fix(gnovm): allow type-switch with sole case nil: (preprocess single-case branch must tag-type-only when ct is nil)

gnovm/pkg/gnolang/preprocess.go

@@ -1042,15 +1042,16 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 							n.Cases[i] = toConstTypeExpr(last, cx, ct)
 							// maybe type-switch def.
 							if ss.VarName != "" {
-								if len(n.Cases) == 1 {
+								if len(n.Cases) == 1 && ct != nil {
 									// If there is only 1 case, the
 									// define applies with type.
 									// (re-definition).
 									last.Define(
 										ss.VarName, anyValue(ct))
 								} else {
 									// If there are 2 or more
-									// cases, the type is the tag type.
+									// cases, or the sole case is nil,
+									// the type is the tag type.
 									tt := evalStaticTypeOf(store, last, ss.X)
 									last.Define(
 										ss.VarName, anyValue(tt))

gnovm/tests/files/typeswitch1.gno

@@ -0,0 +1,85 @@
+// Test simple type switches on basic types, including `case nil:` as
+// the sole case (the regression that motivated this filetest — Gno
+// preprocess previously panicked "name xx not declared" because the
+// single-case branch registered the type-switch var with a nil static
+// type when ct was nil).
+
+package main
+
+import "fmt"
+
+const (
+	a = iota
+	b
+	c
+	d
+	e
+)
+
+var x = []int{1, 2, 3}
+
+func f(x int, len *byte) {
+	*len = byte(x)
+}
+
+func whatis(x interface{}) string {
+	switch xx := x.(type) {
+	default:
+		return fmt.Sprint("default ", xx)
+	case int, int8, int16, int32:
+		return fmt.Sprint("signed ", xx)
+	case int64:
+		return fmt.Sprint("signed64 ", int64(xx))
+	case uint, uint8, uint16, uint32:
+		return fmt.Sprint("unsigned ", xx)
+	case uint64:
+		return fmt.Sprint("unsigned64 ", uint64(xx))
+	case nil:
+		return fmt.Sprint("nil ", xx)
+	}
+	panic("not reached")
+}
+
+func whatis1(x interface{}) string {
+	xx := x
+	switch xx.(type) {
+	default:
+		return fmt.Sprint("default ", xx)
+	case int, int8, int16, int32:
+		return fmt.Sprint("signed ", xx)
+	case int64:
+		return fmt.Sprint("signed64 ", xx.(int64))
+	case uint, uint8, uint16, uint32:
+		return fmt.Sprint("unsigned ", xx)
+	case uint64:
+		return fmt.Sprint("unsigned64 ", xx.(uint64))
+	case nil:
+		return fmt.Sprint("nil ", xx)
+	}
+	panic("not reached")
+}
+
+func check(x interface{}, s string) {
+	w := whatis(x)
+	if w != s {
+		fmt.Println("whatis", x, "=>", w, "!=", s)
+		panic("fail")
+	}
+
+	w = whatis1(x)
+	if w != s {
+		fmt.Println("whatis1", x, "=>", w, "!=", s)
+		panic("fail")
+	}
+}
+
+func main() {
+	check(1, "signed 1")
+	check(uint(1), "unsigned 1")
+	check(int64(1), "signed64 1")
+	check(uint64(1), "unsigned64 1")
+	check(1.5, "default 1.5")
+	check(nil, "nil <nil>")
+}
+
+// Output: