Fix error return when the value is a struct (#1381)

When returning a struct implementing to_string an exception will be
thrown on Enum.to_list, because structs passes the is_map/1 gaurd and
Enum.to_list fails on structs. In these cases it needs to fall back to to_string.

In Elixir 1.17 we can use is_non_struct_map/1, but think it won't be
worth updating the minimum Elixir version for this.

Author: wkirschbaum

lib/absinthe/phase/document/execution/resolution.ex

@@ -409,7 +409,8 @@ defmodule Absinthe.Phase.Document.Execution.Resolution do
     end
   end
 
-  defp split_error_value(error_value) when is_list(error_value) or is_map(error_value) do
+  defp split_error_value(error_value)
+       when is_list(error_value) or (is_map(error_value) and not is_struct(error_value)) do
     Keyword.split(Enum.to_list(error_value), [:message, :path])
   end
 

test/absinthe/resolution_test.exs

@@ -30,6 +30,20 @@ defmodule Absinthe.ResolutionTest do
       field :invalid_resolver, :string do
         resolve("bogus")
       end
+
+      field :map_resolver, :string do
+        resolve fn _, _ ->
+          {:error, %{message: "map"}}
+        end
+      end
+
+      field :struct_resolver, :string do
+        resolve fn _, _ ->
+          # we are using the Date struct here, but it can be any struct
+          # with an implementation for String.Chars
+          {:error, ~D[2025-01-01]}
+        end
+      end
     end
   end
 
@@ -77,4 +91,28 @@ defmodule Absinthe.ResolutionTest do
                    {:ok, _} = Absinthe.run(doc, Schema)
                  end
   end
+
+  test "resolves error with map value" do
+    doc = """
+    { mapResolver }
+    """
+
+    assert {:ok,
+            %{
+              data: %{"mapResolver" => nil},
+              errors: [%{message: "map"}]
+            }} = Absinthe.run(doc, Schema)
+  end
+
+  test "resolves error with struct value implementing String.Chars" do
+    doc = """
+    { structResolver }
+    """
+
+    assert {:ok,
+            %{
+              data: %{"structResolver" => nil},
+              errors: [%{message: "2025-01-01"}]
+            }} = Absinthe.run(doc, Schema)
+  end
 end