Fix layout flags for types that have oddly sized primitive type fields

Author: gbaraldi

src/datatype.c

@@ -702,6 +702,11 @@ void jl_compute_field_offsets(jl_datatype_t *st)
                     // Should never happen
                     throw_ovf(should_malloc, desc, st, fsz);
                 desc[i].isptr = 0;
+                if (fsz > jl_datatype_size(fld)) {
+                    // We have to pad the size to integer size class, but it means this has some padding
+                    isbitsegal = 0;
+                    haspadding = 1;
+                }
                 if (jl_is_uniontype(fld)) {
                     fsz += 1; // selector byte
                     zeroinit = 1;

test/core.jl

@@ -8534,3 +8534,9 @@ module GlobalBindingMulti
     using .M.C
 end
 @test GlobalBindingMulti.S === GlobalBindingMulti.M.C.S
+
+#58434 bitsegal comparison of oddly sized fields
+primitive type ByteString58434 (18 * 8) end
+
+@test Base.datatype_isbitsegal(Tuple{ByteString58434}) == false
+@test Base.datatype_haspadding(Tuple{ByteString58434}) == true