checker: don't disallow defining methods on interfaces (#8335)

vlib/v/checker/checker.v

@@ -1879,7 +1879,12 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 	typ_sym := c.table.get_type_symbol(typ)
 	mut inter_sym := c.table.get_type_symbol(inter_typ)
 	styp := c.table.type_to_str(typ)
-	for imethod in inter_sym.methods {
+	imethods := if inter_sym.kind == .interface_ {
+		(inter_sym.info as table.Interface).methods
+	} else {
+		inter_sym.methods
+	}
+	for imethod in imethods {
 		if method := typ_sym.find_method(imethod.name) {
 			msg := c.table.is_same_method(imethod, method)
 			if msg.len > 0 {
@@ -5263,9 +5268,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	}
 	if node.is_method {
 		mut sym := c.table.get_type_symbol(node.receiver.typ)
-		if sym.kind == .interface_ {
-			c.error('interfaces cannot be used as method receiver', node.receiver_pos)
-		} else if sym.kind == .array && !c.is_builtin_mod && node.name == 'map' {
+		if sym.kind == .array && !c.is_builtin_mod && node.name == 'map' {
 			// TODO `node.map in array_builtin_methods`
 			c.error('method overrides built-in array method', node.pos)
 		} else if sym.kind == .sum_type && node.name == 'type_name' {

vlib/v/checker/tests/no_interface_receiver.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/no_interface_receiver.vv:5:5: error: interfaces cannot be used as method receiver
-    3 | }
-    4 |
-    5 | fn (a Animal) str() {}
-      |     ~~~~~~~~
-

vlib/v/checker/tests/no_interface_receiver.vv

@@ -1,5 +0,0 @@
-interface Animal {
-	speak()
-}
-
-fn (a Animal) str() {}

vlib/v/checker/tests/no_interface_receiver_duplicate_a.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/no_interface_receiver_duplicate_a.vv:5:5: error: interfaces cannot be used as method receiver
-    3 | }
-    4 |
-    5 | fn (a Abc) fun() {
-      |     ~~~~~
-    6 | }

vlib/v/checker/tests/no_interface_receiver_duplicate_a.vv

@@ -1,6 +0,0 @@
-interface Abc {
-	fun()
-}
-
-fn (a Abc) fun() {
-}

vlib/v/checker/tests/no_interface_receiver_duplicate_b.out

@@ -1,5 +0,0 @@
-vlib/v/checker/tests/no_interface_receiver_duplicate_b.vv:1:5: error: interfaces cannot be used as method receiver
-    1 | fn (a Abc) fun() {
-      |     ~~~~~
-    2 | }
-    3 |

vlib/v/checker/tests/no_interface_receiver_duplicate_b.vv

@@ -1,6 +0,0 @@
-fn (a Abc) fun() {
-}
-
-interface Abc {
-	fun()
-}

vlib/v/checker/tests/no_method_on_interface_propagation.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/no_method_on_interface_propagation.vv:18:5: error: unknown method: `Cat.foo`
+   16 |     mut a := new_animal('persian')
+   17 |     if a is Cat {
+   18 |         a.foo()
+      |           ~~~~~
+   19 |     }
+   20 | }

vlib/v/checker/tests/no_method_on_interface_propagation.vv

@@ -0,0 +1,20 @@
+struct Cat {
+	breed string
+}
+
+interface Animal {
+	breed string
+}
+
+fn (a Animal) foo() {}
+
+fn new_animal(breed string) Animal {
+	return &Cat{breed}
+}
+
+fn test_methods_on_interfaces_dont_exist_on_implementers() {
+	mut a := new_animal('persian')
+	if a is Cat {
+		a.foo()
+	}
+}

vlib/v/gen/cgen.v

@@ -2923,7 +2923,7 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 		g.writeln('(*($opt_base_typ*)')
 	}
 	if sym.kind == .interface_ {
-		g.write('*(')
+		g.write('(*(')
 	}
 	if sym.kind == .array_fixed {
 		assert node.field_name == 'len'
@@ -3002,7 +3002,7 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 		g.write('$sum_type_dot$sum_type_deref_field)')
 	}
 	if sym.kind == .interface_ {
-		g.write(')')
+		g.write('))')
 	}
 }
 
@@ -5903,7 +5903,7 @@ fn (mut g Gen) interface_table() string {
 		methods_struct_def.writeln('$methods_struct_name {')
 		mut imethods := map[string]string{} // a map from speak -> _Speaker_speak_fn
 		mut methodidx := map[string]int{}
-		for k, method in ityp.methods {
+		for k, method in inter_info.methods {
 			methodidx[method.name] = k
 			typ_name := '_${interface_name}_${method.name}_fn'
 			ret_styp := g.typ(method.return_type)
@@ -6049,7 +6049,7 @@ $staticprefix $interface_name* I_${cctype}_to_Interface_${interface_name}_ptr($c
 		}
 		// add line return after interface index declarations
 		sb.writeln('')
-		if ityp.methods.len > 0 {
+		if inter_info.methods.len > 0 {
 			sb.writeln(methods_wrapper.str())
 			sb.writeln(methods_typ_def.str())
 			sb.writeln(methods_struct_def.str())

vlib/v/gen/fn.v

@@ -343,7 +343,7 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 	typ_sym := g.table.get_type_symbol(g.unwrap_generic(node.receiver_type))
 	// mut receiver_type_name := util.no_dots(typ_sym.name)
 	mut receiver_type_name := util.no_dots(g.cc_type2(g.unwrap_generic(node.receiver_type)))
-	if typ_sym.kind == .interface_ {
+	if typ_sym.kind == .interface_ && (typ_sym.info as table.Interface).defines_method(node.name) {
 		// Speaker_name_table[s._interface_idx].speak(s._object)
 		$if debug_interface_method_call ? {
 			eprintln('>>> interface typ_sym.name: $typ_sym.name | receiver_type_name: $receiver_type_name')

vlib/v/parser/struct.v

@@ -453,6 +453,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	}
 	typ := table.new_type(reg_idx)
 	mut ts := p.table.get_type_symbol(typ)
+	mut info := ts.info as table.Interface
 	// if methods were declared before, it's an error, ignore them
 	ts.methods = []table.Fn{cap: 20}
 	// Parse fields or methods
@@ -470,7 +471,6 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			is_mut = true
 		}
 		if p.peek_tok.kind == .lpar {
-			ts = p.table.get_type_symbol(typ) // removing causes memory bug visible by `v -silent test-fmt`
 			method_start_pos := p.tok.position()
 			line_nr := p.tok.line_nr
 			name := p.check_name()
@@ -511,13 +511,15 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			method.next_comments = mnext_comments
 			methods << method
 			// println('register method $name')
-			ts.register_method(
+			tmethod := table.Fn{
 				name: name
 				params: args
 				return_type: method.return_type
 				is_variadic: is_variadic
 				is_pub: true
-			)
+			}
+			ts.register_method(tmethod)
+			info.methods << tmethod
 		} else {
 			// interface fields
 			field_pos := p.tok.position()
@@ -540,16 +542,15 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 				comments: comments
 				is_public: true
 			}
-			mut info := ts.info as table.Interface
 			info.fields << table.Field{
 				name: field_name
 				typ: field_typ
 				is_pub: true
 				is_mut: is_mut
 			}
+		}
+	}
 	ts.info = info
-		}
-	}
 	p.top_level_statement_end()
 	p.check(.rcbr)
 	pos.update_last_line(p.prev_tok.line_nr)

vlib/v/table/types.v

@@ -658,6 +658,7 @@ pub struct Interface {
 pub mut:
 	types   []Type
 	fields  []Field
+	methods []Fn
 }
 
 pub struct Enum {
@@ -991,3 +992,12 @@ pub fn (s Struct) get_field(name string) Field {
 	}
 	panic('unknown field `$name`')
 }
+
+pub fn (i Interface) defines_method(name string) bool {
+	for method in i.methods {
+		if method.name == name {
+			return true
+		}
+	}
+	return false
+}

vlib/v/tests/methods_on_interfaces_test.v

@@ -0,0 +1,20 @@
+struct Cat {
+	breed string
+}
+
+interface Animal {
+	breed string
+}
+
+fn (a Animal) info() string {
+	return "I'm a ${a.breed} ${typeof(a).name}"
+}
+
+fn new_animal(breed string) Animal {
+	return &Cat{ breed }
+}
+
+fn test_methods_on_interfaces() {
+	mut a := new_animal('persian')
+	assert a.info() == "I'm a persian Animal"
+}