[flang] Flang seems too restrictive on constant expression usage.

[DanielCChen]

Consider the following code

    type A (p, r)
        integer, kind :: p, r
        real (selected_real_kind(p, r)) :: data
    end type
    type(a(6,23)) :: a1
    print*, a1%data%kind

end

Flang failed at the compile time with

error: Semantic errors in t.f
./t.f:3:15: error: Must be a constant value
          real (selected_real_kind(p, r)) :: data
                ^^^^^^^^^^^^^^^^^^^^^^^^

All ifort, gfortran and XLF compile and execute the code successfully.
The folding of selected_real_kind could occur when the type is instantiated.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code ``` type A (p, r) integer, kind :: p, r real (selected_real_kind(p, r)) :: data end type type(a(6,23)) :: a1 print*, a1%data%kind

end

Flang failed at the compile time with

error: Semantic errors in t.f
./t.f:3:15: error: Must be a constant value
real (selected_real_kind(p, r)) :: data
^^^^^^^^^^^^^^^^^^^^^^^^

All ifort, gfortran and XLF compile and execute the code successfully.
The folding of `selected_real_kind` could occur when the type is instantiated.
</details>

[klausler]

This is a constant expression, not a specification expression. The logic in the compiler that validates constant expressions should accept this usage.