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Detect if expression is valid

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It is possible to detect whether an operator or function can be called on a type. To test if a class has an overload of std::hash, one can do this: 

#include <functional> // for std::hash
#include <type_traits> // for std::false_type and std::true_type
#include <utility> // for std::declval

template<class, class = void>
struct has_hash
    : std::false_type
{};

template<class T>
struct has_hash<T, decltype(std::hash<T>()(std::declval<T>()), void())>
    : std::true_type
{};

Since C++17, std::void_t can be used to simplify this type of construct 

#include <functional> // for std::hash
#include <type_traits> // for std::false_type, std::true_type, std::void_t
#include <utility> // for std::declval

template<class, class = std::void_t<> >
struct has_hash
    : std::false_type
{};

template<class T>
struct has_hash<T, std::void_t< decltype(std::hash<T>()(std::declval<T>())) > >
    : std::true_type
{};

where std::void_t is defined as: 

template< class... > using void_t = void;

For detecting if an operator, such as operator< is defined, the syntax is almost the same: 

template<class, class = void>
struct has_less_than
    : std::false_type
{};

template<class T>
struct has_less_than<T, decltype(std::declval<T>() < std::declval<T>(), void())>
    : std::true_type
{};

These can be used to use a std::unordered_map<T> if T has an overload for std::hash, but otherwise attempt to use a std::map<T>: 

template <class K, class V>
using hash_invariant_map = std::conditional_t<
    has_hash<K>::value,
    std::unordered_map<K, V>,
    std::map<K,V>>;
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Metaprogramming:
* Calculating Factorials
* Iterating over a parameter pack
* Iterating with std::integer_sequence
* Tag dispatching
* Detect if expression is valid
* if-then-else
* Manual distinction of types when given any type T
* Calculating power with C++ 11
* Generic min/max with variable argument count
Table Of Contents
0 Getting started
1 Literals
2 Basic type keywords
3 Operator precedence
4 Floating point arithmetic
5 Bit operators
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11 Loops
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13 friend keyword
14 keywords
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16 auto keyword
17 Pointers
18 Type keywords (class, enum, struct, union)
19 Classes / structs
20 std::string
21 Enumeration
22 std::atomic
23 std::vector
24 std::array
25 std::pair
26 std::map
27 std::unordered_map
28 std::set and std::multiset
29 std::any
30 std::variant
31 std::optional
32 std::integer_sequence
33 std::function
34 std::forward_list
35 std::iomanip
36 Iterators
37 Basic I/O
38 File I/O
39 Streams
40 Stream manipulators
41 Metaprogramming
42 Returning multiple values from a function
43 References
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48 Pointers to class / struct members
49 The this pointer
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62 RAII
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64 Implementation-defined behavior
65 Special member functions
66 Random numbers
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69 Perfect forwarding
70 Virtual member functions
71 Undefined behavior
72 Overload resolution
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74 Pimpl idiom
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76 Fold expressions
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79 ISO C++ Standard
80 User-defined literals
81 Type erasure
82 Memory management
83 Explicit type conversions
84 RTTI
85 Standard library algorithms
86 Expression templates
87 Scopes
88 Atomic types
89 static assert
90 constexpr
91 Date and time with std::chrono
92 Trailing return type
93 Function template overloading
94 Common compile linker errors
95 Design patterns
96 Optimizations
97 Compiling and building
98 Type traits
99 One definition rule
100 Unspecified behavio
101 Argument dependent name lookup
102 Attributes
103 Internationalization
104 Profiling
105 Return type covariance
106 Non-static member functions
107 Recursion
108 Callable objects
109 Constant class member functions
110 C++ vs. C++ 11 vs C++ 17
111 Inline functions
112 Client server examples
113 Header files
114 Const correctness
115 Refactoring techniques
116 Parameter packs
117 Iteration
118 type deduction
119 C++ 11 memory model
120 Build systems
121 Concurrency with OpenMP
122 Type inference
123 Resource management
124 Storage class specifiers
125 Alignment
126 Inline variables
127 Linkage specifications
128 Curiusly recurring template pattern
129 Using declaration
130 Typedef and type aliases
131 Layout of object types
132 C incompatibilities
133 Optimization
134 Semaphore
135 Thread synchronization
136 Debugging
137 Futures and promises
138 More undefined behaviors
139 Mutexes
140 Recursive mutex
141 Unit testing
142 decltype
143 Digit separators
144 C++ Containers
145 Arithmetic meta-programming
146 Contributors