Name unions and fix ambiguous types (backport of #1027)

compiler/model/metamodel.ts

@@ -160,6 +160,14 @@ export abstract class BaseType {
   kind: string
   /** Variant name for externally tagged variants */
   variantName?: string
+  /**
+   * Additional identifiers for use by code generators. Usage depends on the actual type:
+   * - on unions (modeled as alias(union_of)), these are identifiers for the union members
+   * - for additional properties, this is the name of the dict that holds these properties
+   * - for additional property, this is the name of the key and value fields that hold the
+   *   additional property
+   */
+  codegenNames?: string[]
 }
 
 export type Variants = ExternalTag | InternalTag | Container

compiler/model/utils.ts

@@ -627,7 +627,7 @@ export function hoistTypeAnnotations (type: model.TypeDefinition, jsDocs: JSDoc[
   // We want to enforce a single jsDoc block.
   assert(jsDocs, jsDocs.length < 2, 'Use a single multiline jsDoc block instead of multiple single line blocks')
 
-  const validTags = ['class_serializer', 'doc_url', 'behavior', 'variants', 'variant', 'shortcut_property']
+  const validTags = ['class_serializer', 'doc_url', 'behavior', 'variants', 'variant', 'shortcut_property', 'codegen_names']
   const tags = parseJsDocTags(jsDocs)
   if (jsDocs.length === 1) {
     const description = jsDocs[0].getDescription()
@@ -648,6 +648,12 @@ export function hoistTypeAnnotations (type: model.TypeDefinition, jsDocs: JSDoc[
     } else if (tag === 'doc_url') {
       assert(jsDocs, isValidUrl(value), '@doc_url is not a valid url')
       type.docUrl = value
+    } else if (tag === 'codegen_names') {
+      type.codegenNames = value.split(',').map(v => v.trim())
+      assert(jsDocs,
+        type.kind === 'type_alias' && type.type.kind === 'union_of' && type.type.items.length === type.codegenNames.length,
+        '@codegen_names must have the number of items as the union definition'
+      )
     } else {
       assert(jsDocs, false, `Unhandled tag: '${tag}' with value: '${value}' on type ${type.name.name}`)
     }