Index: flang/test/Semantics/collectives05.f90
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+++ flang/test/Semantics/collectives05.f90
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+! RUN: %python %S/test_errors.py %s %flang_fc1
+! XFAIL: *
+! This test checks for semantic errors in co_reduce subroutine calls based on
+! the co_reduce interface defined in section 16.9.49 of the Fortran 2018 standard.
+! To Do: add co_reduce to the list of intrinsics
+
+module foo_m
+  implicit none
+
+  type foo_t
+    integer :: n=0
+  contains
+    procedure :: derived_type_op
+    generic :: operator(+) => derived_type_op
+  end type
+
+contains
+
+  pure function derived_type_op(lhs, rhs) result(lhs_op_rhs)
+    class(foo_t), intent(in) :: lhs, rhs
+    type(foo_t) lhs_op_rhs
+    lhs_op_rhs%n = lhs%n + rhs%n
+  end function
+
+end module foo_m
+
+program main
+  use foo_m, only : foo_t
+  implicit none
+
+  type(foo_t) foo
+  class(foo_t), allocatable :: polymorphic
+  integer i, status, integer_array(1)
+  real x
+  real vector(1)
+  real array(1,1,1, 1,1,1, 1,1,1, 1,1,1, 1,1,1)
+  character(len=1) string, message, character_array(1)
+  integer coindexed[*]
+  logical bool
+
+  ! correct calls, should produce no errors
+  call co_reduce(i,      int_op)
+  call co_reduce(i,      int_op,                            status)
+  call co_reduce(i,      int_op,                            stat=status)
+  call co_reduce(i,      int_op,                                         errmsg=message)
+  call co_reduce(i,      int_op,                            stat=status, errmsg=message)
+  call co_reduce(i,      int_op,            result_image=1, stat=status, errmsg=message)
+  call co_reduce(i,      operation=int_op,  result_image=1, stat=status, errmsg=message)
+  call co_reduce(a=i,    operation=int_op,  result_image=1, stat=status, errmsg=message)
+  call co_reduce(array,  operation=real_op, result_image=1, stat=status, errmsg=message)
+  call co_reduce(vector, operation=real_op, result_image=1, stat=status, errmsg=message)
+  call co_reduce(string, operation=char_op, result_image=1, stat=status, errmsg=message)
+  call co_reduce(foo,    operation=left,    result_image=1, stat=status, errmsg=message)
+
+  allocate(foo_t :: polymorphic)
+
+  ! Test all statically verifiable semantic requirements on co_reduce arguments
+  ! Note: We cannot check requirements that relate to "corresponding references." 
+  ! References can correspond only if they execute on differing images.  A code that
+  ! executes in a single image might be standard-conforming even if the same code
+  ! executing in multiple images is not.
+
+  ! argument 'a' cannot be polymorphic
+  !ERROR: to be determined
+  call co_reduce(polymorphic, derived_type_op)
+
+  ! argument 'a' cannot be coindexed
+  !ERROR: (message to be determined)
+  call co_reduce(coindexed[1], int_op)
+
+  ! argument 'a' is intent(inout)
+  !ERROR: (message to be determined)
+  call co_reduce(i + 1, int_op)
+
+  ! operation must be a pure function
+  !ERROR: (message to be determined)
+  call co_reduce(i, operation=not_pure)
+
+  ! operation must have exactly two arguments
+  !ERROR: (message to be determined)
+  call co_reduce(i, too_many_args)
+
+  ! operation result must be a scalar
+  !ERROR: (message to be determined)
+  call co_reduce(i, array_result)
+
+  ! operation result must be non-allocatable
+  !ERROR: (message to be determined)
+  call co_reduce(i, allocatable_result)
+
+  ! operation result must be non-pointer
+  !ERROR: (message to be determined)
+  call co_reduce(i, pointer_result)
+
+  ! operation's arguments must be scalars
+  !ERROR: (message to be determined)
+  call co_reduce(i, array_args)
+
+  ! operation arguments must be non-allocatable
+  !ERROR: (message to be determined)
+  call co_reduce(i, allocatable_args)
+
+  ! operation arguments must be non-pointer
+  !ERROR: (message to be determined)
+  call co_reduce(i, pointer_args)
+
+  ! operation arguments must be non-polymorphic
+  !ERROR: (message to be determined)
+  call co_reduce(i, polymorphic_args)
+
+  ! operation: type of 'operation' result and arguments must match type of argument 'a'
+  !ERROR: (message to be determined)
+  call co_reduce(i, real_op)
+
+  ! operation: kind type parameter of 'operation' result and arguments must match kind type parameter of argument 'a'
+  !ERROR: (message to be determined)
+  call co_reduce(x, double_precision_op)
+
+  ! arguments must be non-optional
+  !ERROR: (message to be determined)
+  call co_reduce(i, optional_args)
+
+  ! if one argument is asynchronous, the other must be also
+  !ERROR: (message to be determined)
+  call co_reduce(i, asynchronous_mismatch)
+
+  ! if one argument is a target, the other must be also
+  !ERROR: (message to be determined)
+  call co_reduce(i, target_mismatch)
+
+  ! if one argument has the value attribute, the other must have it also
+  !ERROR: (message to be determined)
+  call co_reduce(i, value_mismatch)
+
+  ! result_image argument must be an integer scalar
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=integer_array)
+
+  ! result_image argument must be an integer
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=bool)
+
+  ! stat not allowed to be coindexed
+  !ERROR: to be determined
+  call co_reduce(i, int_op, stat=coindexed[1])
+
+  ! stat argument must be an integer scalar
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=integer_array)
+
+  ! stat argument has incorrect type
+  !ERROR: Actual argument for 'stat=' has bad type 'CHARACTER(KIND=1,LEN=1_8)'
+  call co_reduce(i, int_op, result_image=1, string)
+
+  ! stat argument is intent(out)
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=1+1)
+
+  ! errmsg argument must not be coindexed
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=status, errmsg=conindexed_string[1])
+
+  ! errmsg argument must be a scalar
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=status, errmsg=character_array)
+
+  ! errmsg argument must be a character
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=status, errmsg=i)
+
+  ! errmsg argument is intent(inout)
+  !ERROR: to be determined
+  call co_reduce(i, int_op, result_image=1, stat=status, errmsg="literal constant")
+
+  ! too many arguments to the co_reduce() call
+  !ERROR: too many actual arguments for intrinsic 'co_reduce'
+  call co_reduce(i, int_op, result_image=1, stat=status, errmsg=message, 3.4)
+
+  ! non-existent keyword argument
+  !ERROR: unknown keyword argument to intrinsic 'co_reduce'
+  call co_reduce(fake=3.4)
+
+contains
+
+  pure function left(lhs, rhs) result(lhs_op_rhs)
+    type(foo_t), intent(in)  :: lhs, rhs
+    type(foo_t) :: lhs_op_rhs
+    lhs_op_rhs = lhs
+  end function
+
+  pure function char_op(lhs, rhs) result(lhs_op_rhs)
+    character(len=1), intent(in)  :: lhs, rhs
+    character(len=1) :: lhs_op_rhs
+    lhs_op_rhs = min(lhs, rhs)
+  end function
+
+  pure function real_op(lhs, rhs) result(lhs_op_rhs)
+    real, intent(in) :: lhs, rhs
+    real :: lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function double_precision_op(lhs, rhs) result(lhs_op_rhs)
+    integer, parameter :: double = kind(1.0D0)
+    real(double), intent(in) :: lhs, rhs
+    real(double) lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function int_op(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in) :: lhs, rhs
+    integer :: lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  function not_pure(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in) :: lhs, rhs
+    integer :: lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function too_many_args(lhs, rhs, foo) result(lhs_op_rhs)
+    integer, intent(in) :: lhs, rhs, foo
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function array_result(lhs, rhs)
+    integer, intent(in) :: lhs, rhs
+    integer array_result(1)
+    array_result = lhs + rhs
+  end function
+
+  pure function allocatable_result(lhs, rhs)
+    integer, intent(in) :: lhs, rhs
+    integer, allocatable :: allocatable_result
+    allocatable_result = lhs + rhs
+  end function
+
+  pure function pointer_result(lhs, rhs)
+    integer, intent(in) :: lhs, rhs
+    integer, pointer :: pointer_result
+    allocate(pointer_result, source=lhs + rhs )
+  end function
+
+  pure function array_args(lhs, rhs)
+    integer, intent(in) :: lhs(1), rhs(1)
+    integer array_args
+    array_args = lhs(1) + rhs(1)
+  end function
+
+  pure function allocatable_args(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), allocatable :: lhs, rhs
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function pointer_args(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), pointer :: lhs, rhs
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function polymorphic_args(lhs, rhs) result(lhs_op_rhs)
+    class(foo_t), intent(in) :: lhs, rhs
+    type(foo_t) lhs_op_rhs
+    lhs_op_rhs%n = lhs%n + rhs%n
+  end function
+
+  pure function optional_args(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), optional :: lhs, rhs
+    integer lhs_op_rhs
+    if (present(lhs) .and. present(rhs)) then
+      lhs_op_rhs = lhs + rhs
+    else
+      lhs_op_rhs = 0
+    end if
+  end function
+
+  pure function target_mismatch(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), target  :: lhs
+    integer, intent(in) :: rhs
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function value_mismatch(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), value:: lhs
+    integer, intent(in) :: rhs
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+  pure function asynchronous_mismatch(lhs, rhs) result(lhs_op_rhs)
+    integer, intent(in), asynchronous:: lhs
+    integer, intent(in) :: rhs
+    integer lhs_op_rhs
+    lhs_op_rhs = lhs + rhs
+  end function
+
+end program