Merge pull request #566 from aturner-epcc/aturner-epcc/gcc12

Adds note on how to use GCC 12

docs/user-guide/dev-environment.md

@@ -300,7 +300,6 @@ environments.
 !!! warning
     Intel compilers are not currently available on ARCHER2.
 
-
 ### GNU compiler collection (GCC)
 
 The commonly used open source GNU compiler collection is available and
@@ -368,6 +367,25 @@ Due to MODULEPATH changes, the following have been reloaded:
     `-fallow-argument-mismatch` to reduce the error to a warning. The same
     effect may be achieved via `-std=legacy`.
 
+#### Using GCC 12.x on ARCHER2
+
+GCC 12.x compilers are available on ARCHER2 for users who wish to access 
+newer features (particularly C++ features).
+
+Testing by the CSE service has
+identified that some software regression tests produce different results
+from the reference values when using software compiled with gfortran from
+GCC 12.x so we do not recommend its general use by users. Users should 
+carefully check results from software built using compilers from GCC 12.x
+before using it for their research projects.
+
+You can access GCC 12.x by using the commands:
+
+```
+module load extra-compilers
+module load PrgEnv-gnu
+```
+
 #### Reference material
 
   - C/C++ documentation