Empty union base definitions rejected before extensions are merged (spec violation)

Describe the bug

The spec grammar:

(UnionTypeDefinition : Description? union Name Directives[Const]? UnionMemberTypes?)

makes UnionMemberTypes optional. SchemaParser correctly parses this SDL and TypeDefinitionRegistry correctly shows the union extensions. However, SchemaGenerator.makeExecutableSchema() fails validation with:

SchemaProblem{errors=[Union type 'Pet' must include one or more member types.]}

Root 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.

(GraphQL Java 25.0)

To Reproduce

import graphql.schema.idl.SchemaParser
import graphql.schema.idl.UnExecutableSchemaGenerator
import org.junit.jupiter.api.Test

/**
 * Demonstrates a graphql-java bug: empty union base definitions with members
 * added via extensions are rejected, even though the GraphQL spec allows this.
 *
 * The spec grammar (UnionTypeDefinition : Description? union Name Directives[Const]? UnionMemberTypes?)
 * makes UnionMemberTypes optional. SchemaParser correctly parses this SDL and
 * TypeDefinitionRegistry correctly shows the union extensions. However,
 * SchemaGenerator.makeExecutableSchema() fails validation with:
 *
 *   SchemaProblem{errors=[Union type 'Pet' must include one or more member types.]}
 *
 * The validation checks the base definition before merging extension members.
 */
class UnionTest {
    @Test
    fun `test empty union base`() {
        val sdl = """
            type Cat { meow: String }
            type Dog { bark: String }
            union Pet
            extend union Pet = Cat | Dog
            type Query { pet: Pet }
        """.trimIndent()

        val registry = SchemaParser().parse(sdl)

        println("=== Registry parsed successfully ===")
        println("Types: ${registry.types().keys}")
        println("Union extensions: ${registry.unionTypeExtensions()}")

        try {
            val schema = UnExecutableSchemaGenerator.makeUnExecutableSchema(registry)
            println("=== Schema built successfully ===")
        } catch (e: Exception) {
            println("=== Exception ===")
            println("Class: ${e.javaClass.name}")
            println("Message: ${e.message}")
            e.printStackTrace()
            throw e
        }
    }
}

Java rewrite:

import graphql.schema.idl.SchemaParser;
import graphql.schema.idl.TypeDefinitionRegistry;
import graphql.schema.idl.UnExecutableSchemaGenerator;
import org.junit.jupiter.api.Test;

class UnionTest {

  /**
   * Demonstrates a graphql-java bug: empty union base definitions with members
   * added via extensions are rejected, even though the GraphQL spec allows this.
   *
   * Expected failure (current graphql-java behavior):
   *
   *   SchemaProblem{errors=[Union type 'Pet' must include one or more member types.]}
   */
  @Test
  void testEmptyUnionBase() {
    String sdl = String.join("\n",
        "type Cat { meow: String }",
        "type Dog { bark: String }",
        "union Pet",
        "extend union Pet = Cat | Dog",
        "type Query { pet: Pet }"
    );

    TypeDefinitionRegistry registry = new SchemaParser().parse(sdl);

    System.out.println("=== Registry parsed successfully ===");
    System.out.println("Types: " + registry.types().keySet());
    System.out.println("Union extensions: " + registry.unionTypeExtensions());

    try {
      UnExecutableSchemaGenerator.makeUnExecutableSchema(registry);
      System.out.println("=== Schema built successfully ===");
    } catch (Exception e) {
      System.out.println("=== Exception ===");
      System.out.println("Class: " + e.getClass().getName());
      System.out.println("Message: " + e.getMessage());
      e.printStackTrace();
      throw e;
    }
  }
}

Hello, this issue has been inactive for 60 days, so we're marking it as stale. If you would like to continue this discussion, please comment within the next 30 days or we'll close the issue.

Fix #4200: Allow empty union base definitions when extensions add mem…

Fix #4200: Allow empty union definitions when extensions provide members

Hello, as this issue has been inactive for 90 days, we're closing the issue. If you would like to resume the discussion, please create a new issue.

Fix #4364 and #4200: nullable type bound and empty union extension su…

This is fixed in #4388

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Empty union base definitions rejected before extensions are merged (spec violation) #4200

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Empty union base definitions rejected before extensions are merged (spec violation) #4200

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rstata commented on Jan 4, 2026

github-actions commented on Mar 5, 2026

github-actions commented on Apr 4, 2026

andimarek commented on May 20, 2026

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