cmd/compile/internal/syntax: check fallthrough in CheckBranches mode

The parser CheckBranches mode checked correct use of break, continue,
and labels, but not of fallthrough statements.

This CL adds checking of fallthrough statements as well.

For #51456.

Change-Id: I5000388011973724f80c59a6aaf015e3bb70faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/414134
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

1 files changed, 55 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/syntax/testdata/fallthrough.go b/src/cmd/compile/internal/syntax/testdata/fallthrough.go
new file mode 100644
index 0000000000..851da81ea0
--- /dev/null
+++ b/src/cmd/compile/internal/syntax/testdata/fallthrough.go
@@ -0,0 +1,55 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fallthroughs
+
+func _() {
+	var x int
+	switch x {
+	case 0:
+		fallthrough
+
+	case 1:
+		fallthrough // ERROR fallthrough statement out of place
+		{
+		}
+
+	case 2:
+		{
+			fallthrough // ERROR fallthrough statement out of place
+		}
+
+	case 3:
+		for {
+			fallthrough // ERROR fallthrough statement out of place
+		}
+
+	case 4:
+		fallthrough // trailing empty statements are ok
+		;
+		;
+
+	case 5:
+		fallthrough
+
+	default:
+		fallthrough // ERROR cannot fallthrough final case in switch
+	}
+
+	fallthrough // ERROR fallthrough statement out of place
+
+	if true {
+		fallthrough // ERROR fallthrough statement out of place
+	}
+
+	for {
+		fallthrough // ERROR fallthrough statement out of place
+	}
+
+	var t any
+	switch t.(type) {
+	case int:
+		fallthrough // ERROR cannot fallthrough in type switch
+	}
+}