fix: custom ts3.4 downlevel for transform/type-transform

.changeset/shiny-starfishes-sparkle.md

@@ -0,0 +1,5 @@
+---
+"@smithy/types": patch
+---
+
+custom ts3.4 downlevel for types/transform/type-transform

packages/types/package.json

@@ -6,7 +6,7 @@
     "build:cjs": "tsc -p tsconfig.cjs.json",
     "build:es": "tsc -p tsconfig.es.json",
     "build:types": "tsc -p tsconfig.types.json",
-    "build:types:downlevel": "downlevel-dts dist-types dist-types/ts3.4",
+    "build:types:downlevel": "rimraf dist-types/ts3.4 && downlevel-dts dist-types dist-types/ts3.4 && node scripts/downlevel",
     "stage-release": "rimraf ./.release && yarn pack && mkdir ./.release && tar zxvf ./package.tgz --directory ./.release && rm ./package.tgz",
     "clean": "rimraf ./dist-* && rimraf *.tsbuildinfo",
     "lint": "eslint -c ../../.eslintrc.js \"src/**/*.ts\"",

packages/types/scripts/downlevel.js

@@ -0,0 +1,11 @@
+const fs = require("fs");
+const path = require("path");
+
+const pkgRoot = path.join(__dirname, "..");
+
+function replaceDownlevelFile(pathFromSrc = "") {
+  const code = fs.readFileSync(path.join(pkgRoot, "dist-types", "ts3.4", "downlevel-ts3.4", pathFromSrc), "utf-8");
+  fs.writeFileSync(path.join(pkgRoot, "dist-types", "ts3.4", pathFromSrc), code, "utf-8");
+}
+
+replaceDownlevelFile("transform/type-transform.d.ts");

packages/types/src/downlevel-ts3.4/transform/type-transform.ts

@@ -0,0 +1,34 @@
+/**
+ * @public
+ *
+ * Transforms any members of the object T having type FromType
+ * to ToType. This applies only to exact type matches.
+ *
+ * This is for the case where FromType is a union and only those fields
+ * matching the same union should be transformed.
+ */
+export type Transform<T, FromType, ToType> = RecursiveTransformExact<T, FromType, ToType>;
+
+/**
+ * @internal
+ *
+ * Returns ToType if T matches exactly with FromType.
+ */
+type TransformExact<T, FromType, ToType> = [T] extends [FromType] ? ([FromType] extends [T] ? ToType : T) : T;
+
+/**
+ * @internal
+ *
+ * Applies TransformExact to members of an object recursively.
+ */
+type RecursiveTransformExact<T, FromType, ToType> = T extends Function
+  ? T
+  : T extends object
+  ? {
+      [key in keyof T]: [T[key]] extends [FromType]
+        ? [FromType] extends [T[key]]
+          ? ToType
+          : RecursiveTransformExact<T[key], FromType, ToType>
+        : RecursiveTransformExact<T[key], FromType, ToType>;
+    }
+  : TransformExact<T, FromType, ToType>;

packages/types/src/transform/type-transform.spec.ts

@@ -1,18 +1,32 @@
+import type { Transform as DownlevelTransform } from "../downlevel-ts3.4/transform/type-transform";
 import type { Transform } from "./type-transform";
 
 type Exact<A, B> = [A] extends [B] ? ([B] extends [A] ? true : false) : false;
 
-// It should transform exact unions recursively.
 type A = {
   a: string;
   b: number | string;
   c: boolean | number | string;
   nested: A;
 };
 
-type T = Transform<A, number | string, "enum">;
+{
+  // It should transform exact unions recursively.
 
-const assert1: Exact<T["a"], string> = true as const;
-const assert2: Exact<T["b"], "enum"> = true as const;
+  type T = Transform<A, number | string, "enum">;
 
-const assert3: Exact<T["nested"]["nested"]["nested"]["b"], "enum"> = true as const;
+  const assert1: Exact<T["a"], string> = true as const;
+  const assert2: Exact<T["b"], "enum"> = true as const;
+
+  const assert3: Exact<T["nested"]["nested"]["nested"]["b"], "enum"> = true as const;
+}
+
+{
+  // the downlevel should function similarly
+  type T = DownlevelTransform<A, number | string, "enum">;
+
+  const assert1: Exact<T["a"], string> = true as const;
+  const assert2: Exact<T["b"], "enum"> = true as const;
+
+  const assert3: Exact<T["nested"]["nested"]["nested"]["b"], "enum"> = true as const;
+}