From abf35270cf0114fbb5436a0c1e3b7c4521f28789 Mon Sep 17 00:00:00 2001
From: yuyi <yuyi98@163.com>
Date: Tue, 7 Jun 2022 00:25:02 +0800
Subject: [PATCH] checker: fix json decoder with generic struct (#14700)

---
 vlib/json/json_decode_with_generic_test.v | 24 +++++++++++++++++++++++
 vlib/v/checker/fn.v                       |  7 ++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 vlib/json/json_decode_with_generic_test.v

diff --git a/vlib/json/json_decode_with_generic_test.v b/vlib/json/json_decode_with_generic_test.v
new file mode 100644
index 0000000000..0ec358cca3
--- /dev/null
+++ b/vlib/json/json_decode_with_generic_test.v
@@ -0,0 +1,24 @@
+import json
+
+struct Result<T> {
+	ok     bool
+	result T
+}
+
+struct User {
+	id       int
+	username string
+}
+
+fn func<T>() ?T {
+	text := '{"ok": true, "result":{"id":37467243, "username": "ciao"}}'
+	a := json.decode(Result<T>, text)?
+	return a.result
+}
+
+fn test_decode_with_generic_struct() ? {
+	ret := func<User>()?
+	println(ret)
+	assert ret.id == 37467243
+	assert ret.username == 'ciao'
+}
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 7f6577be54..746fa7b013 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -506,7 +506,12 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 		}
 		expr := node.args[0].expr
 		if expr is ast.TypeNode {
-			sym := c.table.sym(c.unwrap_generic(expr.typ))
+			mut unwrapped_typ := c.unwrap_generic(expr.typ)
+			if c.table.sym(expr.typ).kind == .struct_ && expr.typ.has_flag(.generic) {
+				unwrapped_typ = c.table.unwrap_generic_type(expr.typ, c.table.cur_fn.generic_names,
+					c.table.cur_concrete_types)
+			}
+			sym := c.table.sym(unwrapped_typ)
 			if c.table.known_type(sym.name) && sym.kind != .placeholder {
 				mut kind := sym.kind
 				if sym.info is ast.Alias {