checker: add error when interface i, without a .str() method, have i.str() called (#5788)

vlib/v/checker/checker.v

@@ -936,6 +936,10 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 	}
 	// TODO: str methods
 	if method_name == 'str' {
+		if left_type_sym.kind == .interface_ {
+			iname := left_type_sym.name
+			c.error('interface `$iname` does not have a .str() method. Use typeof() instead', call_expr.pos)
+		}
 		call_expr.receiver_type = left_type
 		call_expr.return_type = table.string_type
 		if call_expr.args.len > 0 {

vlib/v/checker/tests/no_interface_str.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/no_interface_str.v:18:12: error: interface `Animal` does not have a .str() method. Use typeof() instead
+   16 | fn moin() {
+   17 |     a := get_animal()
+   18 |     println(a.str())
+      |               ~~~~~
+   19 | }

vlib/v/checker/tests/no_interface_str.vv

@@ -0,0 +1,19 @@
+interface Animal {
+	speak()
+}
+
+struct Cow {
+}
+
+fn (c Cow)speak() {
+	println('moo')
+}
+
+fn get_animal() Animal {
+	return Cow{}
+}
+
+fn moin() {
+	a := get_animal()
+	println(a.str())
+}