[clangd] Fix a selection tree crash for unmatched-bracket code.

Fixes https://github.com/clangd/clangd/issues/999

Differential Revision: https://reviews.llvm.org/D118322

clang-tools-extra/clangd/Selection.cpp

@@ -306,6 +306,14 @@ public:
       return SelectionTree::Unselected;
     }
 
+    // The eof token is used as a sentinel.
+    // In general, source range from an AST node should not claim the eof token,
+    // but it could occur for unmatched-bracket cases.
+    // FIXME: fix it in TokenBuffer, expandedTokens(SourceRange) should not
+    // return the eof token.
+    if (ExpandedTokens.back().kind() == tok::eof)
+      ExpandedTokens = ExpandedTokens.drop_back();
+
     SelectionTree::Selection Result = NoTokens;
     while (!ExpandedTokens.empty()) {
       // Take consecutive tokens from the same context together for efficiency.

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

@@ -523,6 +523,10 @@ TEST(SelectionTest, CommonAncestor) {
           auto l = [^[[foo = bar]]] { };
         })cpp",
        "VarDecl"},
+      {R"cpp(
+        /*error-ok*/
+        void func() [[{^]])cpp",
+       "CompoundStmt"},
   };
 
   for (const Case &C : Cases) {