fix #40050, handling fields that are pointers due to subtype circularity (#40095)

(cherry picked from commit e3dffb9)

Author: JeffBezanson

src/cgutils.cpp

@@ -608,7 +608,10 @@ static Type *_julia_struct_to_llvm(jl_codegen_params_t *ctx, jl_value_t *jt, jl_
             size_t fsz = 0, al = 0;
             bool isptr = !jl_islayout_inline(ty, &fsz, &al);
             if (jst->layout) {
-                assert(isptr == jl_field_isptr(jst, i));
+                // NOTE: jl_field_isptr can disagree with jl_islayout_inline here if the
+                // struct decided this field must be a pointer due to a type circularity.
+                // Example from issue #40050: `struct B <: Ref{Tuple{B}}; end`
+                isptr = jl_field_isptr(jst, i);
                 assert((isptr ? sizeof(void*) : fsz + jl_is_uniontype(ty)) == jl_field_size(jst, i));
             }
             Type *lty;

test/core.jl

@@ -7224,6 +7224,11 @@ end
 @test_broken isbitstype(Tuple{B33954})
 @test_broken isbitstype(B33954)
 
+struct B40050 <: Ref{Tuple{B40050}}
+end
+@test string((B40050(),)) == "($B40050(),)"
+@test_broken isbitstype(Tuple{B40050})
+
 # Issue #34206/34207
 function mre34206(a, n)
     va = view(a, :)