In order to simplify my problem, I'll be using std::unique_lock as the tool to explain.
std::unique_lock has a template argument, the mutex. However, it's constructor is also a template function unique_lock(TMutex &, const chrono::duration<_Rep, _Period>&).
When one uses this, one can write:
auto lock = std::unique_lock(my_mutex, 5s);
So, the question: How to write out the deduction guide for this (without changing behavior), how to do so?
My best attempt upto now:
template<typename _Mutex>
template<typename _Rep, typename _Period>
unique_lock(_Mutex &, const chrono::duration<_Rep, _Period>&) -> unique_lock<_Mutex>;
Unfortunately, clang doesn't accept this:
error: extraneous template parameter list in template specialization or out-of-line template definition
GCC has a better error message for this:
error: too many template-parameter-lists
You can change it to a single template parameter list, like this:
template<typename _Mutex, typename _Rep, typename _Period>
unique_lock(_Mutex &, const chrono::duration<_Rep, _Period>&) -> unique_lock<_Mutex>;
And it works.
From the comments in your question, you seem to be mixing CTAD and specializations.
You're not specializing anything in unique_lock. Not a member function, not a constructor, you're just defining a deduction guide.
More specifically, from cppreference:
The syntax of a user-defined deduction guide is the syntax of a function declaration with a trailing return type [...] A deduction guide is not a function [...]
Notice that it has a syntax of a declaration, not a specialization. It's just different from what you expected.