checker: fix interpolation recursive str (fix #1905) (#7141)

vlib/v/checker/check_types.v

@@ -390,6 +390,10 @@ pub fn (mut c Checker) string_inter_lit(mut node ast.StringInterLiteral) table.T
 			}
 			node.need_fmts[i] = fmt != c.get_default_fmt(ftyp, typ)
 		}
+		// check recursive str
+		if c.cur_fn.is_method && c.cur_fn.name == 'str' && c.cur_fn.receiver.name == expr.str() {
+			c.error('cannot call `str()` method recursively', expr.position())
+		}
 	}
 	return table.string_type
 }

vlib/v/checker/tests/interpolation_recursive_str_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/interpolation_recursive_str_err.vv:8:8: error: cannot call `str()` method recursively
+    6 |
+    7 | fn (t Test) str() string {
+    8 |     _ = '$t'
+      |           ^
+    9 |     return 'test'
+   10 | }

vlib/v/checker/tests/interpolation_recursive_str_err.vv

@@ -0,0 +1,15 @@
+module main
+
+struct Test {
+	a int
+}
+
+fn (t Test) str() string {
+	_ = '$t'
+	return 'test'
+}
+
+fn main() {
+	a := Test{}
+	println(a)
+}