special case empty object for jtd (#2158)

fixes #2123

lib/types/jtd-schema.ts

@@ -74,7 +74,7 @@ type EnumString<T> = [T] extends [never]
   : null
 
 /** true if type is a union of string literals */
-type IsEnum<T> = null extends EnumString<Exclude<T, null>> ? false : true
+type IsEnum<T> = null extends EnumString<T> ? false : true
 
 /** true only if all types are array types (not tuples) */
 // NOTE relies on the fact that tuples don't have an index at 0.5, but arrays
@@ -88,13 +88,18 @@ type IsElements<T> = false extends IsUnion<T>
   : false
 
 /** true if the the type is a values type */
-type IsValues<T> = false extends IsUnion<Exclude<T, null>>
-  ? TypeEquality<keyof Exclude<T, null>, string>
+type IsValues<T> = false extends IsUnion<T> ? TypeEquality<keyof T, string> : false
+
+/** true if type is a properties type and Union is false, or type is a discriminator type and Union is true */
+type IsRecord<T, Union extends boolean> = Union extends IsUnion<T>
+  ? null extends EnumString<keyof T>
+    ? false
+    : true
   : false
 
-/** true if type is a proeprties type and Union is false, or type is a discriminator type and Union is true */
-type IsRecord<T, Union extends boolean> = Union extends IsUnion<Exclude<T, null>>
-  ? null extends EnumString<keyof Exclude<T, null>>
+/** true if type represents an empty record */
+type IsEmptyRecord<T> = [T] extends [Record<string, never>]
+  ? [T] extends [never]
     ? false
     : true
   : false
@@ -131,7 +136,7 @@ export type JTDSchemaType<T, D extends Record<string, unknown> = Record<string,
       ? {type: "timestamp"}
       : // enums - only accepts union of string literals
       // TODO we can't actually check that everything in the union was specified
-      true extends IsEnum<T>
+      true extends IsEnum<Exclude<T, null>>
       ? {enum: EnumString<Exclude<T, null>>[]}
       : // arrays - only accepts arrays, could be array of unions to be resolved later
       true extends IsElements<Exclude<T, null>>
@@ -140,15 +145,20 @@ export type JTDSchemaType<T, D extends Record<string, unknown> = Record<string,
             elements: JTDSchemaType<E, D>
           }
         : never
+      : // empty properties
+      true extends IsEmptyRecord<Exclude<T, null>>
+      ?
+          | {properties: Record<string, never>; optionalProperties?: Record<string, never>}
+          | {optionalProperties: Record<string, never>}
       : // values
-      true extends IsValues<T>
+      true extends IsValues<Exclude<T, null>>
       ? T extends Record<string, infer V>
         ? {
             values: JTDSchemaType<V, D>
           }
         : never
       : // properties
-      true extends IsRecord<T, false>
+      true extends IsRecord<Exclude<T, null>, false>
       ? ([RequiredKeys<Exclude<T, null>>] extends [never]
           ? {
               properties?: Record<string, never>
@@ -168,15 +178,15 @@ export type JTDSchemaType<T, D extends Record<string, unknown> = Record<string,
             additionalProperties?: boolean
           }
       : // discriminator
-      true extends IsRecord<T, true>
+      true extends IsRecord<Exclude<T, null>, true>
       ? {
           [K in keyof Exclude<T, null>]-?: Exclude<T, null>[K] extends string
             ? {
                 discriminator: K
                 mapping: {
                   // TODO currently allows descriminator to be present in schema
                   [M in Exclude<T, null>[K]]: JTDSchemaType<
-                    Omit<T extends {[C in K]: M} ? T : never, K>,
+                    Omit<T extends Record<K, M> ? T : never, K>,
                     D
                   >
                 }

spec/types/jtd-schema.spec.ts

@@ -1,4 +1,4 @@
-/* eslint-disable @typescript-eslint/no-empty-interface,no-void */
+/* eslint-disable @typescript-eslint/no-empty-interface,no-void,@typescript-eslint/ban-types */
 import _Ajv from "../ajv_jtd"
 import type {JTDSchemaType, SomeJTDSchemaType, JTDDataType} from "../../dist/jtd"
 import chai from "../chai"
@@ -17,8 +17,14 @@ interface B {
   b?: string
 }
 
+interface C {
+  type: "c"
+}
+
 type MyData = A | B
 
+type Missing = A | C
+
 interface LinkedList {
   val: number
   next?: LinkedList
@@ -32,6 +38,14 @@ const mySchema: JTDSchemaType<MyData> = {
   },
 }
 
+const missingSchema: JTDSchemaType<Missing> = {
+  discriminator: "type",
+  mapping: {
+    a: {properties: {a: {type: "float64"}}},
+    c: {properties: {}},
+  },
+}
+
 describe("JTDSchemaType", () => {
   it("validation should prove the data type", () => {
     const ajv = new _Ajv()
@@ -69,6 +83,22 @@ describe("JTDSchemaType", () => {
     serialize(invalidData)
   })
 
+  it("validation should prove the data type for missingSchema", () => {
+    const ajv = new _Ajv()
+    const validate = ajv.compile(missingSchema)
+    const validData: unknown = {type: "c"}
+
+    if (validate(validData)) {
+      validData.type.should.equal("c")
+    }
+    should.not.exist(validate.errors)
+
+    if (ajv.validate(missingSchema, validData)) {
+      validData.type.should.equal("c")
+    }
+    should.not.exist(validate.errors)
+  })
+
   it("should typecheck number schemas", () => {
     const numf: JTDSchemaType<number> = {type: "float64"}
     const numi: JTDSchemaType<number> = {type: "int32"}
@@ -286,12 +316,42 @@ describe("JTDSchemaType", () => {
     const emptyButFull: JTDSchemaType<{a: string}> = {}
     const emptyMeta: JTDSchemaType<unknown> = {metadata: {}}
 
-    // constant null not representable
+    // constant null representable as nullable empty object
     const emptyNull: TypeEquality<JTDSchemaType<null>, never> = true
 
     void [empty, emptyUnknown, falseUnknown, emptyButFull, emptyMeta, emptyNull]
   })
 
+  it("should typecheck empty records", () => {
+    // empty record variants
+    const emptyPro: JTDSchemaType<{}> = {properties: {}}
+    const emptyOpt: JTDSchemaType<{}> = {optionalProperties: {}}
+    const emptyBoth: JTDSchemaType<{}> = {properties: {}, optionalProperties: {}}
+    const emptyRecord: JTDSchemaType<Record<string, never>> = {properties: {}}
+    const notNullable: JTDSchemaType<{}> = {properties: {}, nullable: false}
+
+    // can't be null
+    // @ts-expect-error
+    const nullable: JTDSchemaType<{}> = {properties: {}, nullable: true}
+
+    const emptyNullUnion: JTDSchemaType<null | {}> = {properties: {}, nullable: true}
+    const emptyNullRecord: JTDSchemaType<null | Record<string, never>> = {
+      properties: {},
+      nullable: true,
+    }
+
+    void [
+      emptyPro,
+      emptyOpt,
+      emptyBoth,
+      emptyRecord,
+      notNullable,
+      nullable,
+      emptyNullUnion,
+      emptyNullRecord,
+    ]
+  })
+
   it("should typecheck ref schemas", () => {
     const refs: JTDSchemaType<number[], {num: number}> = {
       definitions: {