A new version of GCC is out, with a lot of improvements:

GCC 5.1 released

Some changes:

C++

• G++ now supports C++14 variable templates.
• -Wnon-virtual-dtor doesn't warn anymore for final classes.
• Excessive template instantiation depth is now a fatal error. This prevents excessive diagnostics that usually do not help to identify the problem.
• G++ and libstdc++ now implement the feature-testing macros from Feature-testing recommendations for C++.
• G++ now allows typename in a template template parameter.

template<template<typename> typename X> struct D; // OK

• G++ now supports C++14 aggregates with non-static data member initializers.

struct A { int i, j = i; };
A a = { 42 }; // a.j is also 42

• G++ now supports C++14 extended constexpr.

constexpr int f (int i)
{
  int j = 0;
  for (; i > 0; --i)
    ++j;
  return j;
}
constexpr int i = f(42); // i is 42

• G++ now supports the C++14 sized deallocation functions.

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

• A new One Definition Rule violation warning (controlled by -Wodr) detects mismatches in type definitions and virtual table contents during link-time optimization.
• New warnings -Wsuggest-final-types and -Wsuggest-final-methods help developers to annotate programs with final specifiers (or anonymous namespaces) to improve code generation. These warnings can be used at compile time, but they are more useful in combination with link-time optimization.
• G++ no longer supports N3639 variable length arrays, as they were removed from the C++14 working paper prior to ratification. GNU VLAs are still supported, so VLA support is now the same in C++14 mode as in C++98 and C++11 modes.
• G++ now allows passing a non-trivially-copyable class via C varargs, which is conditionally-supported with implementation-defined semantics in the standard. This uses the same calling convention as a normal value parameter.
• G++ now defaults to -fabi-version=0 and -fabi-compat-version=2. So various mangling bugs are fixed, but G++ will still emit aliases with the old, wrong mangling where feasible. -Wabi continues to warn about differences.

libstdc++

• A Dual ABI is provided by the library. A new ABI is enabled by default. The old ABI is still supported and can be used by defining the macro _GLIBCXX_USE_CXX11_ABI to 0 before including any C++ standard library headers.
• A new implementation of std::string is enabled by default, using the small string optimization instead of copy-on-write reference counting.
• A new implementation of std::list is enabled by default, with an O(1) size() function;
• Full support for C++11, including the following new features:
  • std::deque and std::vector<bool> meet the allocator-aware container requirements;
  • movable and swappable iostream classes;
  • support for std::align and std::aligned_union;
  • type traits std::is_trivially_copyable, std::is_trivially_constructible, std::is_trivially_assignable etc.;
  • I/O manipulators std::put_time, std::get_time, std::hexfloat and std::defaultfloat;
  • generic locale-aware std::isblank;
  • locale facets for Unicode conversion;
  • atomic operations for std::shared_ptr;
  • std::notify_all_at_thread_exit() and functions for making futures ready at thread exit.
• Support for the C++11 hexfloat manipulator changes how the num_put facet formats floating point types when ios_base::fixed|ios_base::scientific is set in a stream's fmtflags. This change affects all language modes, even though the C++98 standard gave no special meaning to that combination of flags. To prevent the use of hexadecimal notation for floating point types use str.unsetf(std::ios_base::floatfield) to clear the relevant bits in str.flags().
• Full experimental support for C++14, including the following new features:
  • std::is_final type trait;
  • heterogeneous comparison lookup in associative containers.
  • global functions cbegin, cend, rbegin, rend, crbegin, and crend for range access to containers, arrays and initializer lists.
• Improved experimental support for the Library Fundamentals TS, including:
  • class std::experimental::any;
  • function template std::experimental::apply;
  • function template std::experimental::sample;
  • function template std::experimental::search and related searcher types;
  • variable templates for type traits;
  • function template std::experimental::not_fn.
• New random number distributions logistic_distribution and uniform_on_sphere_distribution as extensions.
• GDB Xmethods for containers and std::unique_ptr.