v4.9.0 (3 June 2024)

• Stan now detects if your compiler supports C++17 and will issue a warning if it is not available. Support for pre-C++17 compilers may be removed as early as next version. (#3063)
• Added a new distribution: wiener_full_lpdf. (#2822)
• Replaced static const objects with static constexpr when available. (#2830)
• Gradients for the hypergeometric_pFq function rewritten for increased speed and stability. (#2961)
• Added the Hypergeometric 1F0 function and gradients. (#2962)
• Removed macro usage in the definitions of the require_ templates. (#2965)
• Added a sparse matrix implimentation for arena_matrix. (#2971)
• Added hand-calculated derivatives for the multivariate normal lpdf. (#2980)
• Added derivatives for the multi_student_t_cholesky_lpdf. (#2990)
• Added constrain function for creating matrices holding row or column simplexes. (#2992)
• Updated TBB Windows build rules for compatibility with RTools make. (#2999)
• Improved efficiency of von_mises_lpdf gradient calculation, credit to @venpopov. (#3010)
• Fixed a few additional issues in checking the inputs to distributions defined over cholesky matrices. (#3012)
• Functions which are wrappers around CVODES and IDAS routines from Sundials now throw domain_error, rather than a mix of domain_error and runtime_error. (#3013)
• Fixed the stack_allocator being able to return non-8-byte aligned pointers. (#3014)
• Upgrade bundled Boost headers to 1.84. (#3001, #3018, #3019)
• Fixed a bug where linspaced_array was returning its results truncated to integers. (#3023)
• Fixed the return type of max(int, int) being a double. (#3024)
• Added 'override' to built-in make variables. (#3028)
• Maded sqrt(x) have a finite gradient at x=0. Allows distance(x,y) to work for x=y too. (#3033)
• Improved stability of the von_mises_lpdf function to avoid numeric overflow. Now it's no longer necessary to use the normal_lpdf for kappa>100, allowing vectorizing of the likelihood. (#3036)
• Updated the weibull_cdf and weibull_lcdf functions for numerical stability as well as correct handling of y == 0.0. (#3037)
• Improved speed of inv_Phi and std_normal_qf functions. (#3046)
• Fixed spurious linker issue with csr_matrix_times_vector. (#3048, #3053)
• Improved error messages when variables dimensions do not match in operations that require it. (#3049)
• Added support for Windows ARM64 with RTools ARM64 toolchain. (#3051)
• Fixed clean-all error when using external SUNDIALS libraries. (#3054)
• Fixed several small floating-point accuracy issues with ARM64 platforms. (#3059)
• The RNGs for the multinomial and multinomial_logit distributions now accept a zero total count, resulting in a zero integer vector. (#3061)
• Fixed a data race in profiling in multiple threads. (#3066)
• Backported SUNDIALS bugfix for hashmap integer overflow. (#3072)