Consider the code below

template<typename T> constexpr int foo(T) { return 0; };
template<> int foo(int) { return 1; }

Compiling this code with g++-4.8.0 -Wall -c -std=c++0x bug.cpp throws the following error:

bug.cpp:2:23: error: redeclaration ‘constexpr int foo(T) [with T = int]’ differs in ‘constexpr’
 template<> int foo(int) { return 1; }
                       ^
bug.cpp:1:36: error: from previous declaration ‘constexpr int foo(T) [with T = int]’
 template<typename T> constexpr int foo(T) { return 0; };

Yet, the C++ standard (section 7.1.5) says that an explicit specialization can differ from the template declaration with respect to the constexpr specifier.