Fortran 2018 RESULT statement followed by RETURN in internal functions: portable or unspecified?

Answer

Yes. A RETURN statement immediately after assigning to the RESULT variable in an internal function is portable and standard-compliant in Fortran 2018. The standard does not leave this unspecified.

Confirmed facts from the standard

The value of a function is determined by the value of the variable named in the RESULT clause at the point of termination of the procedure. A RETURN statement explicitly terminates execution of a procedure and returns control to the caller. The RETURN statement does not clear or reset the RESULT variable; it returns using the current state of that variable.

Therefore assigning to RESULT and then executing RETURN is a normal explicit exit path. An implicit return by reaching the end of the function body is functionally equivalent.

Likely explanation and portability

Mainstream compilers treat the sequence as standard. There is no normative conflict between RESULT assignment and RETURN because they perform distinct operations: RESULT assignment updates the designated return storage, RETURN transfers control.

No documented portability risk requiring refactoring has been reported for non-recursive internal functions. Warnings about unreachable code or unused result would appear if the compiler considered the construct invalid.

Verification steps for this case

1. Compile with strict standard mode, e.g. -std=f2018 for gfortran or /std:f18 for Intel Fortran.
2. Enable all warnings, e.g. -Wall -Wextra, and check for unreachable code or unused result alerts.
3. Test value stability: assign to RESULT, execute RETURN, and place a subsequent assignment to the same RESULT variable that would change the return value if execution continued. The caller must receive the first value.

One diagnostic detail that changes the recommendation is whether the internal functions are declared RECURSIVE. If so, confirm the RESULT clause is present and the function does not alias its own name, as RESULT handling is mandatory for recursive functions.