Fix crash encoding non-map data for struct types

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Encoding a struct type (a map type with a `__struct__` field, e.g. from Elixir) with non-map data (a string, integer, list, or atom) crashed with a raw `badmap` error instead of returning `{error, [spectra:error()]}`. Plain map types, list types, record types, and decoding were unaffected — only the struct branch of `to_json/4` was missing its `is_map/1` guard.
+
 ## [0.14.0] - 2026-09-11
 
 ### Fixed

src/spectra_json.erl

@@ -147,7 +147,7 @@ to_json(TypeInfo, #sp_nonempty_list{} = Type, Data, Config) ->
     nonempty_list_to_json(TypeInfo, Type, Data, Config);
 to_json(TypeInfo, #sp_list{} = ListType, Data, Config) when is_list(Data) ->
     list_to_json(TypeInfo, ListType, Data, Config);
-to_json(TypeInfo, #sp_map{struct_name = StructName} = Map, Data, Config) ->
+to_json(TypeInfo, #sp_map{struct_name = StructName} = Map, Data, Config) when is_map(Data) ->
     case StructName of
         undefined ->
             map_to_json(TypeInfo, Map, Data, Config);
@@ -243,9 +243,7 @@ list_to_json(TypeInfo, #sp_list{type = Type} = ListType, Data, Config) when is_l
     Config :: spectra:sp_config()
 ) ->
     {ok, json:encode_value()} | {error, [spectra:error()]}.
-map_to_json(TypeInfo, #sp_map{fields = Fields}, Data, Config) when
-    is_map(Data)
-->
+map_to_json(TypeInfo, #sp_map{fields = Fields}, Data, Config) ->
     %% Check if this is an Elixir struct and remove __struct__ field for JSON serialization
     DataWithoutStruct =
         case maps:take('__struct__', Data) of
@@ -259,9 +257,7 @@ map_to_json(TypeInfo, #sp_map{fields = Fields}, Data, Config) when
             {ok, maps:from_list(MapFields)};
         {error, Errors} ->
             {error, Errors}
-    end;
-map_to_json(_TypeInfo, MapType, Data, _Config) ->
-    {error, [sp_error:type_mismatch(MapType, Data)]}.
+    end.
 
 -spec map_fields_to_json(
     TypeInfo :: spectra:type_info(),

test/elixir_struct_test.erl

@@ -2,6 +2,8 @@
 
 -include_lib("eunit/include/eunit.hrl").
 
+-include("../include/spectra.hrl").
+
 -define(SKIP_IF_NO_ELIXIR(Body),
     case code:is_loaded('Elixir.TestUserStruct') of
         false ->
@@ -209,6 +211,21 @@ run_from_json_field_absent_from_struct_errors_gracefully() ->
     Result = spectra:decode(json, TypeInfo, extra_field_struct_type(), JsonBinary),
     ?assertMatch({error, _}, Result).
 
+%% --- Fix #3: to_json/4 crashed with badmap for non-map data on struct types ---
+
+to_json_non_map_data_returns_error_test() ->
+    TypeInfo = spectra_type_info:new(?MODULE, false),
+    Type = struct_type(),
+    lists:foreach(
+        fun(BadData) ->
+            ?assertMatch(
+                {error, [#sp_error{type = type_mismatch}]},
+                spectra:encode(json, TypeInfo, Type, BadData)
+            )
+        end,
+        [<<"a string">>, 42, [1, 2, 3], an_atom]
+    ).
+
 %% --- Fix #2: apply_only/2 must recurse into #sp_type_with_variables{} ---
 
 schema_only_parameterized_type_test() ->