private void InitializeFullQualifierSymbol(BoundQualifier qualifier)
        {
            if (qualifier.IsInitialized())
            {
                return;
            }
            var result = ArrayBuilder <object> .GetInstance();

            var identifiers = qualifier.Identifiers;
            NamespaceOrTypeSymbol qualifierOpt = null;

            for (int i = 0; i < identifiers.Length; i++)
            {
                bool         last         = i == identifiers.Length - 1;
                var          types        = last ? _types : _nestingTypes;
                var          identifier   = identifiers[i];
                LookupResult lookupResult = LookupResult.GetInstance();
                this.LookupSymbolsSimpleName(lookupResult, new LookupConstraints(identifier.Name, identifier.MetadataName, types, qualifierOpt));
                var symbol = this.ResultSymbol(lookupResult, identifier.Name, identifier.MetadataName, identifier.Syntax, identifier.BoundTree.DiagnosticBag, false, out bool wasError, qualifierOpt, LookupOptions.Default);
                Debug.Assert(symbol != null);
                result.Add(symbol);
                lookupResult.Free();
                qualifierOpt = symbol as NamespaceOrTypeSymbol;
            }
            qualifier.InitializeValues(identifiers, result.ToImmutableAndFree());
        }
        protected override void LookupSymbolsInSingleBinder(LookupResult result, string name, int arity, ConsList <Symbol> basesBeingResolved, LookupOptions options, bool diagnose)
        {
            LookupResult tmp       = LookupResult.GetInstance();
            LookupResult nonViable = LookupResult.GetInstance();

            // Member definitions of different kinds hide each other (field defs hide method defs, etc.).
            // So even if the caller asks only for invocable members find any member first and then reject the result if a non-invokable is found.
            LookupOptions anyMemberLookupOptions = options & ~LookupOptions.MustBeInvocableMember;

            // TODO: optimize lookup (there might be many interactions in the chain)
            for (ICompilation commonSubmission = Compilation.PreviousSubmission; commonSubmission != null; commonSubmission = commonSubmission.PreviousSubmission)
            {
                // TODO (tomat): cross-language binding - for now, skip non-C# submissions
                Compilation submission = commonSubmission as Compilation;
                if (submission == null)
                {
                    continue;
                }

                tmp.Clear();

                Imports imports = GetImports(submission);
                imports.LookupSymbolInAliases(this, tmp, name, arity, basesBeingResolved, anyMemberLookupOptions, diagnose);

                // If a viable using alias and a matching member are both defined in the submission an error is reported elsewhere.
                // Ignore the member in such case.
                if (!tmp.IsMultiViable && (options & LookupOptions.NamespaceAliasesOnly) == 0)
                {
                    this.LookupMembers(tmp, submission.ScriptClass, name, arity, basesBeingResolved, anyMemberLookupOptions, diagnose);
                }

                // found a non-method in the current submission:
                if (tmp.Symbols.Count > 0 && tmp.Symbols.First().Kind != SymbolKind.Method)
                {
                    if (!tmp.IsMultiViable)
                    {
                        // skip non-viable members, but remember them in case no viable members are found in previous submissions:
                        nonViable.MergePrioritized(tmp);
                        continue;
                    }

                    if (result.Symbols.Count == 0)
                    {
                        result.MergeEqual(tmp);
                    }

                    break;
                }

                // merge overloads:
                Debug.Assert(result.Symbols.Count == 0 || result.Symbols.All(s => s.Kind == SymbolKind.Method));
                result.MergeEqual(tmp);
            }

            // Set a proper error if we found a symbol that is not invocable but were asked for invocable only.
            // Only a single non-method can be present in the result; methods are always invocable.
            if ((options & LookupOptions.MustBeInvocableMember) != 0 && result.Symbols.Count == 1)
            {
                Symbol      symbol = result.Symbols.First();
                AliasSymbol alias  = symbol as AliasSymbol;
                if (alias != null)
                {
                    symbol = alias.GetAliasTarget(basesBeingResolved);
                }

                if (IsNonInvocableMember(symbol))
                {
                    result.SetFrom(LookupResult.NotInvocable(symbol, result.Symbols.First(), diagnose));
                }
            }
            else if (result.Symbols.Count == 0)
            {
                result.SetFrom(nonViable);
            }

            tmp.Free();
            nonViable.Free();
        }