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nonportable-shortcircuit-inquiry (C161)

This rule is turned on by default.

What it does

Checks for use of a variable in the same logical expression as "definedness" inquiry.

Why is this bad?

Unlike many other languages, the Fortran standard doesn't mandate (or prohibit) short-circuiting in logical expressions, so different compilers have different behaviour when it comes to evaluating such expressions. This is commonly encountered when using present() with an optional dummy argument and checking its value in the same expression. Without short-circuiting, this can lead to segmentation faults when the expression is evaluated if the argument isn't present.

Instead, you should nest the conditional statements, or use the Fortran 2023 "condtional expression" (also called ternary expressions in other languages). Unfortunately, any else branches may need to be duplicated or refactored to accommodate this change.

This lack of short-circuiting also affects other inquiry functions such as associated and allocated which are used to guard invalid accesses.

Example

Don't do this: 

integer function example(arg1)
  integer, optional, intent(in) :: arg1

  if (present(arg1) .and. arg1 > 2) then
    example = arg1 * arg1
  else
    example = 1
  end if
The compiler may or may not evaluate arg1 > 2 even if present(arg1) is false. This is a runtime error, and may crash your program.

Use instead, noting that we either need to duplicate the else branch, or refactor differently: 

integer function example(arg1)
  integer, optional, intent(in) :: arg1

  if (present(arg1)) then
    if (arg1 > 2) then
      example = arg1 * arg1
    else
      example = 1
    end if
  else
    example = 1
  end if

Or with Fortran 2023 (not currently supported by most compilers!): 

integer function example(arg1)
  integer, optional, intent(in) :: arg1

  example = present(arg1) ? (arg1 > 2 ? arg1 * arg1 : 1) : 1
Note that although the true/false arms of the conditional-expression are lazily evaluated, it's still not possible to use a compound logical expression here, so we still must have a nested expression and duplicate the default value.

References