From d9eff3494e05a137039a33fee267561b1b9dfe15 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:20 +0300
Subject: [PATCH] checker: fix sumtype else branch of if !is should consider
 smartcast (fixes #13587)

---
 vlib/v/checker/if.v                            |  3 +++
 .../sumtype_not_is_else_smartcast_test.v       | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)
 create mode 100644 vlib/v/tests/sumtypes/sumtype_not_is_else_smartcast_test.v

diff --git a/vlib/v/checker/if.v b/vlib/v/checker/if.v
index 182915222..a86231779 100644
--- a/vlib/v/checker/if.v
+++ b/vlib/v/checker/if.v
@@ -751,6 +751,9 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 					c.smartcast(mut first_cond.left, first_cond.left_type, first_cond.left_type.clear_flag(.option), mut
 						scope, false, true, false)
 				}
+			} else if first_cond.left in [ast.Ident, ast.SelectorExpr] && first_cond.op == .not_is {
+				c.smartcast(mut first_cond.left, first_cond.left_type, first_cond.right_type, mut
+					scope, false, false)
 			}
 		}
 	}
diff --git a/vlib/v/tests/sumtypes/sumtype_not_is_else_smartcast_test.v b/vlib/v/tests/sumtypes/sumtype_not_is_else_smartcast_test.v
new file mode 100644
index 000000000..fa85fe07f
--- /dev/null
+++ b/vlib/v/tests/sumtypes/sumtype_not_is_else_smartcast_test.v
@@ -0,0 +1,18 @@
+type NotIsElseSumtype = NotIsElseBb | NotIsElseCc
+
+struct NotIsElseBb {
+	b int
+}
+
+struct NotIsElseCc {
+	c int
+}
+
+fn test_sumtype_not_is_else_smartcast() {
+	aa := NotIsElseSumtype(NotIsElseBb{0})
+	if aa !is NotIsElseBb {
+		assert false
+	} else {
+		assert aa.b == 0
+	}
+}
-- 
2.39.5