[flang] Reverse a reversed type compatibility check

[klausler]

Summary

The semantic test for an intrinsic assignment to a polymorphic
derived type entity from a type that is an extension of its base
type was reversed, so it would allow assignments that it shouldn't
and disallowed some that it should; and the test case for it
incorectly assumed that the invalid semantics were correct.
Fix the code and the test, and add a new test for the invalid
case (LHS type is an extension of the RHS type).

flang/lib/Semantics/tools.cpp

 }
 
 // 7.5.2.4 "same derived type" test -- rely on IsTkCompatibleWith() and its
 // infrastructure to detect and handle comparisons on distinct (but "same")
 // sequence/bind(C) derived types
 static bool MightBeSameDerivedType(
     const std::optional<evaluate::DynamicType> &lhsType,
     const std::optional<evaluate::DynamicType> &rhsType) {
-  return lhsType && rhsType && rhsType->IsTkCompatibleWith(*lhsType);
+  return lhsType && rhsType && lhsType->IsTkCompatibleWith(*rhsType);
 }
 
 Tristate IsDefinedAssignment(
     const std::optional<evaluate::DynamicType> &lhsType, int lhsRank,
     const std::optional<evaluate::DynamicType> &rhsType, int rhsRank) {
   if (!lhsType || !rhsType) {
     return Tristate::No; // error or rhs is untyped
   }

flang/test/Semantics/selecttype03.f90

contains
   function foo(i)
     integer :: i
     class(t1),DIMENSION(:),allocatable :: foo
     integer, dimension(2) :: U
     U = (/ 1,2 /)
     if (i>0) then
       foo = array1(2,U)
     else if (i<0) then
-      !ERROR: No intrinsic or user-defined ASSIGNMENT(=) matches operand types CLASS(t1) and CLASS(t2)
-      foo = array2(2,U)
+      foo = array2(2,U) ! ok: t2 extends t1
     end if
   end function
 
+  function foo2()
+    class(t2),DIMENSION(:),allocatable :: foo2
+    !ERROR: No intrinsic or user-defined ASSIGNMENT(=) matches operand types CLASS(t2) and CLASS(t1)
+    foo2 = array1(2,:)
+  end function
+
   subroutine sub_with_in_and_inout_param(y, z)
     type(t2), INTENT(IN) :: y
     class(t2), INTENT(INOUT) :: z
     z%i = 10
   end subroutine
 
   subroutine sub_with_in_and_inout_param_vector(y, z)
     type(t2),DIMENSION(:), INTENT(IN) :: y
     class(t2),DIMENSION(:), INTENT(INOUT) :: z
     z%i = 10
   end subroutine
 
 end