cgen: fix thread struct name for same fn var name on different scope

vlib/v/gen/c/spawn_and_go.v

@@ -30,6 +30,11 @@ fn (mut g Gen) spawn_and_go_expr(node ast.SpawnExpr, mode SpawnGoMode) {
 	mut use_tmp_fn_var := false
 	tmp_fn := g.new_tmp_var()
 
+	if expr.is_fn_var {
+		// generate a name different for same var fn name declared in another scope
+		name = '${name}_${node.pos.pos}'
+	}
+
 	if expr.concrete_types.len > 0 {
 		name = g.generic_fn_name(expr.concrete_types, name)
 	} else if expr.is_fn_var && expr.fn_var_type.has_flag(.generic) {

vlib/v/tests/chan_same_fn_name_test.v

@@ -0,0 +1,24 @@
+fn a() chan string {
+	ch_out := chan string{}
+	f := fn (a chan string) {
+		a <- 'foo'
+	}
+	spawn f(ch_out)
+	return ch_out
+}
+
+fn b(ch_in chan string) string {
+	f := fn (a chan string, b chan string) {
+		val := <-a
+		{}
+		b <- val
+	}
+	ch_out := chan string{}
+	spawn f(ch_in, ch_out)
+	return <-ch_out
+}
+
+fn test_main() {
+	ch0 := a()
+	assert b(ch0) == 'foo'
+}