From 8948fa0ea35caa767afaf6d0707e5d7006aedb2c Mon Sep 17 00:00:00 2001
From: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Date: Tue, 17 Oct 2023 11:03:42 -0300
Subject: [PATCH] go/printer: fix invalid output for empty decls

The current output for empty declarations such as var, const, import
results in "var", "const", "import" respectively. These are not valid
syntax and the parser will promptly reject them as invalid syntax.

This CL updates this behavior by adding "()" to the output of empty
decls so the syntax becomes valid, e.g "var ()" instead of "var".

Fixes #63566
---
 src/go/printer/nodes.go        |  9 +++++++++
 src/go/printer/printer_test.go | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
index 0a693b6667..b9dac5da64 100644
--- a/src/go/printer/nodes.go
+++ b/src/go/printer/nodes.go
@@ -1739,6 +1739,15 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 	p.setPos(d.Pos())
 	p.print(d.Tok, blank)
 
+	// Empty decls for "var ()", "const ()", "import ()"
+	if len(d.Specs) == 0 {
+		switch d.Tok {
+		case token.VAR, token.CONST, token.IMPORT:
+			p.print(token.LPAREN, token.RPAREN)
+			return
+		}
+	}
+
 	if d.Lparen.IsValid() || len(d.Specs) > 1 {
 		// group of parenthesized declarations
 		p.setPos(d.Lparen)
diff --git a/src/go/printer/printer_test.go b/src/go/printer/printer_test.go
index 8e78bc640e..f5e1419dde 100644
--- a/src/go/printer/printer_test.go
+++ b/src/go/printer/printer_test.go
@@ -848,3 +848,35 @@ func TestSourcePosNewline(t *testing.T) {
 		t.Errorf("unexpected Fprint output:\n%s", buf.Bytes())
 	}
 }
+
+// TestEmptyDecl tests that empty decls for const, var, import are printed with
+// valid syntax e.g "var ()" instead of just "var", which is invalid and cannot
+// be parsed.
+func TestEmptyDecl(t *testing.T) { // issue 63566
+	var tableTests = []struct {
+		tok      token.Token
+		expected string
+	}{
+		{
+			tok:      token.VAR,
+			expected: "var ()",
+		},
+		{
+			tok:      token.CONST,
+			expected: "const ()",
+		},
+		{
+			tok:      token.IMPORT,
+			expected: "import ()",
+		},
+	}
+
+	for _, tt := range tableTests {
+		var buf bytes.Buffer
+		Fprint(&buf, token.NewFileSet(), &ast.GenDecl{Tok: tt.tok})
+		got := buf.String()
+		if got != tt.expected {
+			t.Errorf("got %q, expected %q\n", got, tt.expected)
+		}
+	}
+}