[CodeComplete] Collect visited contexts when scope specifier is invalid.

[ioeric]

Summary

This will allow completion consumers to guess the specified scope by
putting together scopes in the context with the specified scope (e.g. when the
specified namespace is not imported yet).

[ilya-biryukov]

LGTM

[ioeric]

• Cleanup unit tests (forgot to run... sorry)

lib/Sema/SemaCodeComplete.cpp

void Sema::CodeCompleteQualifiedId(Scope *S, CXXScopeSpec &SS,
 
   // We want to keep the scope specifier even if it's invalid (e.g. the scope
   // "a::b::" is not corresponding to any context/namespace in the AST), since
   // it can be useful for global code completion which have information about
   // contexts/symbols that are not in the AST.
   if (SS.isInvalid()) {
     CodeCompletionContext CC(CodeCompletionContext::CCC_Symbol);
     CC.setCXXScopeSpecifier(SS);
-    HandleCodeCompleteResults(this, CodeCompleter, CC, nullptr, 0);
+    // As SS is invalid, we try to collect accessible contexts from the current
+    // scope with a dummy lookup so that the completion consumer can try to
+    // guess what the specified scope is.
+    ResultBuilder DummyResults(*this, CodeCompleter->getAllocator(),
+                               CodeCompleter->getCodeCompletionTUInfo(), CC);
+    if (S->getEntity()) {
+      CodeCompletionDeclConsumer Consumer(DummyResults, S->getEntity(),
+                                          BaseType);
+      LookupVisibleDecls(S, LookupOrdinaryName, Consumer,
+                         /*IncludeGlobalScope=*/false,
+                         /*LoadExternal=*/false);
+    }
+    HandleCodeCompleteResults(this, CodeCompleter,
+                              DummyResults.getCompletionContext(), nullptr, 0);
     return;
   }
   // Always pretend to enter a context to ensure that a dependent type
   // resolves to a dependent record.
   DeclContext *Ctx = computeDeclContext(SS, /*EnteringContext=*/true);
   if (!Ctx)
     return;
 

unittests/Sema/CodeCompleteTest.cpp

 TEST(SemaCodeCompleteTest, VisitedNSForInvalideQualifiedId) {
   auto VisitedNS = runCodeCompleteOnCode(R"cpp(
      namespace ns { foo::^ }
   )cpp")
                        .VisitedNamespaces;
   EXPECT_TRUE(VisitedNS.empty());
 }
 
+TEST(SemaCodeCompleteTest, VisitedNSForInvalidQualifiedId) {
+  auto VisitedNS = runCodeCompleteOnCode(R"cpp(
+     namespace na {}
+     namespace ns1 {
+     using namespace na;
+     foo::^
+     }
+  )cpp")
+                       .VisitedNamespaces;
+  EXPECT_THAT(VisitedNS, UnorderedElementsAre("ns1::", "na::"));
+}
+
 TEST(SemaCodeCompleteTest, VisitedNSWithoutQualifier) {
   auto VisitedNS = runCodeCompleteOnCode(R"cpp(
     namespace n1 {
     namespace n2 {
       void f(^) {}
     }
     }
   )cpp")