v.checker,v.gen: allow for &IAbc(voidptr(0)) where IAbc contains fields

vlib/v/checker/checker.v

@@ -2721,9 +2721,12 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 				}
 				continue
 			}
+			// voidptr is an escape hatch, it should be allowed to be passed
+			if utyp != ast.voidptr_type {
 				c.error("`$styp` doesn't implement field `$ifield.name` of interface `$inter_sym.name`",
 					pos)
 			}
+		}
 		inter_sym.info.types << utyp
 	}
 	return true

vlib/v/gen/c/cgen.v

@@ -6549,6 +6549,7 @@ fn (mut g Gen) interface_table() string {
 					} else {
 						// the field is embedded in another struct
 						cast_struct.write_string('\t\t.$cname = ($field_styp*)((char*)x')
+						if st != ast.voidptr_type {
 							for embed_type in st_sym.struct_info().embeds {
 								embed_sym := g.table.get_type_symbol(embed_type)
 								if _ := embed_sym.find_field(field.name) {
@@ -6556,6 +6557,7 @@ fn (mut g Gen) interface_table() string {
 									break
 								}
 							}
+						}
 						cast_struct.writeln('),')
 					}
 				}

vlib/v/tests/interface_edge_cases/voidptr_casted_as_an_interface_test.v

@@ -0,0 +1,26 @@
+interface IAbc {
+	name string
+	xyz()
+}
+
+struct Abc {
+	name string
+	x    int = 123
+}
+
+fn (a Abc) xyz() {}
+
+fn f(i &IAbc) string {
+	return '$i'
+}
+
+fn test_voidptr_casted_as_an_interface_reference() {
+	mut pi := &IAbc(voidptr(0))
+	dump(pi)
+	assert f(pi) == '&IAbc(0)'
+	//
+	i := IAbc(Abc{})
+	pi = &i
+	dump(pi)
+	assert f(pi).contains('x: 123')
+}