocol.
   This is the function invoked when there is string interpolation.

lib/elixir/test/elixir/kernel_test.exs

@@ -169,7 +169,7 @@ defmodule KernelTest do
 
     assert struct(user, unknown: "key") == user
     assert struct(user, name: "jose") == %User{ name: "jose" }
-    assert struct(user, name: "other") == %User{ name: "other" }
+    assert struct(user, name: "other", __struct__: Post) == %User{ name: "other" }
   end
 
   defmodule Conversions do