Question

Users are allowed to add explicit specializations to the std namespace. However, there are a few templates that I am explicitly forbidden from specializing.

What templates can and can’t I specialize?

Answer 1

Quoting loosely from the standard:

• numeric_limits shall not be specialized for non-arithmetic standard types (e.g. complex<T>)

• "[S]pecializations of shared_ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable [and] convertible to bool."

• "Specializations of weak_ptr shall be CopyConstructible and CopyAssignable."

• "[T]emplate specializations [of std::hash] shall meet the requirements of class template hash."

• Anything in <type_traits>: "The behavior of a program that adds specializations for any of the class templates defined in this subclause is undefined unless otherwise specified." (only some specializations of common_type are explicitly allowed)

• Locales have certain required specializations.

• All specializations of istreambuf_iterator shall have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

• "The effect of instantiating the class template complex for any type other than float, double, or long double is unspecified." I take it that means that defining such other specializations is pointless.

• "Specializations and instantiations of the atomic template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor."

• "The class templates unary_function and binary_function are deprecated. A program shall not declare specializations of these templates."

And of course the overarching clause 17.6.4.2.1, sentence 1 (thanks @sehe and @curiousguy):

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a
namespace within namespace std unless otherwise specified. A program may add a template specialization
for any standard library template to namespace std only if the declaration depends on a user-defined type
and the specialization meets the standard library requirements for the original template and is not explicitly
prohibited.

And sentence 2:

The behavior of a C++ program is undefined if it declares

• an explicit specialization of any member function of a standard library class template, or

• an explicit specialization of any member function template of a standard library class or class template, or

• an explicit or partial specialization of any member class template of a standard library class or class template.

A program may explicitly instantiate a template defined in the standard library only if the declaration
depends on the name of a user-defined type and the instantiation meets the standard library requirements
for the original template.