Add type system tests for SettingsDefinition (#747)

Author: dskloetc

test/config.types.test.ts

@@ -0,0 +1,157 @@
+import { SettingDefinition, Settings } from '../src/config'
+
+// This file has type declarations that test the types of the config system at
+// compile time.
+
+// Declarations with a (@)ts-expect-error comment are expected to produce a
+// type error, and compilation will fail if they don't.
+
+// Declarations are followed by `void foo` to prevent "declared but never used"
+// errors from the TypeScript compiler.
+
+// Utility types:
+
+// This uses a trick proposed on https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-421529650
+type Equal<A, B> =
+  (<T>() => T extends A ? 1 : 2) extends <T>() => T extends B ? 1 : 2 ? true : false
+
+// Without this, the type system sometimes doesn't realize that two types are
+// equal even though they are.
+type Expand<T> = { [K in keyof T]: T[K] }
+
+type ExpectEqual<A, E> =
+  Equal<Expand<A>, Expand<E>> extends true
+    ? true
+    : { error: 'Types are not equal'; actual: A; expected: E }
+
+// Test that the shape of the SettingsDefinition is enforced by the type
+// system.
+
+// @ts-expect-error test
+const _assertStringDefault: SettingDefinition = {
+  type: 'string',
+  description: 'string settings cannot have a number as default',
+  default: 2,
+}
+void _assertStringDefault
+
+// @ts-expect-error test
+const _assertNumberDefault: SettingDefinition = {
+  type: 'string',
+  description: 'number settings cannot have a boolean as default',
+  default: true,
+}
+void _assertNumberDefault
+
+// @ts-expect-error test
+const _assertBooleanDefault: SettingDefinition = {
+  type: 'boolean',
+  description: 'boolean settings cannot have a string as default',
+  default: 'true',
+}
+void _assertBooleanDefault
+
+// @ts-expect-error test
+const _assertEnumNumberDefault: SettingDefinition = {
+  type: 'enum',
+  description: 'enum settings cannot have a number as default',
+  options: ['option1', 'option2', 'option3'],
+  default: 1,
+}
+void _assertEnumNumberDefault
+
+// Ideally this would fail but it doesn't
+const _assertEnumOptionDefault: SettingDefinition = {
+  type: 'enum',
+  description: 'enum settings can have a default that is not a valid option',
+  options: ['option1', 'option2', 'option3'],
+  default: 'option4',
+}
+void _assertEnumOptionDefault
+
+// Test that the type of the settings is correctly inferred from the type of
+// the settings definition.
+
+const testSettingsDefinition = {
+  optionalStringSetting: {
+    type: 'string',
+    description: 'optional string setting',
+  },
+  requiredStringSetting: {
+    type: 'string',
+    description: 'required string setting',
+    required: true,
+  },
+  defaultStringSetting: {
+    type: 'string',
+    description: 'string setting with default value',
+    default: 'default value',
+  },
+  optionalNumberSetting: {
+    type: 'number',
+    description: 'optional number setting',
+  },
+  requiredNumberSetting: {
+    type: 'number',
+    description: 'required number setting',
+    required: true,
+  },
+  defaultNumberSetting: {
+    type: 'number',
+    description: 'number setting with default value',
+    default: 42,
+  },
+  optionalBooleanSetting: {
+    type: 'boolean',
+    description: 'optional boolean setting',
+  },
+  requiredBooleanSetting: {
+    type: 'boolean',
+    description: 'required boolean setting',
+    required: true,
+  },
+  defaultBooleanSetting: {
+    type: 'boolean',
+    description: 'boolean setting with default value',
+    default: true,
+  },
+  optionalEnumSetting: {
+    type: 'enum',
+    description: 'optional enum setting',
+    options: ['option1', 'option2', 'option3'],
+  },
+  requiredEnumSetting: {
+    type: 'enum',
+    description: 'required enum setting',
+    required: true,
+    options: ['option1', 'option2', 'option3'],
+  },
+  defaultEnumSetting: {
+    type: 'enum',
+    description: 'enum setting with default value',
+    options: ['option1', 'option2', 'option3'],
+    default: 'option1',
+  },
+} as const
+void testSettingsDefinition
+
+type ExpectedSettingsType = {
+  optionalStringSetting?: string
+  requiredStringSetting: string
+  defaultStringSetting: string
+  optionalNumberSetting?: number
+  requiredNumberSetting: number
+  defaultNumberSetting: number
+  optionalBooleanSetting?: boolean
+  requiredBooleanSetting: boolean
+  defaultBooleanSetting: boolean
+  optionalEnumSetting?: 'option1' | 'option2' | 'option3'
+  requiredEnumSetting: 'option1' | 'option2' | 'option3'
+  defaultEnumSetting: 'option1' | 'option2' | 'option3'
+}
+
+const _settingsType: ExpectEqual<
+  Settings<typeof testSettingsDefinition>,
+  ExpectedSettingsType
+> = true
+void _settingsType