Deducing return type of map generator helper

[ozars]

Description

map generator helper function requires its return type to be given explicitly if it is different from the type produced by the generator argument. This patch enables deducing the return type from the return type of given lambda function. Example:

This:

auto val = GENERATE(map<std::tuple<int, std::string>>([](int val) {
  return std::make_tuple(val, std::to_string(val));
}, range(-5, 5));

can be optionally written as:

auto val = GENERATE(map([](int val) {
  return std::make_tuple(val, std::to_string(val));
}, range(-5, 5));

[codecov]

Codecov Report

Merging #1576 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1576      +/-   ##
==========================================
+ Coverage   80.58%   80.61%   +0.03%     
==========================================
  Files         121      121              
  Lines        3399     3394       -5     
==========================================
- Hits         2739     2736       -3     
+ Misses        660      658       -2

[codecov]

Codecov Report

Merging #1576 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1576   +/-   ##
=======================================
  Coverage   80.58%   80.58%           
=======================================
  Files         121      121           
  Lines        3399     3399           
=======================================
  Hits         2739     2739           
  Misses        660      660