Index: clangd/CMakeLists.txt
===================================================================
--- clangd/CMakeLists.txt
+++ clangd/CMakeLists.txt
@@ -7,6 +7,7 @@
   ClangdServer.cpp
   ClangdUnit.cpp
   ClangdUnitStore.cpp
+  Context.cpp
   DraftStore.cpp
   GlobalCompilationDatabase.cpp
   JSONExpr.cpp
Index: clangd/Context.h
===================================================================
--- /dev/null
+++ clangd/Context.h
@@ -0,0 +1,123 @@
+//===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Context for storing and retrieving implicit data. Useful for passing implicit
+// parameters on a per-request basis.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
+
+#include "TypedValueMap.h"
+#include "llvm/ADT/DenseMap.h"
+#include <type_traits>
+
+namespace clang {
+namespace clangd {
+
+/// Allows to store and retrieve implicit data. A \p Context is typically
+/// created on a per-request basis. Contexts are chained via the parent
+/// pointers. Any values are first looked up in the current Context and, if
+/// not found, parent Contexts are queried.
+/// Contexts are move-only, but one should be careful to avoid moving Contexts
+/// used as parents for some other Contexts.
+class Context {
+public:
+  Context(Context *Parent, TypedValueMap Data);
+
+  /// Move-only.
+  Context(Context const &) = delete;
+  Context &operator=(const Context &) = delete;
+
+  Context(Context &&) = default;
+  Context &operator=(Context &&) = default;
+
+  /// Get data stored for a typed \p Key. If values are not found
+  /// \returns Pointer to the data associated with \p Key. If no data is
+  /// specified for \p Key, return null.
+  template <class Type> Type *get(Key<Type> &Key) const {
+    if (auto Val = Data.get(Key))
+      return Val;
+    if (Parent)
+      return Parent->get(Key);
+    return nullptr;
+  }
+
+  /// Get data stored for a typed \p Key. Note that this function does not
+  /// distinguish between \returns Stored pointer, or null if data is not found.
+  template <class Type> Type *get(PtrKey<Type> &Key) const {
+    if (auto Val = Data.get(Key))
+      return Val;
+    if (Parent)
+      return Parent->get(Key);
+    return nullptr;
+  }
+
+private:
+  Context *Parent;
+  TypedValueMap Data;
+};
+
+/// Sets global Context object, returned by globalCtx.
+/// Only one session can exist at a time and the session must be initialized
+/// before any clangd functions are called.
+class GlobalSession {
+public:
+  GlobalSession(Context Ctx);
+  ~GlobalSession();
+};
+
+/// If there is an active GlobalSession, returns the Context provided by it.
+/// Otherwise returns an empty Context.
+Context &globalCtx();
+
+/// Fluent Builder API for creating Contexts. Instead of using it directly,
+/// prefer to use the buildCtx factory functions. Provides only an r-value-only
+/// API, i.e. it is intended to be used on unnamed temporaries:
+///
+///     Context Ctx = buildCtx().add(LoggerKey, &MyLogger).add(TracerKey,
+///     &MyTracer);
+class ContextBuilder {
+public:
+  ContextBuilder(Context *Parent);
+
+  ContextBuilder(ContextBuilder &&) = default;
+  ContextBuilder(ContextBuilder const &) = delete;
+
+  template <class T, class... Args>
+  ContextBuilder &&add(Key<T> &Key, Args &&... As) && {
+    bool Added = Data.emplace(Key, std::forward<Args>(As)...);
+    assert(Added && "Value was already in the map");
+    return std::move(*this);
+  }
+
+  template <class T, class... Args>
+  ContextBuilder &&add(PtrKey<T> &Key, Args &&... As) && {
+    bool Added = Data.emplace(Key, std::forward<Args>(As)...);
+    assert(Added && "Value was already in the map");
+    return std::move(*this);
+  }
+
+  operator Context() && { return Context(Parent, std::move(Data)); }
+
+private:
+  Context *Parent;
+  TypedValueMap Data;
+};
+
+/// Creates a new ContextBuilder, using globalCtx() as a parent.
+ContextBuilder buildCtx();
+/// Creates a new ContextBuilder with explicit \p Parent.
+ContextBuilder buildCtx(Context *Parent);
+
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
Index: clangd/Context.cpp
===================================================================
--- /dev/null
+++ clangd/Context.cpp
@@ -0,0 +1,43 @@
+//===--- Context.cpp -----------------------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#include "Context.h"
+#include <cassert>
+
+namespace clang {
+namespace clangd {
+
+static Context *GlobalCtx = nullptr;
+static Context EmptyContext(nullptr, {});
+
+Context::Context(Context *Parent, TypedValueMap Data)
+    : Parent(Parent), Data(std::move(Data)) {}
+
+GlobalSession::GlobalSession(Context Ctx) {
+  assert(!GlobalCtx && "GlobalCtx was already set");
+  GlobalCtx = new Context(std::move(Ctx));
+}
+
+GlobalSession::~GlobalSession() {
+  delete GlobalCtx;
+  GlobalCtx = nullptr;
+}
+
+Context &globalCtx() {
+  if (GlobalCtx)
+    return *GlobalCtx;
+  return EmptyContext;
+}
+
+ContextBuilder::ContextBuilder(Context *Parent) : Parent(Parent) {}
+
+ContextBuilder buildCtx() { return buildCtx(&globalCtx()); }
+ContextBuilder buildCtx(Context *Parent) { return {Parent}; }
+} // namespace clangd
+} // namespace clang
Index: clangd/TypedValueMap.h
===================================================================
--- /dev/null
+++ clangd/TypedValueMap.h
@@ -0,0 +1,123 @@
+//===--- TypedValueMap.h - Type-safe heterogenous key-value map -*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Type-safe heterogenous map.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/DenseMap.h"
+#include <type_traits>
+
+namespace clang {
+namespace clangd {
+
+/// Used as identity for map values. Non-movable and non-copyable. Address of
+/// this object is used internally to as keys in a map.
+template <class Type> class Key {
+public:
+  static_assert(!std::is_reference<Type>::value,
+                "Reference arguments to Key<> are not allowed");
+
+  Key() = default;
+
+  Key(Key const &) = delete;
+  Key &operator=(Key const &) = delete;
+  Key(Key &&) = delete;
+  Key &operator=(Key &&) = delete;
+};
+
+/// Similar to a Key<T*> with a slightly easier to use semantics.
+/// While get(Key<T*>) returns T**, get(PtrKey<T>) returns T*.
+/// Therefore PtrKey<> does not distinguish values missing from the map and
+/// values equal to null.
+template <class Type> class PtrKey {
+public:
+  Key<Type *> UnderlyingKey;
+};
+
+/// A type-safe map from Key<T> to T.
+class TypedValueMap {
+public:
+  TypedValueMap() = default;
+  TypedValueMap(const TypedValueMap &) = delete;
+  TypedValueMap(TypedValueMap &&) = default;
+
+  template <class Type> Type *get(Key<Type> &Key) const {
+    auto It = Map.find(&Key);
+    if (It == Map.end())
+      return nullptr;
+    return static_cast<Type *>(It->second->getValuePtr());
+  }
+
+  template <class Type, class... Args>
+  bool emplace(Key<Type> &Key, Args &&... As) {
+    bool Added =
+        Map.try_emplace(&Key,
+                        llvm::make_unique<
+                            TypedAnyStorage<typename std::decay<Type>::type>>(
+                            std::forward<Args>(As)...))
+            .second;
+    return Added;
+  }
+
+  template <class Type> Type *get(PtrKey<Type> &PtrKey) const {
+    Type **PtrVal = get(PtrKey.UnderlyingKey);
+    if (!PtrVal)
+      return nullptr;
+    return *PtrVal;
+  }
+
+  template <class Type, class Arg> bool emplace(PtrKey<Type> &PtrKey, Arg *A) {
+    return emplace(PtrKey.UnderlyingKey, A);
+  }
+
+  template <class Type> bool emplace(PtrKey<Type> &PtrKey, std::nullptr_t) {
+    return emplace(PtrKey.UnderlyingKey, nullptr);
+  }
+
+  template <class Type> bool remove(Key<Type> &Key) {
+    auto It = Map.find(&Key);
+    if (It == Map.end())
+      return false;
+
+    Map.erase(It);
+    return true;
+  }
+
+  template <class Type> bool remove(PtrKey<Type> &Key) {
+    return remove(Key.UnderlyingKey);
+  }
+
+private:
+  class AnyStorage {
+  public:
+    virtual ~AnyStorage() = default;
+    virtual void *getValuePtr() = 0;
+  };
+
+  template <class T> class TypedAnyStorage : public AnyStorage {
+    static_assert(std::is_same<typename std::decay<T>::type, T>::value,
+                  "Argument to TypedAnyStorage must be decayed");
+
+  public:
+    template <class... Args>
+    TypedAnyStorage(Args &&... As) : Value(std::forward<Args>(As)...) {}
+
+    void *getValuePtr() override { return &Value; }
+
+  private:
+    T Value;
+  };
+
+  // Map keys are addresses of Key<> objects.
+  llvm::DenseMap<void *, std::unique_ptr<AnyStorage>> Map;
+};
+
+} // namespace clangd
+} // namespace clang
Index: unittests/clangd/CMakeLists.txt
===================================================================
--- unittests/clangd/CMakeLists.txt
+++ unittests/clangd/CMakeLists.txt
@@ -10,6 +10,7 @@
 
 add_extra_unittest(ClangdTests
   ClangdTests.cpp
+  ContextTests.cpp
   JSONExprTests.cpp
   TraceTests.cpp
   )
Index: unittests/clangd/ContextTests.cpp
===================================================================
--- /dev/null
+++ unittests/clangd/ContextTests.cpp
@@ -0,0 +1,88 @@
+//===-- ContextTests.cpp - Context tests ------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Context.h"
+
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+
+TEST(TypedValueMapTests, Simple) {
+  Key<int> IntParam;
+  Key<int> ExtraIntParam;
+
+  clangd::TypedValueMap Ctx;
+
+  ASSERT_TRUE(Ctx.emplace(IntParam, 10));
+  ASSERT_TRUE(Ctx.emplace(ExtraIntParam, 20));
+
+  EXPECT_EQ(*Ctx.get(IntParam), 10);
+  EXPECT_EQ(*Ctx.get(ExtraIntParam), 20);
+
+  ASSERT_FALSE(Ctx.emplace(IntParam, 30));
+
+  ASSERT_TRUE(Ctx.remove(IntParam));
+  EXPECT_EQ(Ctx.get(IntParam), nullptr);
+  EXPECT_EQ(*Ctx.get(ExtraIntParam), 20);
+
+  ASSERT_TRUE(Ctx.emplace(IntParam, 30));
+  EXPECT_EQ(*Ctx.get(IntParam), 30);
+  EXPECT_EQ(*Ctx.get(ExtraIntParam), 20);
+}
+
+TEST(TypedValueMapTests, MoveOps) {
+  Key<std::unique_ptr<int>> Param;
+
+  clangd::TypedValueMap Ctx;
+  Ctx.emplace(Param, llvm::make_unique<int>(10));
+  EXPECT_EQ(**Ctx.get(Param), 10);
+
+  clangd::TypedValueMap NewCtx = std::move(Ctx);
+  EXPECT_EQ(**NewCtx.get(Param), 10);
+}
+
+TEST(TypedValueMapTests, PtrKey) {
+  int Value = 10;
+  PtrKey<int> Param;
+
+  clangd::TypedValueMap Ctx;
+  EXPECT_EQ(Ctx.get(Param), nullptr);
+
+  Ctx.emplace(Param, &Value);
+  EXPECT_EQ(*Ctx.get(Param), 10);
+
+  Ctx.remove(Param);
+  EXPECT_EQ(Ctx.get(Param), nullptr);
+
+  Ctx.emplace(Param, nullptr);
+  EXPECT_EQ(Ctx.get(Param), nullptr);
+}
+
+TEST(ContextTests, Builders) {
+  Key<int> ParentParam;
+  Key<int> ParentAndChildParam;
+  Key<int> ChildParam;
+
+  Context ParentCtx =
+      buildCtx().add(ParentParam, 10).add(ParentAndChildParam, 20);
+  Context ChildCtx =
+      buildCtx(&ParentCtx).add(ParentAndChildParam, 30).add(ChildParam, 40);
+
+  EXPECT_EQ(*ParentCtx.get(ParentParam), 10);
+  EXPECT_EQ(*ParentCtx.get(ParentAndChildParam), 20);
+  EXPECT_EQ(ParentCtx.get(ChildParam), nullptr);
+
+  EXPECT_EQ(*ChildCtx.get(ParentParam), 10);
+  EXPECT_EQ(*ChildCtx.get(ParentAndChildParam), 30);
+  EXPECT_EQ(*ChildCtx.get(ChildParam), 40);
+}
+
+} // namespace clangd
+} // namespace clang