Move use of declval into decltype.

[nickpdemarco]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 13.59%. Comparing base (06baff8) to head (2db590a).
Report is 2 commits behind head on main.

❗ Current head 2db590a differs from pull request most recent head dd7a22c. Consider uploading reports for the commit dd7a22c to get more accurate results

Files	Patch %	Lines
test/initial_draft.cpp	20.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
- Coverage   15.04%   13.59%   -1.45%     
==========================================
  Files           5        5              
  Lines         319      353      +34     
  Branches      199      213      +14     
==========================================
  Hits           48       48              
- Misses        198      233      +35     
+ Partials       73       72       -1     

Flag	Coverage Δ
Linux	13.59% <20.00%> (-1.45%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[nickpdemarco]

@sean-parent this is my best attempt at getting result_type to work, so far. If I try to use result_type_helper like this:

    template <class... Args>
    using result_type = 
      decltype(std::declval<chain>().result_type_helper()(std::declval<Args>()...));

We fail by attempting to access a member of an incomplete type. Not sure if there's a cleaner way around this.

[sean-parent]

I'm liking it! What more would you like to do on this before I push.

[nickpdemarco]

If you're happy, I'm happy. Feel free to merge.