fix(gnovm): Improve Error Message in evalStaticTypeOf function

gnovm/pkg/gnolang/nodes.go

@@ -403,7 +403,7 @@ type IndexExpr struct { // X[Index]
 	Attributes
 	X     Expr // expression
 	Index Expr // index expression
-	HasOK bool // if true, is form: `value, ok := <X>[<Key>]
+	HasOK bool // if true, is form: `value, ok := <X>[<Key>]`
 }
 
 type SelectorExpr struct { // X.Sel

gnovm/pkg/gnolang/preprocess.go

@@ -2449,16 +2449,33 @@
 	t := evalStaticTypeOfRaw(store, last, x)
 	if tt, ok := t.(*tupleType); ok {
 		if len(tt.Elts) != 1 {
-			panic(fmt.Sprintf(
-				"evalStaticTypeOf() only supports *CallExpr with 1 result, got %s",
-				tt.String(),
-			))
-		} else {
-			return tt.Elts[0]
+			loc := last.GetLocation()
+			funcName := getFunctionName(x)
+			valueCount := len(tt.Elts)
+
+			var msg string
+			if valueCount == 0 {
+				msg = fmt.Sprintf("%s: %s() (no value) used as value", loc, funcName)
+			} else {
+				msg = fmt.Sprintf("%s: %s() (%d values) used as single value", loc, funcName, valueCount)
+			}
+
+			panic(fmt.Errorf("%s\nHint: Ensure the function returns a single value, or use multiple assignment", msg))
+		}
+		return tt.Elts[0]
+	}
+	return t
+}
+
+// getFunctionName attempts to extract the function name from the expression
+func getFunctionName(x Expr) string {
+	switch expr := x.(type) {
+	case *CallExpr:
+		if funcName, ok := expr.Func.(*NameExpr); ok {
+			return string(funcName.Name)
 		}
-	} else {
-		return t
 	}
+	return "<unknown function>"
 }
 
 // like evalStaticTypeOf() but returns the raw *tupleType for *CallExpr.

gnovm/pkg/gnolang/preprocess_test.go

@@ -60,3 +60,62 @@ func main() {
 	initStaticBlocks(store, pn, n)
 	Preprocess(store, pn, n)
 }
+
+func TestEvalStaticTypeOf_MultipleValues(t *testing.T) {
+	pn := NewPackageNode("main", "main", nil)
+	pv := pn.NewPackage()
+	store := gonativeTestStore(pn, pv)
+	store.SetBlockNode(pn)
+
+	const src = `package main
+func multipleReturns() (int, string) {
+	return 1, "hello"
+}
+func main() {
+	x := multipleReturns()
+}`
+	n := MustParseFile("main.go", src)
+
+	initStaticBlocks(store, pn, n)
+
+	defer func() {
+		err := recover()
+		assert.NotNil(t, err, "Expected panic")
+		errMsg := fmt.Sprint(err)
+		assert.Contains(t, errMsg, "multipleReturns() (2 values) used as single value", "Unexpected error message")
+		assert.Contains(t, errMsg, "Hint: Ensure the function returns a single value, or use multiple assignment", "Missing hint in error message")
+	}()
+
+	Preprocess(store, pn, n)
+}
+
+func TestEvalStaticTypeOf_NoValue(t *testing.T) {
+	pn := NewPackageNode("main", "main", nil)
+	pv := pn.NewPackage()
+	store := gonativeTestStore(pn, pv)
+	store.SetBlockNode(pn)
+
+	const src = `package main
+
+func main() {
+	n := f()
+}
+
+func f() {
+	println("hello!")
+}
+`
+	n := MustParseFile("main.go", src)
+
+	initStaticBlocks(store, pn, n)
+
+	defer func() {
+		err := recover()
+		assert.NotNil(t, err, "Expected panic")
+		errMsg := fmt.Sprint(err)
+		assert.Contains(t, errMsg, "f() (no value) used as value", "Unexpected error message")
+		assert.Contains(t, errMsg, "Hint: Ensure the function returns a single value, or use multiple assignment", "Missing hint in error message")
+	}()
+
+	Preprocess(store, pn, n)
+}