checker: fix json decoder with generic struct (#14700)

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'
+}

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 {