GNU Fortran implements a number of extensions over standard Fortran. This chapter contains information on their syntax and meaning. There are currently two categories of GNU Fortran extensions, those that provide functionality beyond that provided by any standard, and those that are supported by GNU Fortran purely for backward compatibility with legacy compilers. By default, -std=gnu allows the compiler to accept both types of extensions, but to warn about the use of the latter. Specifying either -std=f95, -std=f2003 or -std=f2008 disables both types of extensions, and -std=legacy allows both without warning.
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https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gfortran/Extensions-implemented-in-GNU-Fortran.html