constexpr does not imply const in c++14

[mumbleskates]

OS: ubuntu 21.04
compiler: clang-14 nightly

fast_float::span<T>::len() is marked constexpr but not const, and the compiler emits a fatal warning in the default build.

commands:

mkdir build && cd build
cmake -DFASTFLOAT_TEST=ON ..
cmake --build . --verbose

error:

[ 11%] Building CXX object tests/CMakeFiles/basictest.dir/basictest.cpp.o
cd /home/widders/repos/fast_float/build/tests && /usr/bin/c++ -DSUPPLEMENTAL_TEST_DATA_DIR=\"/home/widders/repos/fast_float/build/_deps/supplemental_test_files-build/data\" -I/home/widders/repos/fast_float/include -I/home/widders/repos/fast_float/build/_deps/doctest-src -O3 -DNDEBUG -Werror -Wall -Wextra -Weffc++ -Wsign-compare -Wshadow -Wwrite-strings -Wpointer-arith -Winit-self -Wconversion -Wsign-conversion -std=gnu++11 -o CMakeFiles/basictest.dir/basictest.cpp.o -c /home/widders/repos/fast_float/tests/basictest.cpp
In file included from /home/widders/repos/fast_float/tests/basictest.cpp:5:
In file included from /home/widders/repos/fast_float/include/fast_float/fast_float.h:77:
In file included from /home/widders/repos/fast_float/include/fast_float/parse_number.h:4:
In file included from /home/widders/repos/fast_float/include/fast_float/ascii_number.h:18:
/home/widders/repos/fast_float/include/fast_float/float_common.h:127:20: error: 'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const' to avoid a change in behavior [-Werror,-Wconstexpr-not-const]
  constexpr size_t len() noexcept {
                   ^
                         const
1 error generated.

fix:

diff --git a/include/fast_float/float_common.h b/include/fast_float/float_common.h
--- a/include/fast_float/float_common.h	(revision 1b9150913e07bc199ea7bc25fc1609a748bd301c)
+++ b/include/fast_float/float_common.h	(date 1631605731861)
@@ -124,7 +124,7 @@
   span(const T* _ptr, size_t _length) : ptr(_ptr), length(_length) {}
   span() : ptr(nullptr), length(0) {}
 
-  constexpr size_t len() noexcept {
+  constexpr size_t len() const noexcept {
     return length;
   }

[lemire]

This was supposedly fixed in a commit, but I made a mistake. I will create a patch release soon. Thank you.

[lemire]

Fixed in release 3.1.

The fix is just as you suggested.

[mumbleskates]

Thanks!

[lemire]

Thanks to you.