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(g77-295.info)Not My Type

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Not My Type
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   A fruitful source of bugs in Fortran source code is use, or mis-use,
of Fortran's implicit-typing feature, whereby the type of a variable,
array, or function is determined by the first character of its name.

   Simple cases of this include statements like `LOGX=9.227', without a
statement such as `REAL LOGX'.  In this case, `LOGX' is implicitly
given `INTEGER(KIND=1)' type, with the result of the assignment being
that it is given the value `9'.

   More involved cases include a function that is defined starting with
a statement like `DOUBLE PRECISION FUNCTION IPS(...)'.  Any caller of
this function that does not also declare `IPS' as type `DOUBLE
PRECISION' (or, in GNU Fortran, `REAL(KIND=2)') is likely to assume it
returns `INTEGER', or some other type, leading to invalid results or
even program crashes.

   The `-Wimplicit' option might catch failures to properly specify the
types of variables, arrays, and functions in the code.

   However, in code that makes heavy use of Fortran's implicit-typing
facility, this option might produce so many warnings about cases that
are working, it would be hard to find the one or two that represent
bugs.  This is why so many experienced Fortran programmers strongly
recommend widespread use of the `IMPLICIT NONE' statement, despite it
not being standard FORTRAN 77, to completely turn off implicit typing.
(`g77' supports `IMPLICIT NONE', as do almost all FORTRAN 77 compilers.)

   Note that `-Wimplicit' catches only implicit typing of *names*.  It
does not catch implicit typing of expressions such as `X**(2/3)'.  Such
expressions can be buggy as well--in fact, `X**(2/3)' is equivalent to
`X**0', due to the way Fortran expressions are given types and then
evaluated.  (In this particular case, the programmer probably wanted
`X**(2./3.)'.)