C++14: floats, bits, and constant expressions

Re:C++14: floats, bits, and constant expressions by Kristine1975 on Monday March 28, 2016 @05:14AM (#11373335)

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)

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Re:C++14: floats, bits, and constant expressions by satbyy on Monday March 28, 2016 @06:06AM (#11373453)

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.

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