diff --git a/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.h b/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.h
@@ -0,0 +1,46 @@
+//===--- AvoidCastInConditionalCheck.h - clang-tidy -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVM_AVOIDCASTINCONDITIONALCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVM_AVOIDCASTINCONDITIONALCHECK_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace llvm {
+
+/// \brief Finds cases of cast<> used in conditional statements which will
+/// assert on failure in Debug builds.
+///
+/// Finds cases like these:
+/// \code
+///   if (auto x = cast<X>(y)) <...>
+///   if (cast<X>(y)) <...>
+/// \endcode
+/// But not cases like these:
+/// \code
+///   if (auto f = cast<Z>(y)->foo()) <...>
+///   if (cast<Z>(y)->foo()) <...>
+/// \endcode
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/llvm-avoid-cast-in-conditional.html
+class AvoidCastInConditionalCheck : public ClangTidyCheck {
+public:
+  AvoidCastInConditionalCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace llvm
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVM_AVOIDCASTINCONDITIONALCHECK_H
diff --git a/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.cpp b/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvm/AvoidCastInConditionalCheck.cpp
@@ -0,0 +1,43 @@
+//===--- AvoidCastInConditionalCheck.cpp - clang-tidy ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "AvoidCastInConditionalCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace llvm {
+
+void AvoidCastInConditionalCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      ifStmt(anyOf(
+          hasConditionVariableStatement(declStmt(containsDeclaration(
+              0, varDecl(hasInitializer(
+                     callExpr(callee(namedDecl(hasName("cast"))))
+                         .bind("cast-call")))))),
+          has(implicitCastExpr(has(callExpr(callee(namedDecl(hasName("cast"))))
+                                       .bind("cast-call")))))),
+      this);
+}
+
+void AvoidCastInConditionalCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  if (const auto *MatchedDecl = Result.Nodes.getNodeAs<CallExpr>("cast-call")) {
+    diag(MatchedDecl->getBeginLoc(),
+         "Found cast<> in conditional statement; consider using dyn_cast<> if "
+         "it can fail, or removal of conditional if it can't.");
+    ;
+  }
+}
+
+} // namespace llvm
+} // namespace tidy
+} // namespace clang
diff --git a/clang-tools-extra/clang-tidy/llvm/CMakeLists.txt b/clang-tools-extra/clang-tidy/llvm/CMakeLists.txt
--- a/clang-tools-extra/clang-tidy/llvm/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/llvm/CMakeLists.txt
@@ -1,6 +1,7 @@
 set(LLVM_LINK_COMPONENTS support)
 
 add_clang_library(clangTidyLLVMModule
+  AvoidCastInConditionalCheck.cpp
   HeaderGuardCheck.cpp
   IncludeOrderCheck.cpp
   LLVMTidyModule.cpp
diff --git a/clang-tools-extra/clang-tidy/llvm/LLVMTidyModule.cpp b/clang-tools-extra/clang-tidy/llvm/LLVMTidyModule.cpp
--- a/clang-tools-extra/clang-tidy/llvm/LLVMTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/llvm/LLVMTidyModule.cpp
@@ -10,6 +10,7 @@
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 #include "../readability/NamespaceCommentCheck.h"
+#include "AvoidCastInConditionalCheck.h"
 #include "HeaderGuardCheck.h"
 #include "IncludeOrderCheck.h"
 #include "TwineLocalCheck.h"
@@ -21,6 +22,8 @@
 class LLVMModule : public ClangTidyModule {
 public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
+    CheckFactories.registerCheck<AvoidCastInConditionalCheck>(
+        "llvm-avoid-cast-in-conditional");
     CheckFactories.registerCheck<LLVMHeaderGuardCheck>("llvm-header-guard");
     CheckFactories.registerCheck<IncludeOrderCheck>("llvm-include-order");
     CheckFactories.registerCheck<readability::NamespaceCommentCheck>(
diff --git a/clang-tools-extra/clang-tidy/utils/HeaderFileExtensionsUtils.h b/clang-tools-extra/clang-tidy/utils/HeaderFileExtensionsUtils.h
--- a/clang-tools-extra/clang-tidy/utils/HeaderFileExtensionsUtils.h
+++ b/clang-tools-extra/clang-tidy/utils/HeaderFileExtensionsUtils.h
@@ -18,7 +18,7 @@
 namespace tidy {
 namespace utils {
 
-typedef llvm::SmallSet<llvm::StringRef, 5> HeaderFileExtensionsSet;
+using HeaderFileExtensionsSet = SmallSet<StringRef, 5>;
 
 /// \brief Checks whether expansion location of \p Loc is in header file.
 bool isExpansionLocInHeaderFile(
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -130,6 +130,12 @@
   <clang-tidy/checks/modernize-use-override>` now supports `OverrideSpelling`
   and `FinalSpelling` options.
 
+- New :doc:`llvm-avoid-cast-in-conditional
+  <clang-tidy/checks/llvm-avoid-cast-in-conditional>` check.
+
+  Finds cases of ``cast<>`` used as condition in conditional statements
+  which will assert on failure in Debug builds.
+
 - New :doc:`openmp-exception-escape
   <clang-tidy/checks/openmp-exception-escape>` check.
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -175,6 +175,7 @@
    hicpp-use-nullptr (redirects to modernize-use-nullptr) <hicpp-use-nullptr>
    hicpp-use-override (redirects to modernize-use-override) <hicpp-use-override>
    hicpp-vararg (redirects to cppcoreguidelines-pro-type-vararg) <hicpp-vararg>
+   llvm-avoid-cast-in-conditional
    llvm-header-guard
    llvm-include-order
    llvm-namespace-comment
diff --git a/clang-tools-extra/docs/clang-tidy/checks/llvm-avoid-cast-in-conditional.rst b/clang-tools-extra/docs/clang-tidy/checks/llvm-avoid-cast-in-conditional.rst
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/llvm-avoid-cast-in-conditional.rst
@@ -0,0 +1,19 @@
+.. title:: clang-tidy - llvm-avoid-cast-in-conditional
+
+llvm-avoid-cast-in-conditional
+==============================
+
+Finds cases of ``cast<>`` used as the condition of a conditional
+statements which will assert on failure in Debug builds. The use of
+cast<> implies that the operation cannot fail, and should not be used
+as the condition of a conditional statement.
+
+.. code-block:: c++
+
+  // Finds cases like these:
+  if (auto x = cast<X>(y)) <...>
+  if (cast<X>(y)) <...>
+
+  // But not cases like these:
+  if (auto f = cast<Z>(y)->foo()) <...>
+  if (cast<Z>(y)->foo()) <...>
diff --git a/clang-tools-extra/test/clang-tidy/llvm-avoid-cast-in-conditional.cpp b/clang-tools-extra/test/clang-tidy/llvm-avoid-cast-in-conditional.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/llvm-avoid-cast-in-conditional.cpp
@@ -0,0 +1,25 @@
+// RUN: %check_clang_tidy %s llvm-avoid-cast-in-conditional %t
+
+struct X;
+struct Y;
+struct Z {
+  int foo();
+};
+
+template <class X, class Y>
+X* cast(Y*);
+
+bool foo(Y* y) {
+  if (auto x = cast<X>(y))
+    // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: {{.*llvm-avoid-cast-in-conditional}}
+    return true;
+  if (cast<X>(y))
+    // CHECK-MESSAGES: :[[@LINE-1]]:7: warning:  {{.*llvm-avoid-cast-in-conditional}}
+    return true;
+
+  // Does not trigger warning.
+  if (cast<Z>(y)->foo())
+    return true;
+  return false;
+}
+
diff --git a/clang/include/clang/Basic/LLVM.h b/clang/include/clang/Basic/LLVM.h
--- a/clang/include/clang/Basic/LLVM.h
+++ b/clang/include/clang/Basic/LLVM.h
@@ -31,6 +31,7 @@
   template<typename T> class ArrayRef;
   template<typename T> class MutableArrayRef;
   template<typename T> class OwningArrayRef;
+  template <typename T, unsigned N, typename C> class SmallSet;
   template<unsigned InternalLen> class SmallString;
   template<typename T, unsigned N> class SmallVector;
   template<typename T> class SmallVectorImpl;
@@ -66,6 +67,7 @@
   using llvm::Optional;
   using llvm::OwningArrayRef;
   using llvm::SaveAndRestore;
+  using llvm::SmallSet;
   using llvm::SmallString;
   using llvm::SmallVector;
   using llvm::SmallVectorImpl;