checker: fix struct init update with generics (fix #20136) (#20139)

vlib/v/checker/struct.v

@@ -874,9 +874,10 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 	if node.has_update_expr {
 		update_type := c.expr(mut node.update_expr)
 		node.update_expr_type = update_type
+		expr_sym := c.table.final_sym(c.unwrap_generic(update_type))
 		if node.update_expr is ast.ComptimeSelector {
 			c.error('cannot use struct update syntax in compile time expressions', node.update_expr_pos)
-		} else if c.table.final_sym(update_type).kind != .struct_ {
+		} else if expr_sym.kind != .struct_ {
 			s := c.table.type_to_str(update_type)
 			c.error('expected struct, found `${s}`', node.update_expr.pos())
 		} else if update_type != node.typ {

vlib/v/tests/struct_init_update_with_generics_test.v

@@ -0,0 +1,20 @@
+struct Foo {
+	a      int
+	b      string
+	isbool bool
+}
+
+fn do[T](f T) T {
+	return T{
+		...f
+		isbool: true
+	}
+}
+
+fn test_main() {
+	foo := do(Foo{
+		a: 1
+		b: '2'
+	})
+	assert foo.a == 1 && foo.b == '2' && foo.isbool
+}