[J3] Simple functions in Constant Expressions

[Brad Richardson]

Hi all,

There have been requests in a few places asking for something equivalent to C++'s `constexpr`. Looking at the recently added `simple` feature, I think the constraints are sufficient for `simple function`s to be effectively `constexpr`. Would it be possible to add `simple` functions to the section on Constant Expressions (Section 10.1.12). I.e. the first paragraph would read:

A constant expression is an expression with limitations that make it suitable for use as a kind type parameter,
initializer, or named constant. It is an expression in which each operation is intrinsic or simple, and each primary is

I believe the restrictions listed in that section combined with the constraints for simple would be sufficient. Am I missing something? Is there some constraint that could be added to `simple` procedures to enable this? If so I would be in favor of adding it. My biggest motivation would be declaring constants of derived types with private components. For example

interface operator(.unit.)
module procedure from_units
end interface
interface
simple module function from_units(value, unit) result(acceleration)
real, intent(in) :: value
type(acceleration_unit_t), intent(in) :: unit
type(acceleration_t) :: acceleration
end function
end interface
type(acceleration_unit_t), parameter :: METERS_PER_SQUARE_SECOND = ...

! In a different module
type(acceleration_t), parameter :: GRAVITY = 9.81 .unit. METERS_PER_SQUARE_SECOND

Regards,
Brad Richardson
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