With clang 3.7.0 this results in "constexpr function never produces a constant expression", since the C-style pointer cast is essentially a reinterpret_cast (C++14 Standard §5.4p4.4), which is not a "core constant expression" (C++14 Standard §5.20p2.13) and thus must not appear in a constexpr function: C++14 Standard §7.1.5p5:

... if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, the program is ill-formed; no diagnostic required.

clang seems to go the extra mile and produces a diagnostic.

Incidentally the pointer cast would violate C++'s aliasing rules and thus invoke undefined behavior.

P.S: The C++14 Standard is available here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n429... (never mind the "draft" status)

Interestingly, g++ gives out a warning (correctly) when compiled with -O2, but not by default (-O0):

    warning: dereferencing type-punned pointer will break strict-aliasing rules  [-Wstrict-aliasing]

So, you're right indeed.

This is illegal (undefined), but casting to char* and reading out the values is legal.

Please stop using C-style casts. It's 2016. C++ != C. Unlearn what you have learned.

'idiomatic C++' != C

Idiomatic C++ is not well defined. There are so many possible ways to write C++ code that you should be skeptical if somebody promotes one version is the true one.