Fix a regression that was exposed by PR llvm#164279, which also requires to rebox the RHS to the LHS type iif LHS is polymorphic.

Author: DanielCChen

flang/lib/Lower/Bridge.cpp

@@ -4778,8 +4778,7 @@ class FirConverter : public Fortran::lower::AbstractConverter {
       mlir::Value shape = builder->genShape(loc, lbounds, extents);
       rhsBox = fir::ReboxOp::create(*builder, loc, lhsBoxType, rhsBox, shape,
                                     /*slice=*/mlir::Value{});
-    } else if (fir::isClassStarType(lhsBoxType) &&
-               !fir::ConvertOp::canBeConverted(rhsBoxType, lhsBoxType)) {
+    } else if (fir::isPolymorphicType(lhsBoxType)) {
       rhsBox = fir::ReboxOp::create(*builder, loc, lhsBoxType, rhsBox,
                                     mlir::Value{}, mlir::Value{});
     }

flang/test/Lower/forall-polymorphic.f90

@@ -41,7 +41,7 @@ subroutine forallPolymorphic()
 ! CHECK: %[[V_43:[0-9]+]] = fir.field_index ptr, !fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>
 ! CHECK: %[[V_44:[0-9]+]] = fir.coordinate_of %[[V_42]], ptr : (!fir.class<!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>>) -> !fir.ref<!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>>>>>
 ! CHECK: %[[V_45:[0-9]+]] = fir.embox %[[V_18]](%[[V_17]]) : (!fir.ref<!fir.array<10x!fir.type<_QFforallpolymorphicTdt1{dt:!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>}>>>, !fir.shape<1>) -> !fir.box<!fir.ptr<!fir.array<10x!fir.type<_QFforallpolymorphicTdt1{dt:!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>}>>>>
-! CHECK: %[[V_46:[0-9]+]] = fir.convert %[[V_45]] : (!fir.box<!fir.ptr<!fir.array<10x!fir.type<_QFforallpolymorphicTdt1{dt:!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>}>>>>) -> !fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>>>>
+! CHECK: %[[V_46:[0-9]+]] = fir.rebox %[[V_45]] : (!fir.box<!fir.ptr<!fir.array<10x!fir.type<_QFforallpolymorphicTdt1{dt:!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>}>>>>) -> !fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>>>>
 ! CHECK: fir.store %[[V_46]] to %[[V_44]] : !fir.ref<!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt{ptr:!fir.class<!fir.ptr<!fir.array<?x!fir.type<_QFforallpolymorphicTdt>>>>}>>>>>
 ! CHECK: }