Fix const in async regression (nim-lang#21898)

* Add test case for a const being used inside an async proc

* Use `typeof` to get the type of the block instead of overloaded templates

This removes the problem with the symbol having different types

I am unsure why I didn't use this in the first place. IIRC I had problems with `typeof` when I first tried to use it in the original implementation

lib/pure/asyncmacro.nim

@@ -221,13 +221,11 @@ proc asyncSingleProc(prc: NimNode): NimNode =
     procBody = newStmtList()
     let resultIdent = ident"result"
     procBody.add quote do:
-      template nimAsyncDispatchSetResult(x: `subRetType`) {.used.} =
-        # If the proc has implicit return then this will get called
-        `resultIdent` = x
-      template nimAsyncDispatchSetResult(x: untyped) {.used.} =
-        # If the proc doesn't have implicit return then this will get called
-        x
-    procBody.add newCall(ident"nimAsyncDispatchSetResult", blockStmt)
+      # Check whether there is an implicit return
+      when typeof(`blockStmt`) is void:
+        `blockStmt`
+      else:
+        `resultIdent` = `blockStmt`
     procBody.add(createFutureVarCompletions(futureVarIdents, nil))
     procBody.insert(0): quote do:
       {.push warning[resultshadowed]: off.}

tests/async/t21893.nim

@@ -0,0 +1,13 @@
+discard """
+output: "@[97]\ntrue"
+"""
+
+import asyncdispatch
+
+proc test(): Future[bool] {.async.} =
+  const S4 = @[byte('a')]
+  echo S4
+  return true
+
+echo waitFor test()
+