Bump minimum language standard to C++14 (#788)

* Bump minimum language standard to C++14

* Add warning message in config

* Update readme and keep C++14 in GCC5 drone run

.drone.star

@@ -17,7 +17,7 @@ def main(ctx):
 
   things_to_test = [ "special_fun", "distribution_tests", "mp", "misc", "interpolators", "quadrature", "autodiff", "long-running-tests", "float128_tests" ]
   sanitizer_test = [ "special_fun", "distribution_tests", "misc", "interpolators", "quadrature", "float128_tests" ]
-  gnu_5_stds = [ "gnu++11" ]
+  gnu_5_stds = [ "gnu++11", "gnu++14" ]
   gnu_6_stds = [ "gnu++11", "gnu++14" ]
   gnu_8_stds = [ "gnu++11", "gnu++14", "gnu++17" ]
   gnu_10_stds = [ "gnu++11", "gnu++14", "gnu++17", "gnu++20" ]

.github/workflows/ci.yml

@@ -210,7 +210,7 @@ jobs:
       fail-fast: false
       matrix:
         compiler: [ g++-6, clang++-6.0, g++-7, g++-8, clang++-7, clang++-8 ]
-        standard: [ c++11, c++14, c++17 ]
+        standard: [ c++14, c++17 ]
         suite: [ float128_tests, special_fun, distribution_tests, misc, quadrature, mp, interpolators, autodiff, ../example//examples, ../tools ]
     steps:
       - uses: actions/checkout@v2
@@ -322,7 +322,7 @@ jobs:
       fail-fast: false
       matrix:
         toolset: [ gcc, msvc-14.0, msvc-14.2 ]
-        standard: [ 11, 14, 17 ]
+        standard: [ 14, 17 ]
         suite: [ float128_tests, special_fun, distribution_tests, misc, quadrature, mp, interpolators, autodiff, ../example//examples, ../tools ]
     steps:
       - uses: actions/checkout@v2
@@ -542,7 +542,7 @@ jobs:
       fail-fast: false
       matrix:
         compiler: [ g++-10 ]
-        standard: [ c++11, c++14, c++17, c++2a ]
+        standard: [ c++14, c++17, c++20 ]
         suite: [ float128_tests, special_fun, distribution_tests, misc, quadrature, interpolators, autodiff, ../example//examples, ../tools ]
     steps:
       - uses: actions/checkout@v2

README.md

@@ -2,8 +2,8 @@ Boost Math Library
 [![Build Status](https://drone.cpp.al/api/badges/boostorg/math/status.svg)](https://drone.cpp.al/boostorg/math)[![Build Status](https://github.com/boostorg/math/workflows/CI/badge.svg?branch=develop)](https://github.com/boostorg/math/actions)
 ==================
 
->ANNOUNCEMENT: Support for C++03 is now deprecated in this library and will be supported in existing features
->only until March 2021.  New features will require *at least* C++11, as will existing features from next year.
+>ANNOUNCEMENT: Support for C++11 will be deprecated in this library starting in July 2023 (Boost 1.82).  
+>New features will require *at least* C++14, as will existing features starting with the deprecation release.
 
 This library is divided into several interconnected parts:
 

include/boost/math/tools/config.hpp

@@ -13,6 +13,17 @@
 
 #include <boost/math/tools/is_standalone.hpp>
 
+// Minimum language standard transition
+#ifdef _MSVC_LANG
+#  if _MSVC_LANG < 201402L
+#    pragma warning("The minimum language standard to use Boost.Math will be C++14 starting in July 2023 (Boost 1.82 release)");
+#  endif
+#else
+#  if __cplusplus < 201402L
+#    warning "The minimum language standard to use Boost.Math will be C++14 starting in July 2023 (Boost 1.82 release)"
+#  endif
+#endif
+
 #ifndef BOOST_MATH_STANDALONE
 #include <boost/config.hpp>