[flang] Avoid crash in lowering for unlimited polymorphic function return

[clementval]

Summary

The dynamic type of an unlimited polymorphic entity has the
derived category but does not have derived type spec. This leads
to a crash for a nullptr dereference. This patch avoids this crash
by checking if that the dynamic type is not unlimited polymorphic
before dereferencing the derived type spec.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/CallInterface.cpp

const Fortran::evaluate::characteristics::TypeAndShape *typeAndShape =
       result.GetTypeAndShape();
   assert(typeAndShape && "no result type");
   Fortran::evaluate::DynamicType dynamicType = typeAndShape->type();
   // Visit result length specification expressions that are explicit.
   if (dynamicType.category() == Fortran::common::TypeCategory::Character) {
     if (std::optional<Fortran::evaluate::ExtentExpr> length =
             dynamicType.GetCharLength())
       visitor(toEvExpr(*length));
-  } else if (dynamicType.category() == common::TypeCategory::Derived) {
+  } else if (dynamicType.category() == common::TypeCategory::Derived &&
+             !dynamicType.IsUnlimitedPolymorphic()) {
     const Fortran::semantics::DerivedTypeSpec &derivedTypeSpec =
         dynamicType.GetDerivedTypeSpec();
     if (Fortran::semantics::CountLenParameters(derivedTypeSpec) > 0)
       TODO(converter.getCurrentLocation(),
            "function result with derived type length parameters");
   }
 }
 

flang/test/Lower/polymorphic.f90

subroutine polymorphic_to_nonpolymorphic(p)
     class(p1), pointer :: p(:)
     type(p1), allocatable, target :: t(:)
     t = p
   end subroutine
 
 ! CHECK-LABEL: func.func @_QMpolymorphic_testPpolymorphic_to_nonpolymorphic
 ! Just checking that FIR is generated without error.
 
+! Test that lowering does not crash for function return with unlimited
+! polymoprhic value.
+
+  function up_ret()
+    class(*), pointer :: up_ret(:)
+  end function
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPup_ret() -> !fir.class<!fir.ptr<!fir.array<?xnone>>> {
+
+  subroutine call_up_ret()
+    class(*), pointer :: p(:)
+    p => up_ret()
+  end subroutine
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPcall_up_ret() {
+! CHECK:         %{{.*}} = fir.call @_QMpolymorphic_testPup_ret() {{.*}} : () -> !fir.class<!fir.ptr<!fir.array<?xnone>>>
+
 end module