scanner: warn about interpolation for `$f(expr)` (#6179)

vlib/v/scanner/scanner.v

@@ -725,7 +725,11 @@ fn (mut s Scanner) text_scan() token.Token {
 			}
 			// end of `$expr`
 			// allow `'$a.b'` and `'$a.c()'`
-			if s.is_inter_start && next_char != `.` && next_char != `(` {
+			if s.is_inter_start && next_char == `(` {
+				if s.look_ahead(2) != `)` {
+					s.warn('use e.g. `\${f(expr)}` or `\$name\\(` instead of `\$f(expr)`')
+				}
+			} else if s.is_inter_start && next_char != `.` {
 				s.is_inter_end = true
 				s.is_inter_start = false
 			}
@@ -1220,14 +1224,14 @@ fn (mut s Scanner) ident_string() string {
 			}
 		}
 		// ${var} (ignore in vfmt mode)
-		if c == `{` && prevc == `$` && !is_raw && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {
+		if prevc == `$` && c == `{` && !is_raw && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {
 			s.is_inside_string = true
 			// so that s.pos points to $ at the next step
 			s.pos -= 2
 			break
 		}
 		// $var
-		if util.is_name_char(c) && prevc == `$` && !is_raw &&
+		if prevc == `$` && util.is_name_char(c) && !is_raw &&
 			s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {
 			s.is_inside_string = true
 			s.is_inter_start = true
@@ -1372,6 +1376,14 @@ fn (mut s Scanner) inc_line_number() {
 	}
 }
 
+pub fn (s &Scanner) warn(msg string) {
+	pos := token.Position{
+		line_nr: s.line_nr
+		pos: s.pos
+	}
+	eprintln(util.formatted_error('warning:', msg, s.file_path, pos))
+}
+
 pub fn (s &Scanner) error(msg string) {
 	pos := token.Position{
 		line_nr: s.line_nr

vlib/v/tests/string_interpolation_test.v

@@ -182,3 +182,12 @@ fn test_method_interpolation() {
 	assert '>${y.f().a}<' == '>2<'
 	assert '>$y.f().a<' == '>2<'
 }
+
+fn f(i int) int {
+	return i
+}
+
+fn test_call() {
+	s := '${f(4)}'
+	assert s == '4'
+}