Index: clang-tools-extra/clangd/index/dex/Trigram.h
===================================================================
--- clang-tools-extra/clangd/index/dex/Trigram.h
+++ clang-tools-extra/clangd/index/dex/Trigram.h
@@ -62,7 +62,11 @@
 ///
 /// For short queries (less than 3 characters with Head or Tail roles in Fuzzy
 /// Matching segmentation) this returns a single trigram with the first
-/// characters (up to 3) to perfrom prefix match.
+/// characters (up to 3) to perfrom prefix match. However, if the query is short
+/// but it contains two HEAD symbols then the returned trigram would be an
+/// incomplete bigram with those two HEADs (unless query starts with '_' which
+/// is treated as an additional information). This would help to match
+/// "unique_ptr" and similar symbols with "u_p" query
 std::vector<Token> generateQueryTrigrams(llvm::StringRef Query);
 
 } // namespace dex
Index: clang-tools-extra/clangd/index/dex/Trigram.cpp
===================================================================
--- clang-tools-extra/clangd/index/dex/Trigram.cpp
+++ clang-tools-extra/clangd/index/dex/Trigram.cpp
@@ -116,10 +116,16 @@
 
   // Additional pass is necessary to count valid identifier characters.
   // Depending on that, this function might return incomplete trigram.
+  unsigned Heads = 0;
   unsigned ValidSymbolsCount = 0;
-  for (size_t I = 0; I < Roles.size(); ++I)
-    if (Roles[I] == Head || Roles[I] == Tail)
+  for (size_t I = 0; I < Roles.size(); ++I) {
+    if (Roles[I] == Head) {
+      ++ValidSymbolsCount;
+      ++Heads;
+    } else if (Roles[I] == Tail) {
       ++ValidSymbolsCount;
+    }
+  }
 
   std::string LowercaseQuery = Query.lower();
 
@@ -128,9 +134,21 @@
   // If the number of symbols which can form fuzzy matching trigram is not
   // sufficient, generate a single incomplete trigram for query.
   if (ValidSymbolsCount < 3) {
-    std::string Chars =
-        LowercaseQuery.substr(0, std::min<size_t>(3UL, Query.size()));
-    Chars.append(3 - Chars.size(), END_MARKER);
+    std::string Chars;
+    // If the query is not long enough to form a trigram but contains two heads
+    // the returned trigram should be "xy$" where "x" and "y" are the heads.
+    // This might be particulary important for cases like "u_p" to match
+    // "unique_ptr" and similar symbols from the C++ Standard Library.
+    if (Heads == 2 && !Query.startswith("_")) {
+      for (size_t I = 0; I < LowercaseQuery.size(); ++I)
+        if (Roles[I] == Head)
+          Chars += LowercaseQuery[I];
+
+      Chars += END_MARKER;
+    } else {
+      Chars = LowercaseQuery.substr(0, std::min<size_t>(3UL, Query.size()));
+      Chars.append(3 - Chars.size(), END_MARKER);
+    }
     UniqueTrigrams.insert(Token(Token::Kind::Trigram, Chars));
   } else {
     std::deque<char> Chars;
Index: clang-tools-extra/unittests/clangd/DexIndexTests.cpp
===================================================================
--- clang-tools-extra/unittests/clangd/DexIndexTests.cpp
+++ clang-tools-extra/unittests/clangd/DexIndexTests.cpp
@@ -321,6 +321,9 @@
   EXPECT_THAT(generateQueryTrigrams("__"), trigramsAre({"__$"}));
   EXPECT_THAT(generateQueryTrigrams("___"), trigramsAre({"___"}));
 
+  EXPECT_THAT(generateQueryTrigrams("u_p"), trigramsAre({"up$"}));
+  EXPECT_THAT(generateQueryTrigrams("_u_p"), trigramsAre({"_u_"}));
+
   EXPECT_THAT(generateQueryTrigrams("X86"), trigramsAre({"x86"}));
 
   EXPECT_THAT(generateQueryTrigrams("clangd"),