test/ft/constexpr.cpp uses C++17 features but is not guarded in make test

[PavelGuzenfeld]

Description

test/ft/constexpr.cpp uses two C++17-only features:

• Constexpr lambdas (added in C++17)
• static_assert without message (added in C++17)

However, the root Makefile passes it unconditionally to g++ in the C++14
test target, causing make test c++14 to fail:

constexpr.cpp:12:30: error: lambda expressions are not allowed in unevaluated operands
constexpr.cpp:14:7: error: static assertion without a message is a C++17 extension

Scope

• Makefile: no guard — make test c++14 always attempts to build constexpr.cpp
• CMakeLists.txt: already fixed by PR fix: replace if constexpr with C++14 tag-dispatch in update_composite_states (PR #684 regression) #692 — test is skipped when CMAKE_CXX_STANDARD < 17

Fix

Add a C++14 guard in the Makefile:

# In the make test loop, skip constexpr.cpp when STD=c++14

Or exclude constexpr.cpp from the list when standard is c++14.

Reproduction

make test c++14
# → build fails on constexpr.cpp