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 golang#63566

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)

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)
+		}
+	}
+}