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diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
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index 00000000000..2c5676e76a2
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+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
@@ -0,0 +1,70 @@
+//===--- NarrowingConversionsCheck.cpp - clang-tidy------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "NarrowingConversionsCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace cppcoreguidelines {
+
+// FIXME: Check double -> float truncation. Pay attention to casts:
+void NarrowingConversionsCheck::registerMatchers(MatchFinder *Finder) {
+  // ceil() and floor() are guaranteed to return integers, even though the type
+  // is not integral.
+  const auto IsCeilFloorCall = callExpr(callee(functionDecl(
+      hasAnyName("::ceil", "::std::ceil", "::floor", "::std::floor"))));
+
+  const auto IsFloatExpr =
+      expr(hasType(realFloatingPointType()), unless(IsCeilFloorCall));
+
+  // casts:
+  //   i = 0.5;
+  //   void f(int); f(0.5);
+  Finder->addMatcher(implicitCastExpr(hasImplicitDestinationType(isInteger()),
+                                      hasSourceExpression(IsFloatExpr),
+                                      unless(hasParent(castExpr())),
+                                      unless(isInTemplateInstantiation()))
+                         .bind("cast"),
+                     this);
+
+  // Binary operators:
+  //   i += 0.5;
+  Finder->addMatcher(
+      binaryOperator(isAssignmentOperator(),
+                     // The `=` case generates an implicit cast which is covered
+                     // by the previous matcher.
+                     unless(hasOperatorName("=")),
+                     hasLHS(hasType(isInteger())), hasRHS(IsFloatExpr),
+                     unless(isInTemplateInstantiation()))
+          .bind("op"),
+      this);
+}
+
+void NarrowingConversionsCheck::check(const MatchFinder::MatchResult &Result) {
+  if (const auto *Op = Result.Nodes.getNodeAs<BinaryOperator>("op")) {
+    if (Op->getLocStart().isMacroID())
+      return;
+    diag(Op->getOperatorLoc(), "narrowing conversion from %0 to %1")
+        << Op->getRHS()->getType() << Op->getLHS()->getType();
+    return;
+  }
+  const auto *Cast = Result.Nodes.getNodeAs<ImplicitCastExpr>("cast");
+  if (Cast->getLocStart().isMacroID())
+    return;
+  diag(Cast->getExprLoc(), "narrowing conversion from %0 to %1")
+      << Cast->getSubExpr()->getType() << Cast->getType();
+}
+
+} // namespace cppcoreguidelines
+} // namespace tidy
+} // namespace clang