more details about current and future compiler requirements

doc/src/Build_basics.rst

@@ -196,13 +196,18 @@ LAMMPS.
 
    .. tab:: CMake build
 
-      By default CMake will use the compiler it finds according to
+      By default CMake will use the compiler it finds according to its
       internal preferences, and it will add optimization flags
       appropriate to that compiler and any :doc:`accelerator packages
       <Speed_packages>` you have included in the build.  CMake will
       check if the detected or selected compiler is compatible with the
       C++ support requirements of LAMMPS and stop with an error, if this
-      is not the case.
+      is not the case.  A C++11 compatible compiler is currently
+      required, but a transition to require C++17 is in progess and
+      planned to be completed in Summer 2025. Currently, setting
+      ``-DLAMMPS_CXX11=yes`` is required when configuring with CMake while
+      using a C++11 compatible compiler that does not support C++17,
+      otherwise setting ``-DCMAKE_CXX_STANDARD=17`` is preferred.
 
       You can tell CMake to look for a specific compiler with setting
       CMake variables (listed below) during configuration.  For a few
@@ -223,6 +228,8 @@ LAMMPS.
          -D CMAKE_C_COMPILER=name              # name of C compiler
          -D CMAKE_Fortran_COMPILER=name        # name of Fortran compiler
 
+         -D CMAKE_CXX_STANDARD=17              # put compiler in C++17 mode
+         -D LAMMPS_CXX11=yes                   # enforce compilation in C++11 mode
          -D CMAKE_CXX_FLAGS=string             # flags to use with C++ compiler
          -D CMAKE_C_FLAGS=string               # flags to use with C compiler
          -D CMAKE_Fortran_FLAGS=string         # flags to use with Fortran compiler
@@ -321,6 +328,14 @@ LAMMPS.
          you would have to install a newer compiler that supports C++11;
          either as a binary package or through compiling from source.
 
+      While a C++11 compatible compiler is currently sufficient to compile
+      LAMMPS, a transition to require C++17 is in progess and planned to
+      be completed in Summer 2025. Currently, setting ``-DLAMMPS_CXX11``
+      in the ``LMP_INC =`` line in the machine makefile is required when
+      using a C++11 compatible compiler that does not support C++17.
+      Otherwise, to enable C++17 support (if not enabled by default) using
+      a compiler flag like ``-std=c++17`` in CCFLAGS may needed.
+
       If you build LAMMPS with any :doc:`Speed_packages` included,
       there may be specific compiler or linker flags that are either
       required or recommended to enable required features and to

doc/src/Developer_code_design.rst

@@ -313,6 +313,11 @@ way than the varargs mechanism of the C library.  Finally, {fmt} has
 been included into the C++20 language standard, so changes to adopt it
 are future-proof.
 
+The long-term plan is to switch to using ``std::format()`` instead of
+``fmt::format()`` when the minimum C++ standard required for LAMMPS will
+be set to C++20. See the :ref:`basic build instructions <compile>` for
+more details.
+
 Formatted strings are frequently created by calling the
 ``fmt::format()`` function, which will return a string as a
 ``std::string`` class instance.  In contrast to the ``%`` placeholder in