improve asserts type annotations (#12341)

packages/babel-types/scripts/generators/typescript.js

@@ -8,7 +8,7 @@ const toFunctionName = require("../utils/toFunctionName");
 const ts3_7 = process.argv.includes("--ts3.7");
 
 // TypeScript 3.7: https://github.com/microsoft/TypeScript/pull/32695 will allow assert declarations
-const asserts = ts3_7 ? assertion => `asserts ${assertion}` : () => `boolean`;
+const asserts = ts3_7 ? assertion => `asserts ${assertion}` : () => `void`;
 
 let code = `// NOTE: This file is autogenerated. Do not modify.
 // See packages/babel-types/scripts/generators/typescript.js for script used.
@@ -319,9 +319,10 @@ lines.push(
   // the MemberExpression implication is incidental, but it follows from the implementation
   // eslint-disable-next-line max-len
   `export function matchesPattern(node: Node | null | undefined, match: string | ReadonlyArray<string>, allowPartial?: boolean): node is MemberExpression`,
-  // TypeScript 3.7: ": asserts n is T"
   // eslint-disable-next-line max-len
-  `export function validate<T extends Node, K extends keyof T>(n: Node | null | undefined, key: K, value: T[K]): void`,
+  `export function validate<T extends Node, K extends keyof T>(n: Node | null | undefined, key: K, value: T[K]): ${asserts(
+    "n is T"
+  )}`,
   `export function validate(n: Node, key: string, value: any): void;`
 );