all: implement `type_name()` for interfaces too (#8767)

vlib/v/checker/checker.v

@@ -1356,7 +1356,7 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 		c.error('optional type cannot be called directly', call_expr.left.position())
 		return table.void_type
 	}
-	if left_type_sym.kind == .sum_type && method_name == 'type_name' {
+	if left_type_sym.kind in [.sum_type, .interface_] && method_name == 'type_name' {
 		return table.string_type
 	}
 	mut has_generic_generic := false // x.foo<T>() instead of x.foo<int>()

vlib/v/gen/c/cgen.v

@@ -484,7 +484,7 @@ pub fn (mut g Gen) finish() {
 
 pub fn (mut g Gen) write_typeof_functions() {
 	g.writeln('')
-	g.writeln('// >> typeof() support for sum types')
+	g.writeln('// >> typeof() support for sum types / interfaces')
 	for typ in g.table.types {
 		if typ.kind == .sum_type {
 			sum_info := typ.info as table.SumType
@@ -508,6 +508,15 @@ pub fn (mut g Gen) write_typeof_functions() {
 				g.writeln('\t}')
 			}
 			g.writeln('}')
+		} else if typ.kind == .interface_ {
+			inter_info := typ.info as table.Interface
+			g.writeln('static char * v_typeof_interface_${typ.cname}(int sidx) { /* $typ.name */ ')
+			for t in inter_info.types {
+				subtype := g.table.get_type_symbol(t)
+				g.writeln('\tif (sidx == _${typ.cname}_${subtype.cname}_index) return "${util.strip_main_name(subtype.name)}";')
+			}
+			g.writeln('\treturn "unknown ${util.strip_main_name(typ.name)}";')
+			g.writeln('}')
 		}
 	}
 	g.writeln('// << typeof() support for sum types')

vlib/v/gen/c/fn.v

@@ -537,6 +537,12 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 		g.write(').typ ))')
 		return
 	}
+	if left_sym.kind == .interface_ && node.name == 'type_name' {
+		g.write('tos3( /* $left_sym.name */ v_typeof_interface_${typ_sym.cname}( (')
+		g.expr(node.left)
+		g.write(')._interface_idx ))')
+		return
+	}
 	if node.name == 'str' {
 		g.gen_str_for_type(node.receiver_type)
 	}

vlib/v/parser/struct.v

@@ -475,6 +475,10 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 			method_start_pos := p.tok.position()
 			line_nr := p.tok.line_nr
 			name := p.check_name()
+			if name == 'type_name' {
+				p.error_with_pos('cannot override built-in method `type_name`', method_start_pos)
+				return ast.InterfaceDecl{}
+			}
 			if ts.has_method(name) {
 				p.error_with_pos('duplicate method `$name`', method_start_pos)
 				return ast.InterfaceDecl{}