g77 treats procedure references to possible intrinsic names as always enabling their intrinsic nature, regardless of whether the form of the reference is valid for that intrinsic.
For example, CALL SQRT is interpreted by g77 as
an invalid reference to the
SQRT intrinsic function,
because the reference is a subroutine invocation.
First, g77 recognizes the statement CALL SQRT as a reference to a procedure named SQRT, not to a variable with that name (as it would for a statement such as V = SQRT).
Next, g77 establishes that, in the program unit being compiled,
SQRT is an intrinsic—not a subroutine that
happens to have the same name as an intrinsic (as would be
the case if, for example, EXTERNAL SQRT was present).
Finally, g77 recognizes that the form of the
reference is invalid for that particular intrinsic.
That is, it recognizes that it is invalid for an intrinsic
function, such as
SQRT, to be invoked as
At that point, g77 issues a diagnostic.
Some users claim that it is “obvious” that CALL SQRT references an external subroutine of their own, not an intrinsic function.
However, g77 knows about intrinsic subroutines, not just functions, and is able to support both having the same names, for example.
As a result of this, g77 rejects calls to intrinsics that are not subroutines, and function invocations of intrinsics that are not functions, just as it (and most compilers) rejects invocations of intrinsics with the wrong number (or types) of arguments.
So, use the EXTERNAL SQRT statement in a program unit that calls a user-written subroutine named SQRT.