1 files changed, 34 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/typecheck/typecheck.go b/src/cmd/compile/internal/typecheck/typecheck.go
index 519d8ddfd9..4b5c3198ca 100644
--- a/src/cmd/compile/internal/typecheck/typecheck.go
+++ b/src/cmd/compile/internal/typecheck/typecheck.go
@@ -171,6 +171,7 @@ func FuncBody(n *ir.Func) {
 	decldepth = 1
 	errorsBefore := base.Errors()
 	Stmts(n.Body)
+	CheckUnused(n)
 	CheckReturn(n)
 	if base.Errors() > errorsBefore {
 		n.Body.Set(nil) // type errors; do not compile
@@ -2203,6 +2204,39 @@ func isTermNode(n ir.Node) bool {
 	return false
 }
 
+// CheckUnused checks for any declared variables that weren't used.
+func CheckUnused(fn *ir.Func) {
+	// Only report unused variables if we haven't seen any type-checking
+	// errors yet.
+	if base.Errors() != 0 {
+		return
+	}
+
+	// Propagate the used flag for typeswitch variables up to the NONAME in its definition.
+	for _, ln := range fn.Dcl {
+		if ln.Op() == ir.ONAME && ln.Class_ == ir.PAUTO && ln.Used() {
+			if guard, ok := ln.Defn.(*ir.TypeSwitchGuard); ok {
+				guard.Used = true
+			}
+		}
+	}
+
+	for _, ln := range fn.Dcl {
+		if ln.Op() != ir.ONAME || ln.Class_ != ir.PAUTO || ln.Used() {
+			continue
+		}
+		if defn, ok := ln.Defn.(*ir.TypeSwitchGuard); ok {
+			if defn.Used {
+				continue
+			}
+			base.ErrorfAt(defn.Tag.Pos(), "%v declared but not used", ln.Sym())
+			defn.Used = true // suppress repeats
+		} else {
+			base.ErrorfAt(ln.Pos(), "%v declared but not used", ln.Sym())
+		}
+	}
+}
+
 // CheckReturn makes sure that fn terminates appropriately.
 func CheckReturn(fn *ir.Func) {
 	if fn.Type().NumResults() != 0 && len(fn.Body) != 0 {