Make source maps plain objects for use with t.valueToNode

packages/babel-core/src/transformation/file/merge-map.ts

@@ -30,7 +30,13 @@ export default function mergeSourceMap(
   if (typeof inputMap.sourceRoot === "string") {
     result.sourceRoot = inputMap.sourceRoot;
   }
-  return result;
+
+  // remapping returns a SourceMap class type, but this breaks code downstream in
+  // @babel/traverse and @babel/types that relies on data being plain objects.
+  // When it encounters the sourcemap type it outputs a "don't know how to turn
+  // this value into a node" error. As a result, we are converting the merged
+  // sourcemap to a plain js object.
+  return { ...result };
 }
 
 function rootless(map: SourceMap): SourceMap {

packages/babel-core/test/merge-map.js

@@ -0,0 +1,27 @@
+import _mergeSourceMap from "../lib/transformation/file/merge-map.js";
+const mergeSourceMap = _mergeSourceMap.default;
+
+describe("merge-map", () => {
+  it("returns a plain js object", () => {
+    const inputMap = {
+      file: "file.js",
+      mappings: [],
+      names: [],
+      sources: ["file.ts"],
+      version: 3,
+    };
+
+    const outputMap = {
+      file: "file.transpiled.js",
+      mappings: [],
+      names: [],
+      sources: ["file.js"],
+      version: 3,
+    };
+
+    const map = mergeSourceMap(inputMap, outputMap, "file.transpiled.js");
+    expect(typeof map).toBe("object");
+    expect(Object.prototype.toString.call(map)).toBe("[object Object]");
+    expect(Object.getPrototypeOf(map)).toBe(Object.prototype);
+  });
+});