allow Union deref w/o field name

src/data.cpp

@@ -715,7 +715,7 @@ void Value::traverse(const std::string &expr, bool modify, bool dothrow)
 
             maybedot = false;
 
-            if(expr.size()-pos >= 3 && expr[pos]=='-' && expr[pos+1]=='>') {
+            if(expr.size()-pos >= 2 && expr[pos]=='-' && expr[pos+1]=='>') {
                 pos += 2; // skip past "->"
 
                 if(desc->code.code==TypeCode::Any) {
@@ -727,10 +727,10 @@ void Value::traverse(const std::string &expr, bool modify, bool dothrow)
                     size_t sep = expr.find_first_of("<[-.", pos);
 
                     decltype (desc->mlookup)::const_iterator it;
+                    auto& fld = store->as<Value>();
 
                     if(sep>0 && (it=desc->mlookup.find(expr.substr(pos, sep-pos)))!=desc->mlookup.end()) {
                         // found it.
-                        auto& fld = store->as<Value>();
 
                         if(modify || fld.desc==&desc->members[it->second]) {
                             // will select, or already selected
@@ -750,6 +750,16 @@ void Value::traverse(const std::string &expr, bool modify, bool dothrow)
                             if(dothrow)
                                 throw LookupError(SB()<<"traversing const Value, can't select Union in '"<<expr<<"'");
                         }
+
+                    } else if(fld.desc) {
+                        // deref selected
+                        *this = fld;
+
+                    } else {
+                        store.reset();
+                        desc = nullptr;
+                        if(dothrow)
+                            throw LookupError(SB()<<"can't deref. empty Union '"<<expr<<"'");
                     }
                 }
             } else {

test/testdata.cpp

@@ -67,6 +67,29 @@ void testAssign()
     testOk1(!val["alarm"].isMarked(true, false));
 }
 
+void testAssignUnion()
+{
+    testDiag("%s", __func__);
+
+    auto val = TypeDef(TypeCode::Union, {
+                           members::UInt16("u16"),
+                           members::String("s"),
+                       }).create();
+
+    val["->u16"] = 42;
+    testEq(val.as<std::string>(), "42");
+    val["->s"] = "test";
+    testEq(val.as<std::string>(), "test");
+
+    testEq(val.nameOf(val["->"]), "s");
+
+    testThrows<std::invalid_argument>([&val](){
+        val["->u16"] = "hello";
+    });
+
+    //val = nullptr;
+}
+
 void testName()
 {
     testDiag("%s", __func__);
@@ -314,10 +337,11 @@ void testAssignSimilar()
 
 MAIN(testdata)
 {
-    testPlan(93);
+    testPlan(97);
     testSetup();
     testTraverse();
     testAssign();
+    testAssignUnion();
     testName();
     testIterStruct();
     testIterUnion();