Change error to deprecation

Author: nordlow

src/dmd/escape.d

@@ -1284,8 +1284,8 @@ private bool checkReturnEscapeImpl(Scope* sc, Expression e, bool refs, bool gag)
              * Hence, the annotation needs to be explicit.
              */
             if (!gag)
-                error(e.loc, "`pure` function `%s` returns parameter `%s`, annotate with `return`",
-                    sc.func.ident.toChars(), v.toChars());
+                deprecation(e.loc, "`pure` function `%s` returns parameter `%s`, annotate with `return`",
+                            sc.func.ident.toChars(), v.toChars());
             result = true;
         }
         else