From 7d6ba2d07dc4e97c5054f2b260423cce564c929d Mon Sep 17 00:00:00 2001
From: pancake <pancake@nowsecure.com>
Date: Fri, 10 Jul 2020 21:47:29 +0200
Subject: [PATCH] checker: add error when interface i, without a .str() method,
 have i.str() called (#5788)

---
 vlib/v/checker/checker.v                  |  4 ++++
 vlib/v/checker/tests/no_interface_str.out |  6 ++++++
 vlib/v/checker/tests/no_interface_str.vv  | 19 +++++++++++++++++++
 3 files changed, 29 insertions(+)
 create mode 100644 vlib/v/checker/tests/no_interface_str.out
 create mode 100644 vlib/v/checker/tests/no_interface_str.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 9fb0fb24e7..599ef708b4 100644
--- a/vlib/v/checker/checker.v
+++ b/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 {
diff --git a/vlib/v/checker/tests/no_interface_str.out b/vlib/v/checker/tests/no_interface_str.out
new file mode 100644
index 0000000000..871c9a8c7d
--- /dev/null
+++ b/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 | }
diff --git a/vlib/v/checker/tests/no_interface_str.vv b/vlib/v/checker/tests/no_interface_str.vv
new file mode 100644
index 0000000000..b0da84aa33
--- /dev/null
+++ b/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())
+}