[flang] Accept NULL() actual for optional allocatable dummy argument (llvm#66251)

A NULL() pointer is an acceptable actual argument for association with an (absent) optional allocatable dummy argument. Semantics was unconditionally emitting an error that the actual argument is not allocatable.

Author: klausler

flang/lib/Semantics/check-call.cpp

@@ -657,8 +657,11 @@ static void CheckExplicitDataArg(const characteristics::DummyDataObject &dummy,
 
  // 15.5.2.6 -- dummy is ALLOCATABLE
  bool actualIsAllocatable{evaluate::IsAllocatableDesignator(actual)};
+ bool dummyIsOptional{
+ dummy.attrs.test(characteristics::DummyDataObject::Attr::Optional)};
+ bool actualIsNull{evaluate::IsNullPointer(actual)};
  if (dummyIsAllocatable) {
- if (!actualIsAllocatable) {
+ if (!actualIsAllocatable && !(actualIsNull && dummyIsOptional)) {
  messages.Say(
  "ALLOCATABLE %s must be associated with an ALLOCATABLE actual argument"_err_en_US,
  dummyName);
@@ -788,9 +791,6 @@ static void CheckExplicitDataArg(const characteristics::DummyDataObject &dummy,
  }
 
  // NULL(MOLD=) checking for non-intrinsic procedures
- bool dummyIsOptional{
- dummy.attrs.test(characteristics::DummyDataObject::Attr::Optional)};
- bool actualIsNull{evaluate::IsNullPointer(actual)};
  if (!intrinsic && !dummyIsPointer && !dummyIsOptional && actualIsNull) {
  messages.Say(
  "Actual argument associated with %s may not be null pointer %s"_err_en_US,

flang/test/Semantics/null01.f90

@@ -12,6 +12,9 @@ subroutine canbenull(x, y)
  integer, intent(in), optional :: x
  real, intent(in), pointer :: y
  end
+ subroutine optionalAllocatable(x)
+ integer, intent(in), allocatable, optional :: x
+ end
  function f0()
  real :: f0
  end function
@@ -95,6 +98,7 @@ function f3()
  dt4x = dt4(null(dt2x%pps0))
  call canbenull(null(), null()) ! fine
  call canbenull(null(mold=ip0), null(mold=rp0)) ! fine
+ call optionalAllocatable(null(mold=ip0)) ! fine
  !ERROR: Null pointer argument requires an explicit interface
  call implicit(null())
  !ERROR: Null pointer argument requires an explicit interface