cmd/compile: fix detection of duplicate cases for integer ranges

Previously, the check to make sure we only considered constant cases
for duplicates was skipping past integer ranges, because those use
n.List instead of n.Left. Thanks to Emmanuel Odeke for investigating
and helping to identify the root cause.

Fixes #17517.

Change-Id: I46fcda8ed9c346ff3a9647d50b83f1555587b740
Reviewed-on: https://go-review.googlesource.com/31716
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/swt.go

@@ -600,18 +600,21 @@ func (s *exprSwitch) checkDupCases(cc []caseClause) {
 	if !s.exprname.Type.IsInterface() {
 		seen := make(map[interface{}]*Node)
 		for _, c := range cc {
-			// Can't check for duplicates that aren't constants, per the spec. Issue 15896.
-			// Don't check for duplicate bools. Although the spec allows it,
-			// (1) the compiler hasn't checked it in the past, so compatibility mandates it, and
-			// (2) it would disallow useful things like
-			//       case GOARCH == "arm" && GOARM == "5":
-			//       case GOARCH == "arm":
-			//     which would both evaluate to false for non-ARM compiles.
-			if ct := consttype(c.node.Left); ct < 0 || ct == CTBOOL {
-				continue
-			}
-			if c.node.Left != nil {
+			switch {
+			case c.node.Left != nil:
 				// Single constant.
+
+				// Can't check for duplicates that aren't constants, per the spec. Issue 15896.
+				// Don't check for duplicate bools. Although the spec allows it,
+				// (1) the compiler hasn't checked it in the past, so compatibility mandates it, and
+				// (2) it would disallow useful things like
+				//       case GOARCH == "arm" && GOARM == "5":
+				//       case GOARCH == "arm":
+				//     which would both evaluate to false for non-ARM compiles.
+				if ct := consttype(c.node.Left); ct < 0 || ct == CTBOOL {
+					continue
+				}
+
 				val := c.node.Left.Val().Interface()
 				prev, dup := seen[val]
 				if !dup {
@@ -619,10 +622,9 @@ func (s *exprSwitch) checkDupCases(cc []caseClause) {
 					continue
 				}
 				setlineno(c.node)
-				yyerror("duplicate case %v in switch\n\tprevious case at %v", prev.Left, prev.Line())
-				continue
-			}
-			if c.node.List.Len() == 2 {
+				yyerror("duplicate case %#v in switch\n\tprevious case at %v", val, prev.Line())
+
+			case c.node.List.Len() == 2:
 				// Range of integers.
 				low := c.node.List.Index(0).Int64()
 				high := c.node.List.Index(1).Int64()
@@ -633,11 +635,12 @@ func (s *exprSwitch) checkDupCases(cc []caseClause) {
 						continue
 					}
 					setlineno(c.node)
-					yyerror("duplicate case %v in switch\n\tprevious case at %v", prev.Left, prev.Line())
+					yyerror("duplicate case %d in switch\n\tprevious case at %v", i, prev.Line())
 				}
-				continue
+
+			default:
+				Fatalf("bad caseClause node in checkDupCases: %v", c.node)
 			}
-			Fatalf("bad caseClause node in checkDupCases: %v", c.node)
 		}
 		return
 	}

test/switch5.go

@@ -90,3 +90,12 @@ func f6() int {
 	}
 	return 2
 }
+
+// Ensure duplicates in ranges are detected (issue #17517).
+func f7(a int) {
+	switch a {
+	case 0:
+	case 0, 1: // ERROR "duplicate case 0"
+	case 1, 2, 3, 4: // ERROR "duplicate case 1"
+	}
+}