6.2.3 Alternate complex function syntax

Some Fortran compilers, including g77, let the user declare complex functions with the syntax COMPLEX FUNCTION name*16(), as well as COMPLEX*16 FUNCTION name(). Both are non-standard, legacy extensions. gfortran accepts the latter form, which is more common, but not the former.

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https://gcc.gnu.org/onlinedocs/gcc-9.3.0/gfortran/Alternate-complex-function-syntax.html