checker: fix generics with nested generic fn inst call (#13059)

vlib/v/checker/fn.v

@@ -957,7 +957,14 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			node.concrete_list_pos)
 	}
 	if func.generic_names.len > 0 {
-		return node.return_type
+		if has_generic {
+			return node.return_type
+		} else if typ := c.table.resolve_generic_to_concrete(func.return_type, func.generic_names,
+			concrete_types)
+		{
+			node.return_type = typ
+			return typ
+		}
 	}
 	return func.return_type
 }

vlib/v/tests/generics_with_nested_generics_fn_inst_call_test.v

@@ -0,0 +1,30 @@
+fn test_generics_with_nested_generic_fn_inst_call() {
+	value := [byte(105), 116, 32, 119, 111, 114, 107, 115, 33, 33]
+	decoded := decode_arr<string>(value)
+	dump(decoded)
+	assert decoded.len == 4
+	assert decoded[0] == 'it works!!'
+	assert decoded[1] == 'it works!!'
+	assert decoded[2] == 'it works!!'
+	assert decoded[3] == 'it works!!'
+}
+
+fn decode_arr<T>(buf []byte) []T {
+	arr_size := decode<u32>(buf[0..4])
+	mut ret := []T{cap: int(arr_size)}
+
+	for _ in 0 .. arr_size {
+		ret << decode<T>(buf[..])
+	}
+	return ret
+}
+
+fn decode<T>(buf []byte) T {
+	$if T is u32 {
+		return u32(buf.len)
+	} $else $if T is string {
+		return buf.bytestr()
+	} $else {
+		panic('"$T.name" is not supported for `decode`')
+	}
+}