[clangd] Fix an assertion crash in "ExtractVariable" tweak

[hokein]

Summary

GetTypePtr requires that the type should not be null, otherwise we hit
an assertion, we should use getTypePtrOrNull instead.

[hokein]

clang-tools-extra/clangd/Selection.cpp

367

sorry, the previous patch rL365751 didn't fix the crash, and I re-read the code more thoroughly, I think we will never hit this point, need you confirmation, @sammccall.

[sammccall]

clang-tools-extra/clangd/Selection.cpp

361

can we just rewrite this as

while (Root->Children.size() == 1 && !Root->Selected)
  Ancestor = Ancestor->Children.front();
return Root;

seems a lot clearer, not sure what I was thinking when I wrote this...

368

there's no termination condition for the loop. If there were, the compiler would warn us.
I think we don't need this.

[hokein]

Address comment.

clang-tools-extra/clangd/Selection.cpp

 }
 
 SelectionTree::SelectionTree(ASTContext &AST, unsigned Offset)
     : SelectionTree(AST, Offset, Offset) {}
 
 const Node *SelectionTree::commonAncestor() const {
   if (!Root)
     return nullptr;
   for (const Node *Ancestor = Root;; Ancestor = Ancestor->Children.front()) {
+    assert(Ancestor && "Ancestor can not be null");
     if (Ancestor->Selected || Ancestor->Children.size() > 1)
       return Ancestor;
     // The tree only contains ancestors of the interesting nodes.
     assert(!Ancestor->Children.empty() && "bad node in selection tree");
   }
-  return nullptr;
+  llvm_unreachable("must not reach here!");
 }
 
 } // namespace clangd
 } // namespace clang

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

computeReferencedDecls(const clang::Expr *Expr) {
   Visitor.TraverseStmt(const_cast<Stmt *>(dyn_cast<Stmt>(Expr)));
   return Visitor.ReferencedDecls;
 }
 
 // An expr is not extractable if it's null or an expression of type void
 // FIXME: Ignore assignment (a = 1) Expr since it is extracted as dummy = a =
 static bool isExtractableExpr(const clang::Expr *Expr) {
   if (Expr) {
-    const Type *ExprType = Expr->getType().getTypePtr();
+    const Type *ExprType = Expr->getType().getTypePtrOrNull();
     // FIXME: check if we need to cover any other types
     if (ExprType)
       return !ExprType->isVoidType();
   }
   return false;
 }
 
 ExtractionContext::ExtractionContext(const SelectionTree::Node *Node,

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

void f() {
       while(a < ^1)
         ^a++;
       // do while 
       do
         a = ^1;
       while(a < ^3);
     }
   )cpp");
+  // Should not crash.
+  checkNotAvailable(ID, R"cpp(
+    template<typename T, typename ...Args>
+    struct Test<T, Args...> {
+    Test(const T &v) :val(^) {}
+      T val;
+    };
+  )cpp");
   checkNotAvailable(ID, R"cpp(
     int xyz(int a = ^1) {
       return 1;
       class T {
         T(int a = ^1) {};
         int xyz = ^1;
       };
     }