parser: duplicate method declaration on interface (#5825)

vlib/v/checker/tests/no_interface_receiver_duplicate_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/no_interface_receiver_duplicate_a.v: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

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

vlib/v/checker/tests/no_interface_receiver_duplicate_b.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/no_interface_receiver_duplicate_b.v: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

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

vlib/v/parser/fn.v

@@ -196,7 +196,9 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		if language == .v && !p.pref.translated && util.contains_capital(name) {
 			p.error('function names cannot contain uppercase letters, use snake_case instead')
 		}
-		if is_method && p.table.get_type_symbol(rec_type).has_method(name) {
+		type_sym := p.table.get_type_symbol(rec_type)
+		// interfaces are handled in the checker, methods can not be defined on them this way
+		if is_method && (type_sym.has_method(name) && type_sym.kind != .interface_) {
 			p.error('duplicate method `$name`')
 		}
 	}
@@ -476,7 +478,8 @@ fn (mut p Parser) fn_args() ([]table.Arg, bool, bool) {
 						p.check_fn_mutable_arguments(typ, pos)
 					}
 				} else if is_shared || is_atomic {
-					p.error_with_pos('generic object cannot be `atomic` or `shared`', pos)
+					p.error_with_pos('generic object cannot be `atomic` or `shared`',
+						pos)
 				}
 				typ = typ.set_nr_muls(1)
 				if is_shared {
@@ -527,7 +530,8 @@ fn (mut p Parser) check_fn_mutable_arguments(typ table.Type, pos token.Position)
 fn (mut p Parser) check_fn_shared_arguments(typ table.Type, pos token.Position) {
 	sym := p.table.get_type_symbol(typ)
 	if sym.kind !in [.array, .struct_, .map, .placeholder] && !typ.is_ptr() {
-		p.error_with_pos('shared arguments are only allowed for arrays, maps, and structs\n', pos)
+		p.error_with_pos('shared arguments are only allowed for arrays, maps, and structs\n',
+			pos)
 	}
 }
 

vlib/v/parser/struct.v

@@ -359,12 +359,17 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	}
 	typ := table.new_type(reg_idx)
 	ts := p.table.get_type_symbol(typ)
+	// if methods were declared before, it's an error, ignore them
+	ts.methods.clear()
 	// Parse methods
 	mut methods := []ast.FnDecl{}
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
 		method_start_pos := p.tok.position()
 		line_nr := p.tok.line_nr
 		name := p.check_name()
+		if ts.has_method(name) {
+			p.error_with_pos('duplicate method `$name`', method_start_pos)
+		}
 		if util.contains_capital(name) {
 			p.error('interface methods cannot contain uppercase letters, use snake_case instead')
 		}

vlib/v/parser/tests/interface_duplicate_method.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/interface_duplicate_method.v:3:2: error: duplicate method `fun`
+    1 | interface Abc {
+    2 |     fun()
+    3 |     fun()
+      |     ~~~
+    4 | }

vlib/v/parser/tests/interface_duplicate_method.vv

@@ -0,0 +1,4 @@
+interface Abc {
+	fun()
+	fun()
+}