Is it possible to produce a compile-time boolean value based on whether or not a C++11 expression is a constant expression (i.e. constexpr) in C++11? A few questions on SO relate to this, but I don't see a straight answer anywhere.
As of 2017, is_constexpr is not possible in C++11. That sounds like an odd thing to say, so let me explain a bit of the history.
First, we added this feature to resolve a defect: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1129
Johannes Schaub - litb posted a constexpr detection macro that relied on the provision that constant expressions are implicitly noexcept. This worked in C++11, but was never implemented by at least some compilers (for instance, clang). Then, as part of C++17, we evaluated Removing Deprecated Exception Specifications from C++17. As a side-effect of that wording, we accidentally removed that provision. When the Core Working Group discussed adding the provision back in, they realized that there were some serious problems with doing so. You can see the full details in the LLVM bug report. So rather than adding it back in, we decided to consider it a defect against all versions of standard and retroactively removed it.
The effect of this is that there is, to my knowledge, no way to detect whether an expression is usable as a constant expression.