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

vlib/v/checker/checker.v

@@ -2721,8 +2721,11 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 				}
 				continue
 			}
-			c.error("`$styp` doesn't implement field `$ifield.name` of interface `$inter_sym.name`",
-				pos)
+			// voidptr is an escape hatch, it should be allowed to be passed
+			if utyp != ast.voidptr_type {
+				c.error("`$styp` doesn't implement field `$ifield.name` of interface `$inter_sym.name`",
+					pos)
+			}
 		}
 		inter_sym.info.types << utyp
 	}

vlib/v/gen/c/cgen.v

@@ -6549,11 +6549,13 @@ 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')
-						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) {
-								cast_struct.write_string(' + __offsetof_ptr(x, $cctype, $embed_sym.embed_name()) + __offsetof_ptr(x, $embed_sym.cname, $cname)')
-								break
+						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) {
+									cast_struct.write_string(' + __offsetof_ptr(x, $cctype, $embed_sym.embed_name()) + __offsetof_ptr(x, $embed_sym.cname, $cname)')
+									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')
+}