Export top-level defs as function functions, not arrow functions.

This is necessary for exported Scala classes to be instantiable
with `new ExportedClass()`.

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/ClassEmitter.scala

@@ -1109,14 +1109,11 @@ class ClassEmitter(coreSpec: CoreSpec) {
       resultType = AnyType
     )
 
-    if (method.restParam.isEmpty) {
-      ctx.addExport(wamod.Export.Function(exportedName, functionName))
-    } else {
-      /* We cannot directly export the function. We will create a closure
-       * wrapper in the start function and export that instead.
-       */
-      genDelayedTopLevelExport(exportedName)
-    }
+    /* We cannot directly export the function because it would not be considered
+     * a `function`. Instead, we will explicitly create a closure wrapper in the
+     * start function and export that instead.
+     */
+    genDelayedTopLevelExport(exportedName)
   }
 
   private def transformTopLevelFieldExportDef(

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/CoreWasmLib.scala

@@ -377,8 +377,10 @@ object CoreWasmLib {
       List(RefType.func, anyref, Int32),
       List(RefType.any)
     )
+
+    addHelperImport(genFunctionName.makeExportedDef, List(RefType.func), List(RefType.any))
     addHelperImport(
-      genFunctionName.closureRestNoData,
+      genFunctionName.makeExportedDefRest,
       List(RefType.func, Int32),
       List(RefType.any)
     )

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/Emitter.scala

@@ -190,13 +190,14 @@ final class Emitter(config: Emitter.Config) {
           instrs += wa.CALL(genFunctionName.loadModule(tle.owningClass))
           instrs += wa.GLOBAL_SET(genGlobalName.forTopLevelExport(tle.exportName))
         case TopLevelMethodExportDef(_, methodDef) =>
-          // We only need initialization if there is a restParam
+          instrs += ctx.refFuncWithDeclaration(genFunctionName.forExport(tle.exportName))
           if (methodDef.restParam.isDefined) {
-            instrs += ctx.refFuncWithDeclaration(genFunctionName.forExport(tle.exportName))
             instrs += wa.I32_CONST(methodDef.args.size)
-            instrs += wa.CALL(genFunctionName.closureRestNoData)
-            instrs += wa.GLOBAL_SET(genGlobalName.forTopLevelExport(tle.exportName))
+            instrs += wa.CALL(genFunctionName.makeExportedDefRest)
+          } else {
+            instrs += wa.CALL(genFunctionName.makeExportedDef)
           }
+          instrs += wa.GLOBAL_SET(genGlobalName.forTopLevelExport(tle.exportName))
         case TopLevelFieldExportDef(_, _, _) =>
           // Nothing to do
           ()

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/LoaderContent.scala

@@ -119,7 +119,10 @@ const scalaJSHelpers = {
   closureThis: (f, data) => function(...args) { return f(data, this, ...args); },
   closureRest: (f, data, n) => ((...args) => f(data, ...args.slice(0, n), args.slice(n))),
   closureThisRest: (f, data, n) => function(...args) { return f(data, this, ...args.slice(0, n), args.slice(n)); },
-  closureRestNoData: (f, n) => ((...args) => f(...args.slice(0, n), args.slice(n))),
+
+  // Top-level exported defs -- they must be `function`s but have no actual `this` nor `data`
+  makeExportedDef: (f) => function(...args) { return f(...args); },
+  makeExportedDefRest: (f, n) => function(...args) { return f(...args.slice(0, n), args.slice(n)); },
 
   // Strings
   emptyString: "",

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/VarGen.scala

@@ -153,7 +153,9 @@ object VarGen {
     val closureThis = make("closureThis")
     val closureRest = make("closureRest")
     val closureThisRest = make("closureThisRest")
-    val closureRestNoData = make("closureRestNoData")
+
+    val makeExportedDef = make("makeExportedDef")
+    val makeExportedDefRest = make("makeExportedDefRest")
 
     val stringLength = make("stringLength")
     val stringCharAt = make("stringCharAt")