Return types are not matched with method return type.

[RollingCat]

Hi there. I'm new to D language and make a simple software 
monitoring new file with specified extension.

class FileList
{
        str_CurrentDir;
	bool ReturnMatchedFile(string str_arg_filename, string 
str_Extension)
        {
        return endsWith(str_arg_filename, '.'~str_Extension);
        }
        void GetCurrentList(string str_arg_Extension)
        {
		auto 
filter_CurrentList=filter!ReturnMatchedFile(str_arg_Extension, 
dirEntries(str_CurrentDir, SpanMode.shallow));
        }
}

with this FileList class, after making instance of FileList and 
assigning str_CurrentDir variable, when I call 
GetCurrentList("d"), it says that std.algorithm.filter! ~~ does 
not match any function template declaration.
OK. So I modified above code like below just for a test.

class FileList
{
        str_CurrentDir;
        bool ReturnMatchedFile(string str_arg_filename)
        {
        return endsWith(str_arg_filename, ".d");
        }

        void GetCurrentList()
        {
		auto 
filter_CurrentList=filter!ReturnMatchedFile(dirEntries(str_CurrentDir, 
SpanMode.shallow));
        }
}

And it says that
this for ReturnMatchedFile needs to be type FileList not type 
FileResult!(ReturnMatchedFile, DirIterator).
I can't understand it!

Anyway, to solve the problem, again, I modified above code like 
below.

static bool ReturnMatchedFile(string str_arg_filename)
        {
        return endsWith(str_arg_filename, ".d");
        }

class FileList
{
        str_CurrentDir;
        
        void GetCurrentList()
        {
		auto 
filter_CurrentList=filter!ReturnMatchedFile(dirEntries(str_CurrentDir, 
SpanMode.shallow));
        }
}

and it works.

What is the problem?

Best regards and happy new year! :)