[clangd] Fix a crash bug in AddUsing tweak around template handling.

[adamcz]

Summary

The crash happened on cases like:
template<typename TT> using one = two::three<T^T>;
because we tried to call getName() on getBaseTypeIdentifier(), which can
be nullptr.

Ideally we would support this use case as well, but for now not crashing
will do.

[adamcz]

Removed unnecessary change.

[sammccall]

Whoops, indeed.
There should be a version of falsehoods programmers believe about names for the clang AST :-)

• Decls have names
• well, NamedDecls have names
• Decls that are not NamedDecls don't have names
• if a NamedDecl has a name, that name is an identifier
• ...

clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp

212

BaseTypeIdentifier->getName()

[adamcz]

review comment

[adamcz]

Can you submit this? Thanks!

clang-tools-extra/clangd/refactor/tweaks/AddUsing.cpp

bool AddUsing::prepare(const Selection &Inputs) {
   if (Node == nullptr)
     return false;
 
   if (auto *D = Node->ASTNode.get<DeclRefExpr>()) {
     QualifierToRemove = D->getQualifierLoc();
     Name = D->getDecl()->getName();
   } else if (auto *T = Node->ASTNode.get<TypeLoc>()) {
     if (auto E = T->getAs<ElaboratedTypeLoc>()) {
+      if (auto *BaseTypeIdentifier =
+              E.getType().getUnqualifiedType().getBaseTypeIdentifier()) {
+        Name = BaseTypeIdentifier->getName();
         QualifierToRemove = E.getQualifierLoc();
-      Name =
-          E.getType().getUnqualifiedType().getBaseTypeIdentifier()->getName();
+      }
     }
   }
 
   // FIXME: This only supports removing qualifiers that are made up of just
   // namespace names. If qualifier contains a type, we could take the longest
   // namespace prefix and remove that.
   if (!QualifierToRemove.hasQualifier() ||
       !QualifierToRemove.getNestedNameSpecifier()->getAsNamespace() ||

clang-tools-extra/clangd/unittests/TweakTests.cpp

 }
 
 TWEAK_TEST(AddUsing);
 TEST_F(AddUsingTest, Prepare) {
   const std::string Header = R"cpp(
 #define NS(name) one::two::name
 namespace one {
 void oo() {}
+template<typename TT> class tt {};
 namespace two {
 enum ee {};
 void ff() {}
 class cc {
 public:
   struct st {};
   static void mm() {}
 };
 }
 })cpp";
 
   EXPECT_AVAILABLE(Header + "void fun() { o^n^e^:^:^t^w^o^:^:^f^f(); }");
   EXPECT_AVAILABLE(Header + "void fun() { o^n^e^::^o^o(); }");
   EXPECT_AVAILABLE(Header + "void fun() { o^n^e^:^:^t^w^o^:^:^e^e E; }");
   EXPECT_AVAILABLE(Header + "void fun() { o^n^e^:^:^t^w^o:^:^c^c C; }");
   EXPECT_UNAVAILABLE(Header +
                      "void fun() { o^n^e^:^:^t^w^o^:^:^c^c^:^:^m^m(); }");
   EXPECT_UNAVAILABLE(Header +
                      "void fun() { o^n^e^:^:^t^w^o^:^:^c^c^:^:^s^t inst; }");
   EXPECT_UNAVAILABLE(Header +
                      "void fun() { o^n^e^:^:^t^w^o^:^:^c^c^:^:^s^t inst; }");
   EXPECT_UNAVAILABLE(Header + "void fun() { N^S(c^c) inst; }");
+  // This used to crash. Ideally we would support this case, but for now we just
+  // test that we don't crash.
+  EXPECT_UNAVAILABLE(Header +
+                     "template<typename TT> using foo = one::tt<T^T>;");
 }
 
 TEST_F(AddUsingTest, Apply) {
   FileName = "test.cpp";
   struct {
     llvm::StringRef TestSource;
     llvm::StringRef ExpectedSource;
   } Cases[]{{