checker: fix generics with pointer index (fix #15810) (#15815)

vlib/v/checker/checker.v

@@ -3485,8 +3485,12 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 			node.is_farray = true
 		}
 		.any {
-			typ = c.unwrap_generic(typ)
+			unwrapped_typ := c.unwrap_generic(typ)
-			typ_sym = c.table.final_sym(typ)
+			unwrapped_sym := c.table.final_sym(unwrapped_typ)
+			if unwrapped_sym.kind in [.map, .array, .array_fixed] {
+				typ = unwrapped_typ
+				typ_sym = unwrapped_sym
+			}
 		}
 		else {}
 	}

vlib/v/tests/generics_with_pointer_index_test.v

@@ -0,0 +1,62 @@
+module main
+
+pub struct Vec<T> {
+mut:
+	data &T
+	cap  int
+	len  int
+}
+
+pub struct Iter<T> {
+mut:
+	v   &Vec<T>
+	pos int
+}
+
+pub fn with_cap<T>(cap int) Vec<T> {
+	new_data := unsafe { C.malloc(cap * int(sizeof(T))) }
+	unsafe { C.memset(new_data, 0, cap * int(sizeof(T))) }
+
+	return Vec<T>{
+		data: new_data
+		cap: cap
+		len: 0
+	}
+}
+
+pub fn (ar &Vec<T>) iter() Iter<T> {
+	return Iter<T>{
+		v: unsafe { ar }
+	}
+}
+
+pub fn (mut iter Iter<T>) next() ?&T {
+	if iter.pos >= iter.v.len {
+		return none
+	}
+	res := unsafe { &iter.v.data[iter.pos] }
+	iter.pos++
+	return res
+}
+
+pub fn (mut ar Vec<T>) push(elm T) {
+	unsafe {
+		ar.data[ar.len - 1] = elm
+	}
+}
+
+struct Product {
+	price f64
+}
+
+fn test_generic_with_pointer_index() {
+	vec1 := with_cap<Product>(5)
+	println(vec1)
+	assert vec1.len == 0
+	assert vec1.cap == 5
+
+	vec2 := with_cap<int>(5)
+	println(vec2)
+	assert vec2.len == 0
+	assert vec2.cap == 5
+}