From e5100a8f7366043b9ad58ad145703ec81d3ae576 Mon Sep 17 00:00:00 2001
From: Swastik Baranwal <swstkbaranwal@gmail.com>
Date: Tue, 12 Aug 2025 22:01:34 +0530
Subject: [PATCH] cgen: allow alias sumtype smartcasting (fix #25085) (#25096)

---
 vlib/v/checker/if.v                                |  5 +++--
 vlib/v/checker/infix.v                             |  2 +-
 vlib/v/gen/c/cgen.v                                |  2 +-
 vlib/v/gen/c/infix.v                               |  2 +-
 .../tests/sumtypes/alias_sumtype_smartcast_test.v  | 14 ++++++++++++++
 5 files changed, 20 insertions(+), 5 deletions(-)
 create mode 100644 vlib/v/tests/sumtypes/alias_sumtype_smartcast_test.v

diff --git a/vlib/v/checker/if.v b/vlib/v/checker/if.v
index 1c2be9e9c..3f48fe033 100644
--- a/vlib/v/checker/if.v
+++ b/vlib/v/checker/if.v
@@ -707,6 +707,7 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 				right_sym := c.table.sym(right_type)
 				mut expr_type := c.unwrap_generic(node.left_type)
 				left_sym := c.table.sym(expr_type)
+				left_final_sym := c.table.final_sym(expr_type)
 				if left_sym.kind == .aggregate {
 					expr_type = (left_sym.info as ast.Aggregate).sum_type
 				}
@@ -714,7 +715,7 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 					if right_sym.kind != .interface {
 						c.type_implements(right_type, expr_type, node.pos)
 					}
-				} else if !c.check_types(right_type, expr_type) && left_sym.kind != .sum_type {
+				} else if !c.check_types(right_type, expr_type) && left_final_sym.kind != .sum_type {
 					expect_str := c.table.type_to_str(right_type)
 					expr_str := c.table.type_to_str(expr_type)
 					c.error('cannot use type `${expect_str}` as type `${expr_str}`', node.pos)
@@ -740,7 +741,7 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 									node.left.pos)
 							}
 						}
-						if left_sym.kind in [.interface, .sum_type] {
+						if left_final_sym.kind in [.interface, .sum_type] {
 							c.smartcast(mut node.left, node.left_type, right_type, mut
 								scope, is_comptime, false)
 						}
diff --git a/vlib/v/checker/infix.v b/vlib/v/checker/infix.v
index 6378677a5..1493278d7 100644
--- a/vlib/v/checker/infix.v
+++ b/vlib/v/checker/infix.v
@@ -736,7 +736,7 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					if typ != ast.none_type_idx {
 						c.error('`${op}` can only be used to test for none in sql', node.pos)
 					}
-				} else if left_sym.kind !in [.interface, .sum_type]
+				} else if left_final_sym.kind !in [.interface, .sum_type]
 					&& !c.comptime.is_comptime(node.left) {
 					c.error('`${op}` can only be used with interfaces and sum types',
 						node.pos) // can be used in sql too, but keep err simple
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 9e8ee8c8b..f45150d7c 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -5341,7 +5341,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 			}
 			is_option = is_option || node.obj.orig_type.has_flag(.option)
 			if node.obj.smartcasts.len > 0 {
-				obj_sym := g.table.sym(g.unwrap_generic(node.obj.typ))
+				obj_sym := g.table.final_sym(g.unwrap_generic(node.obj.typ))
 				if !prevent_sum_type_unwrapping_once {
 					nested_unwrap := node.obj.smartcasts.len > 1
 					unwrap_sumtype := is_option && nested_unwrap && obj_sym.kind == .sum_type
diff --git a/vlib/v/gen/c/infix.v b/vlib/v/gen/c/infix.v
index e82905e27..1173607fb 100644
--- a/vlib/v/gen/c/infix.v
+++ b/vlib/v/gen/c/infix.v
@@ -845,7 +845,7 @@ fn (mut g Gen) infix_expr_in_optimization(left ast.Expr, left_type ast.Type, rig
 
 // infix_expr_is_op generates code for `is` and `!is`
 fn (mut g Gen) infix_expr_is_op(node ast.InfixExpr) {
-	mut left_sym := g.table.sym(g.unwrap_generic(g.type_resolver.get_type_or_default(node.left,
+	mut left_sym := g.table.final_sym(g.unwrap_generic(g.type_resolver.get_type_or_default(node.left,
 		node.left_type)))
 	is_aggregate := node.left is ast.Ident && g.comptime.get_ct_type_var(node.left) == .aggregate
 	right_sym := g.table.sym(node.right_type)
diff --git a/vlib/v/tests/sumtypes/alias_sumtype_smartcast_test.v b/vlib/v/tests/sumtypes/alias_sumtype_smartcast_test.v
new file mode 100644
index 000000000..70bade250
--- /dev/null
+++ b/vlib/v/tests/sumtypes/alias_sumtype_smartcast_test.v
@@ -0,0 +1,14 @@
+type Sum = int | string
+type SumAlias = Sum
+
+fn test_alias_sumtype_smartcast() {
+	a_int := SumAlias(Sum(10))
+	if a_int is int {
+		assert a_int == 10
+	}
+
+	a_str := SumAlias('foo')
+	if a_str is string {
+		assert a_str == 'foo'
+	}
+}
-- 
2.39.5