Merge branches 'develop' and 'mkdocs' of https://github.com/nlohmann/json into mkdocs

.circleci/config.yml

 version: 2
 jobs:
-  build:
+  build_stable:
     docker:
       - image: debian:stretch
 
@@ -22,3 +22,33 @@ jobs:
       - run:
           name: Execute test suite
           command: 'cd build ; ctest --output-on-failure -j 2'
+
+  build_bleeding_edge:
+    docker:
+      - image: archlinux
+
+    steps:
+      - checkout
+
+      - run:
+          name: Install required tools
+          command: 'pacman -Sy --noconfirm base base-devel gcc git cmake'
+      - run:
+          name: Show versions
+          command: 'g++ --version ; uname -a; cmake --version'
+      - run:
+          name: Run CMake
+          command: 'mkdir build ; cd build ; cmake ..'
+      - run:
+          name: Compile
+          command: 'cmake --build build'
+      - run:
+          name: Execute test suite
+          command: 'cd build ; ctest --output-on-failure -j 2'
+
+workflows:
+  version: 2
+  build_and_test_all:
+    jobs:
+      - build_stable
+      - build_bleeding_edge

.github/CONTRIBUTING.md

@@ -66,6 +66,6 @@ To make changes, you need to edit the following files:
 
 The following areas really need contribution:
 
-- Extending the **continuous integration** toward more exotic compilers such as Android NDK, Intel's Compiler, or the bleeding-edge versions of GCC or Clang.
+- Extending the **continuous integration** toward more exotic compilers such as Android NDK, Intel's Compiler, or the bleeding-edge versions Clang.
 - Improving the efficiency of the **JSON parser**. The current parser is implemented as a naive recursive descent parser with hand coded string handling. More sophisticated approaches like LALR parsers would be really appreciated. That said, parser generators like Bison or ANTLR do not play nice with single-header files -- I really would like to keep the parser inside the `json.hpp` header, and I am not aware of approaches similar to [`re2c`](http://re2c.org) for parsing.
 - Extending and updating existing **benchmarks** to include (the most recent version of) this library. Though efficiency is not everything, speed and memory consumption are very important characteristics for C++ developers, so having proper comparisons would be interesting.

README.md

@@ -544,6 +544,11 @@ for (auto& [key, value] : o.items()) {
 }
 
 // find an entry
+if (o.contains("foo")) {
+  // there is an entry with key "foo"
+}
+
+// or via find and an iterator
 if (o.find("foo") != o.end()) {
   // there is an entry with key "foo"
 }
@@ -1085,7 +1090,7 @@ auto cbor = json::to_msgpack(j); // 0xD5 (fixext2), 0x10, 0xCA, 0xFE
 
 Though it's 2020 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:
 
-- GCC 4.8 - 10.0 (and possibly later)
+- GCC 4.8 - 10.1 (and possibly later)
 - Clang 3.4 - 10.0 (and possibly later)
 - Intel C++ Compiler 17.0.2 (and possibly later)
 - Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0 (and possibly later)
@@ -1142,6 +1147,7 @@ The following compilers are currently used in continuous integration at [Travis]
 | GCC 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)                         | Ubuntu 18.04.4 LTS | GitHub Actions |
 | GCC 8.4.0 (Ubuntu 8.4.0-1ubuntu1~14.04)                         | Ubuntu 14.04.5 LTS | Travis         |
 | GCC 9.3.0 (Ubuntu 9.3.0-11ubuntu0~14.04)                        | Ubuntu 14.04.5 LTS | Travis         |
+| GCC 10.1.0 (Arch Linux latest)                                  | Arch Linux         | Circle CI      |
 | MSVC 19.0.24241.7 (Build Engine version 14.0.25420.1)           | Windows-6.3.9600   | AppVeyor       |
 | MSVC 19.16.27035.0 (15.9.21+g9802d43bc3 for .NET Framework)     | Windows-10.0.14393 | AppVeyor       |
 | MSVC 19.25.28614.0 (Build Engine version 16.5.0+d4cbfca49 for .NET Framework) | Windows-10.0.17763  | AppVeyor       |

include/nlohmann/json.hpp

@@ -3768,7 +3768,8 @@ class basic_json
     @since version 1.0.0
     */
     template<class ValueType, typename std::enable_if<
-                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
+                 std::is_convertible<basic_json_t, ValueType>::value
+                 and not std::is_same<value_t, ValueType>::value, int>::type = 0>
     ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
     {
         // at only works for objects

single_include/nlohmann/json.hpp

@@ -19560,7 +19560,8 @@ class basic_json
     @since version 1.0.0
     */
     template<class ValueType, typename std::enable_if<
-                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
+                 std::is_convertible<basic_json_t, ValueType>::value
+                 and not std::is_same<value_t, ValueType>::value, int>::type = 0>
     ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
     {
         // at only works for objects