An advantage compared with earlier Fortran standards is that
the standard now requires that the compiler can signal if the
user deviates from the permitted standard.
It is required that a Fortran 90 compiler can signal
- use of syntax not defined in the standard.
- violation of the syntax rules.
- use of kinds not available.
- use of obsolete constructs (or statements).
- use of non-Fortran characters (for example, Swedish or Cyrillic
characters) outside of character strings or comments.
- violation of the area of validity for variable names, names of
the DO-loops and the corresponding names like IF, CASE and operators.
- the reason that a program is not accepted by the compiler.
The above means that it is permitted to include extensions to
Fortran 90. It has to be possible to ask the program to signal for
any extensions outside the Fortran 90 standard.