[ASTimporter] Remove decl from lookup only if it has decl context

[steakhal]

Summary

In the case of TypedefDecls we set the DeclContext after we imported it.
It turns out, it could lead to null pointer dereferences during the cleanup part of a failed import.

This patch demonstrates this issue and fixes it by checking if the DeclContext is available or not.

[shafik]

This is a good catch, thank you for fixing this!

[steakhal]

In D102640#2764759, @shafik wrote:

This is a good catch, thank you for fixing this!

Thanks for the quick review!

[martong]

Post commit looks good to me as well.
@steakhal Nice work! @shafik Thanks for the review!

clang/lib/AST/ASTImporter.cpp

if (Pos != ImportedDecls.end()) {
       // times.
 
       // The Lookuptable contains decls only which are in the 'to' context.
       // Remove from the Lookuptable only if it is *imported* into the 'to'
       // context (and do not remove it if it was added during the initial
       // traverse of the 'to' context).
       auto PosF = ImportedFromDecls.find(ToD);
       if (PosF != ImportedFromDecls.end()) {
+        // In the case of TypedefNameDecl we create the Decl first and only
+        // then we import and set its DeclContext. So, the DC might not be set
+        // when we reach here.
+        if (ToD->getDeclContext())
           SharedState->removeDeclFromLookup(ToD);
         ImportedFromDecls.erase(PosF);
       }
 
       // FIXME: AST may contain remaining references to the failed object.
       // However, the ImportDeclErrors in the shared state contains all the
       // failed objects together with their error.
     }
 

clang/unittests/AST/ASTImporterTest.cpp

auto *FromOK = FirstDeclMatcher<FunctionDecl>().match(
       FromTU, functionDecl(hasName("ok")));
   OptErr = Importer->getImportDeclErrorIfAny(FromOK);
   EXPECT_FALSE(OptErr);
   // And we should be able to import.
   FunctionDecl *ImportedOK = Import(FromOK, Lang_CXX03);
   EXPECT_TRUE(ImportedOK);
 }
 
+TEST_P(ErrorHandlingTest, ODRViolationWithinTypedefDecls) {
+  // Importing `z` should fail - instead of crashing - due to an ODR violation.
+  // The `bar::e` typedef sets it's DeclContext after the import is done.
+  // However, if the importation fails, it will be left as a nullptr.
+  // During the cleanup of the failed import, we should check whether the
+  // DeclContext is null or not - instead of dereferencing that unconditionally.
+  constexpr auto ToTUCode = R"(
+      namespace X {
+        struct bar {
+          int odr_violation;
+        };
+      })";
+  constexpr auto FromTUCode = R"(
+      namespace X {
+        enum b {};
+        struct bar {
+          typedef b e;
+          static e d;
+        };
+      }
+      int z = X::bar::d;
+      )";
+  Decl *ToTU = getToTuDecl(ToTUCode, Lang_CXX11);
+  static_cast<void>(ToTU);
+  Decl *FromTU = getTuDecl(FromTUCode, Lang_CXX11);
+  auto *FromZ =
+      FirstDeclMatcher<VarDecl>().match(FromTU, varDecl(hasName("z")));
+  ASSERT_TRUE(FromZ);
+  ASSERT_TRUE(FromZ->hasInit());
+
+  auto *ImportedZ = Import(FromZ, Lang_CXX11);
+  EXPECT_FALSE(ImportedZ);
+}
+
 // An error should be set for a class if it had a previous import with an error
 // from another TU.
 TEST_P(ErrorHandlingTest,
        ImportedDeclWithErrorShouldFailTheImportOfDeclWhichMapToIt) {
   // We already have a fwd decl.
   TranslationUnitDecl *ToTU = getToTuDecl("class X;", Lang_CXX03);
   // Then we import a definition.
   {