From 04ba0e26c045fa45ef3a882f590ef5aabdca0a04 Mon Sep 17 00:00:00 2001
From: Leonard Lyubich <leonard@morphbits.io>
Date: Fri, 14 Nov 2025 22:35:38 +0300
Subject: [PATCH] compiler: Fix expression `switch` with non-last `default`

According to https://go.dev/ref/spec#Expression_switches:

> There can be at most one default case and it may appear anywhere in
the "switch" statement.

Previously, `default` statements executed even if they are followed by a
matching `case`.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>
---
 pkg/compiler/codegen.go     |  8 ++++++++
 pkg/compiler/switch_test.go | 40 +++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/pkg/compiler/codegen.go b/pkg/compiler/codegen.go
index 12af57da52..ab3ed94027 100644
--- a/pkg/compiler/codegen.go
+++ b/pkg/compiler/codegen.go
@@ -878,6 +878,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		c.currentSwitch = label
 		c.pushStackLabel(label, 1)
 
+		last := len(n.Body.List) - 1
+		for i := range last {
+			if n.Body.List[i].(*ast.CaseClause).List == nil { // early default
+				n.Body.List[i], n.Body.List[last] = n.Body.List[last], n.Body.List[i]
+				break
+			}
+		}
+
 		startLabels := make([]uint16, len(n.Body.List))
 		for i := range startLabels {
 			startLabels[i] = c.newLabel()
diff --git a/pkg/compiler/switch_test.go b/pkg/compiler/switch_test.go
index 21b62785aa..d59849edd1 100644
--- a/pkg/compiler/switch_test.go
+++ b/pkg/compiler/switch_test.go
@@ -122,6 +122,46 @@ var switchTestCases = []testCase{
 		`,
 		big.NewInt(1),
 	},
+	{
+		"case after first default",
+		`func F%d() int {
+			a := 5
+			switch a {
+			default: return 4
+			case 5: return 2
+			}
+		}
+		`,
+		big.NewInt(2),
+	},
+	{
+		"case after intermediate default",
+		`func F%d() int {
+			a := 6
+			switch a {
+			case 5: return 2
+			default: return 4
+			case 6:
+			}
+			return 1
+		}
+		`,
+		big.NewInt(1),
+	},
+	{
+		"intermediate default",
+		`func F%d() int {
+			a := 3
+			switch a {
+			case 5: return 2
+			default: return 4
+			case 6:
+			}
+			return 1
+		}
+		`,
+		big.NewInt(4),
+	},
 	{
 		"expression in case clause",
 		`func F%d() int {