cgen: fix generic for_in using iteration (#15979)

vlib/v/gen/c/for.v

@@ -358,7 +358,7 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 			g.writeln('for (size_t $node.key_var = 0;; ++$node.key_var) {')
 		}
 		t_var := g.new_tmp_var()
-		receiver_typ := next_fn.params[0].typ
+		receiver_typ := g.unwrap_generic(next_fn.params[0].typ)
 		receiver_styp := g.typ(receiver_typ)
 		mut fn_name := receiver_styp.replace_each(['*', '', '.', '__']) + '_next'
 		receiver_sym := g.table.sym(receiver_typ)

vlib/v/tests/generics_for_in_iterate_test.v

@@ -0,0 +1,50 @@
+pub struct Vec<T> {
+mut:
+	data &T    [required]
+	cap  usize [required]
+	len  usize [required]
+}
+
+pub fn new<T>() Vec<T> {
+	return Vec<T>{
+		data: unsafe { nil }
+		cap: 0
+		len: 0
+	}
+}
+
+pub fn (ar &Vec<T>) iter() Iter<T> {
+	return Iter<T>{
+		v: unsafe { ar }
+	}
+}
+
+pub struct Iter<T> {
+mut:
+	v   &Vec<T> [required]
+	pos usize
+}
+
+pub fn (mut iter Iter<T>) next() ?&T {
+	if iter.pos >= iter.v.len {
+		return none
+	}
+	defer {
+		iter.pos++
+	}
+	return unsafe { &iter.v.data[iter.pos] }
+}
+
+fn test_generics_for_in_iterate() {
+	mut goods := new<int>()
+	goods.call_generic_fn(fn (a &int) bool {
+		return *a > 1
+	})
+	assert true
+}
+
+fn (arr Vec<T>) call_generic_fn(cb fn (&T) bool) {
+	for val in arr.iter() {
+		println(cb(val))
+	}
+}