Build failures with 0.9.9.3 on non-SIMD architectures #865
Description
On non-SIMD architectures, GLM_CONSTEXPR is defined to constexpr. However, this causes a problem in some cases, such as these overloaded declarations of operator[] in glm/detail/type_vec2.hpp:
GLM_FUNC_DECL GLM_CONSTEXPR T& operator[](length_type i);
GLM_FUNC_DECL GLM_CONSTEXPR T const& operator[](length_type i) const;
The issue is that functions declared constexpr are also implicitly const, so the above two declarations become identical. For example, on a 32 bit ARM platform without NEON instructions, gcc 8.2.0 gives these errors:
In file included from /usr/include/glm/ext/vector_bool2.hpp:5,
from /usr/include/glm/vec2.hpp:5,
from /usr/include/glm/glm.hpp:116,
from IOGfxDisplayGL2.cpp:29:
/usr/include/glm/detail/type_vec2.hpp:90:40: error: 'constexpr const T& glm::vec<2, T, Q>::operator[](glm::vec<2, T, Q>::length_type) const' cannot be overloaded with 'constexpr T& glm::vec<2, T, Q>::operator[](glm::vec<2, T, Q>::length_type) const'
GLM_FUNC_DECL GLM_CONSTEXPR T const& operator[](length_type i) const;
^~~~~~~~
/usr/include/glm/detail/type_vec2.hpp:89:34: note: previous declaration 'constexpr T& glm::vec<2, T, Q>::operator[](glm::vec<2, T, Q>::length_type) const'
GLM_FUNC_DECL GLM_CONSTEXPR T& operator[](length_type i);
^~~~~~~~
Either GLM_CONSTEXPR should not be used, or one of the declarations (and the corresponding implementation) should be put inside #if GLM_HAS_CONSTEXPR.
The issue was found by the Debian autobuilders failing to build packages depending on GLM on all architectures except amd64 and arm64. See for example: https://buildd.debian.org/status/fetch.php?pkg=freedink&arch=armel&ver=109.4-3&stamp=1548775669&raw=0