Fix compilation of input_adapter(container) in edge cases

This fixes a compilation issue with the library if trying to parse JSON from a
custom container type that doesn't support detecting `begin()` and `end()`
functions via ADL.

`include/nlohmann/detail/input/input_adapters.hpp` defines convenience function
for creating input adapter for STL-like containers. It should support iterators
both via `std::begin()` and `std::end()`, as well as `begin()` and `end()` found
via ADL. However, the former doesn't actually work for iterable types not
defined in the `std` namespace, due to the way the return type deduction is
implemented:

    $ cat test.cc
    #include <iostream>
    #include <nlohmann/json.hpp>

    const char DATA[] = R"("Hello, world!")";

    struct MyIterable {
        using iterator = const char*;
        using const_iterator = iterator;
        const_iterator begin() const { return DATA; }
        const_iterator end() const { return DATA + sizeof(DATA); }
    };

    int main()
    {
        const auto c = MyIterable {};
        std::cout << nlohmann::json::parse(c) << "\n";
        return 0;
    }
    $ g++ --version
    g++ (Ubuntu 10.2.0-13ubuntu1) 10.2.0
    Copyright (C) 2020 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

    $ g++ -std=c++11 test.cc
    ...
    /usr/include/nlohmann/detail/input/input_adapters.hpp:375:6: note: candidate: ‘template<class ContainerType> decltype (nlohmann::detail::input_adapter(begin(container), end(container))) nlohmann::detail::input_adapter(const ContainerType&)’
      375 | auto input_adapter(const ContainerType& container) -> decltype(input_adapter(begin(container), end(container)))
          |      ^~~~~~~~~~~~~
    /usr/include/nlohmann/detail/input/input_adapters.hpp:375:6: note:   template argument deduction/substitution failed:
    /usr/include/nlohmann/detail/input/input_adapters.hpp: In substitution of ‘template<class ContainerType> decltype (nlohmann::detail::input_adapter(begin(container), end(container))) nlohmann::detail::input_adapter(const ContainerType&) [with ContainerType = MyIterable]’:
    ...

In the above example the trailing type deduction won't look for the `begin()`
and `end()` functions in the `std` namespace, although the function body would.

I don't know about a more elegant fix in C++11 except to spell out the return
type verbatim.

include/nlohmann/detail/input/input_adapters.hpp

@@ -372,7 +372,7 @@ typename iterator_input_adapter_factory<IteratorType>::adapter_type input_adapte
 
 // Convenience shorthand from container to iterator
 template<typename ContainerType>
-auto input_adapter(const ContainerType& container) -> decltype(input_adapter(begin(container), end(container)))
+typename iterator_input_adapter_factory<typename ContainerType::const_iterator>::adapter_type input_adapter(const ContainerType& container)
 {
     // Enable ADL
     using std::begin;

single_include/nlohmann/json.hpp

@@ -5179,7 +5179,7 @@ typename iterator_input_adapter_factory<IteratorType>::adapter_type input_adapte
 
 // Convenience shorthand from container to iterator
 template<typename ContainerType>
-auto input_adapter(const ContainerType& container) -> decltype(input_adapter(begin(container), end(container)))
+typename iterator_input_adapter_factory<typename ContainerType::const_iterator>::adapter_type input_adapter(const ContainerType& container)
 {
     // Enable ADL
     using std::begin;
@@ -16793,7 +16793,7 @@ class basic_json
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
         const bool ignore_comments = false
-                                 )
+    )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
                 std::move(cb), allow_exceptions, ignore_comments);
@@ -25338,7 +25338,7 @@ template<>
 inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcept(
     is_nothrow_move_constructible<nlohmann::json>::value&&
     is_nothrow_move_assignable<nlohmann::json>::value
-                              )
+)
 {
     j1.swap(j2);
 }

test/src/unit-user_defined_input.cpp

@@ -50,6 +50,8 @@ TEST_CASE("Use arbitrary stdlib container")
 
 struct MyContainer
 {
+    using iterator = const char*;
+    using const_iterator = iterator;
     const char* data;
 };