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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Empty union base definitions rejected before extensions are merged (spec violation)","articleBody":"**Describe the bug**\n\nThe spec grammar:\n```\n(UnionTypeDefinition : Description? union Name Directives[Const]? UnionMemberTypes?)\n```\nmakes UnionMemberTypes optional. SchemaParser correctly parses this SDL and TypeDefinitionRegistry correctly shows the union extensions. However, SchemaGenerator.makeExecutableSchema() fails validation with:\n```\nSchemaProblem{errors=[Union type 'Pet' must include one or more member types.]}\n```\nRoot cause is that the validation checks the base definition before merging extension members (which isn't done for other types like interfaces).  For deeper dive on root cause see [this link](https://chatgpt.com/s/dr_6959b10af4308191bd5c920346071456).\n\n(GraphQL Java 25.0)\n\n**To Reproduce**\n```\nimport graphql.schema.idl.SchemaParser\nimport graphql.schema.idl.UnExecutableSchemaGenerator\nimport org.junit.jupiter.api.Test\n\n/**\n * Demonstrates a graphql-java bug: empty union base definitions with members\n * added via extensions are rejected, even though the GraphQL spec allows this.\n *\n * The spec grammar (UnionTypeDefinition : Description? union Name Directives[Const]? UnionMemberTypes?)\n * makes UnionMemberTypes optional. SchemaParser correctly parses this SDL and\n * TypeDefinitionRegistry correctly shows the union extensions. However,\n * SchemaGenerator.makeExecutableSchema() fails validation with:\n *\n *   SchemaProblem{errors=[Union type 'Pet' must include one or more member types.]}\n *\n * The validation checks the base definition before merging extension members.\n */\nclass UnionTest {\n    @Test\n    fun `test empty union base`() {\n        val sdl = \"\"\"\n            type Cat { meow: String }\n            type Dog { bark: String }\n            union Pet\n            extend union Pet = Cat | Dog\n            type Query { pet: Pet }\n        \"\"\".trimIndent()\n\n        val registry = SchemaParser().parse(sdl)\n\n        println(\"=== Registry parsed successfully ===\")\n        println(\"Types: ${registry.types().keys}\")\n        println(\"Union extensions: ${registry.unionTypeExtensions()}\")\n\n        try {\n            val schema = UnExecutableSchemaGenerator.makeUnExecutableSchema(registry)\n            println(\"=== Schema built successfully ===\")\n        } catch (e: Exception) {\n            println(\"=== Exception ===\")\n            println(\"Class: ${e.javaClass.name}\")\n            println(\"Message: ${e.message}\")\n            e.printStackTrace()\n            throw e\n        }\n    }\n}\n```\n","author":{"url":"https://github.com/rstata","@type":"Person","name":"rstata"},"datePublished":"2026-01-04T00:13:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/4200/graphql-java/issues/4200"}