Issue 1578 reproduction (strict server additionalProps marshal failure)

internal/test/issues/issue-1578/config.yaml

@@ -0,0 +1,6 @@
+package: issue1578
+generate:
+  models: true
+  echo-server: true
+  strict-server: true
+output: issue1578.gen.go

internal/test/issues/issue-1578/doc.go

@@ -0,0 +1,3 @@
+package issue1578
+
+//go:generate go run github.com/deepmap/oapi-codegen/v2/cmd/oapi-codegen --config=config.yaml issue1578.yaml

internal/test/issues/issue-1578/issue1578.yaml

@@ -0,0 +1,20 @@
+openapi: "3.0.1"
+components:
+  schemas:
+    ComplexObject:
+      type: object
+      properties:
+        field1:
+         type: string
+      additionalProperties: {}
+paths:
+  /test:
+    get:
+      operationId: test
+      responses:
+        200:
+          description: test
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ComplexObject"

internal/test/issues/issue-1578/issue1578_test.go

@@ -0,0 +1,36 @@
+package issue1578_test
+
+import (
+	"bytes"
+	"context"
+	issue1578 "github.com/deepmap/oapi-codegen/v2/internal/test/issues/issue-1578"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"io"
+	"net/http/httptest"
+	"testing"
+)
+
+var _ issue1578.StrictServerInterface = &testStrictServerInterface{}
+
+type testStrictServerInterface struct {
+}
+
+func (t testStrictServerInterface) Test(_ context.Context, _ issue1578.TestRequestObject) (issue1578.TestResponseObject, error) {
+	return issue1578.Test200JSONResponse(issue1578.ComplexObject{
+		AdditionalProperties: map[string]interface{}{
+			"message": "hello",
+		},
+	}), nil
+}
+
+func TestIssue1578(t *testing.T) {
+	responseObject, _ := testStrictServerInterface{}.Test(context.Background(), issue1578.TestRequestObject{})
+	response := &httptest.ResponseRecorder{
+		Body: new(bytes.Buffer),
+	}
+	err := responseObject.VisitTestResponse(response)
+	require.NoError(t, err)
+	data, _ := io.ReadAll(response.Body)
+	assert.JSONEq(t, `{"message":"hello"}`, string(data))
+}

internal/test/issues/issue-1578/ugly_workaround.go

@@ -0,0 +1,8 @@
+package issue1578
+
+// This is an ugly workaround for the issue
+//var _ json.Marshaler = Test200JSONResponse{}
+//
+//func (response Test200JSONResponse) MarshalJSON() ([]byte, error) {
+//	return json.Marshal(ComplexObject(response))
+//}