[flang] Check ArrayRef base for contiguity in IsSimplyContiguousHelper

Previous code was returning true for `x(:)` where x is a pointer without
the contiguous attribute.
In case the array ref is a whole array section, check the base for contiguity
to solve the issue.

Differential Revision: https://reviews.llvm.org/D114084

Author: jeanPerier

flang/lib/Evaluate/check-expression.cpp

@@ -678,8 +678,15 @@ class IsSimplyContiguousHelper
     if (!(*this)(symbol).has_value()) {
       return false;
     } else if (auto rank{CheckSubscripts(x.subscript())}) {
-      // a(:)%b(1,1) is not contiguous; a(1)%b(:,:) is
-      return *rank > 0 || x.Rank() == 0;
+      if (x.Rank() == 0) {
+        return true;
+      } else if (*rank > 0) {
+        // a(1)%b(:,:) is contiguous if an only if a(1)%b is contiguous.
+        return (*this)(x.base());
+      } else {
+        // a(:)%b(1,1) is not contiguous.
+        return false;
+      }
     } else {
       return false;
     }

flang/test/Semantics/assign03.f90

@@ -277,6 +277,8 @@ subroutine s12
     logical, parameter :: l4 = is_contiguous(x%a(:,v))
     !ERROR: Must be a constant value
     logical, parameter :: l5 = is_contiguous(y(v,1)%a(1,1))
+    !ERROR: Must be a constant value
+    logical, parameter :: l6 = is_contiguous(p(:))
   end
   subroutine test3(b)
     integer, intent(inout) :: b(..)