Support main callback with ref parameter

Author: andrey-zherikov

source/argparse/api/cli.d

@@ -122,7 +122,7 @@ template CLI(Config config, COMMAND)
     }
 
     static int parseArgs(alias newMain)(string[] args, COMMAND initialValue = COMMAND.init)
-    if(__traits(compiles, { newMain(COMMAND.init); }))
+    if(__traits(compiles, { newMain(initialValue); }))
     {
         alias value = initialValue;
 
@@ -140,7 +140,7 @@ template CLI(Config config, COMMAND)
     }
 
     static int parseArgs(alias newMain)(string[] args, COMMAND initialValue = COMMAND.init)
-    if(__traits(compiles, { newMain(COMMAND.init, string[].init); }))
+    if(__traits(compiles, { newMain(initialValue, string[].init); }))
     {
         alias value = initialValue;
 
@@ -205,11 +205,20 @@ alias CLI(COMMANDS...) = CLI!(Config.init, COMMANDS);
 
 unittest
 {
+    // Ensure that CLI.main is compilable
     struct Args {}
 
-    mixin CLI!({
-        Config cfg;
-        cfg.errorHandler = (string s) {};
-        return cfg;
-    }(), Args).main!((_){});
+    enum Config cfg = { errorHandler: (string s) {} };
+
+    mixin CLI!(cfg, Args).main!((_){});
+}
+
+unittest
+{
+    // Ensure that CLI.main is compilable with reference parameter in callback
+    struct Args {}
+
+    enum Config cfg = { errorHandler: (string s) {} };
+
+    mixin CLI!(cfg, Args).main!((ref _){});
 }