[flang] Disallow pointer constants

None of the other Fortran compilers allow them.

Differential Revision: https://reviews.llvm.org/D89581

Author: psteinfeld

flang/lib/Semantics/check-declarations.cpp

@@ -1280,6 +1280,12 @@ void CheckHelper::CheckPointer(const Symbol &symbol) { // C852
   CheckConflicting(symbol, Attr::POINTER, Attr::TARGET);
   CheckConflicting(symbol, Attr::POINTER, Attr::ALLOCATABLE); // C751
   CheckConflicting(symbol, Attr::POINTER, Attr::INTRINSIC);
+  // Prohibit constant pointers.  The standard does not explicitly prohibit
+  // them, but the PARAMETER attribute requires a entity-decl to have an
+  // initialization that is a constant-expr, and the only form of
+  // initialization that allows a constant-expr is the one that's not a "=>"
+  // pointer initialization.  See C811, C807, and section 8.5.13.
+  CheckConflicting(symbol, Attr::POINTER, Attr::PARAMETER);
   if (symbol.Corank() > 0) {
     messages_.Say(
         "'%s' may not have the POINTER attribute because it is a coarray"_err_en_US,

flang/test/Semantics/resolve90.f90

@@ -1,10 +1,13 @@
 ! RUN: %S/test_errors.sh %s %t %f18
+! Testing for pointer constant, along with :
 ! C751 A component shall not have both the ALLOCATABLE and POINTER attributes.
 ! C752 If the CONTIGUOUS attribute is specified, the component shall be an 
 !   array with the POINTER attribute.
 ! C753 The * char-length option is permitted only if the component is of type 
 !   character.
 subroutine s()
+  !ERROR: 'nullint' may not have both the POINTER and PARAMETER attributes
+  integer, pointer, parameter :: nullint => null()
   type derivedType
     !ERROR: 'pointerallocatablefield' may not have both the POINTER and ALLOCATABLE attributes
     real, pointer, allocatable :: pointerAllocatableField