Index: clang-tidy/misc/CMakeLists.txt
===================================================================
--- clang-tidy/misc/CMakeLists.txt
+++ clang-tidy/misc/CMakeLists.txt
@@ -12,6 +12,7 @@
   MiscTidyModule.cpp
   MoveConstructorInitCheck.cpp
   NoexceptMoveConstructorCheck.cpp
+  SizeofContainerCheck.cpp
   StaticAssertCheck.cpp
   SwappedArgumentsCheck.cpp
   UndelegatedConstructor.cpp
Index: clang-tidy/misc/MiscTidyModule.cpp
===================================================================
--- clang-tidy/misc/MiscTidyModule.cpp
+++ clang-tidy/misc/MiscTidyModule.cpp
@@ -20,6 +20,7 @@
 #include "MacroRepeatedSideEffectsCheck.h"
 #include "MoveConstructorInitCheck.h"
 #include "NoexceptMoveConstructorCheck.h"
+#include "SizeofContainerCheck.h"
 #include "StaticAssertCheck.h"
 #include "SwappedArgumentsCheck.h"
 #include "UndelegatedConstructor.h"
@@ -54,6 +55,8 @@
         "misc-move-constructor-init");
     CheckFactories.registerCheck<NoexceptMoveConstructorCheck>(
         "misc-noexcept-move-constructor");
+    CheckFactories.registerCheck<SizeofContainerCheck>(
+        "misc-sizeof-container");
     CheckFactories.registerCheck<StaticAssertCheck>(
         "misc-static-assert");
     CheckFactories.registerCheck<SwappedArgumentsCheck>(
Index: clang-tidy/misc/SizeofContainerCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/misc/SizeofContainerCheck.h
@@ -0,0 +1,35 @@
+//===--- SizeofContainerCheck.h - clang-tidy---------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_SIZEOF_CONTAINER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_SIZEOF_CONTAINER_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+
+/// Find usages of sizeof on expressions of STL container types. Most likely the
+/// user wanted to use `.size()` instead.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/misc-sizeof-container.html
+class SizeofContainerCheck : public ClangTidyCheck {
+public:
+  SizeofContainerCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_SIZEOF_CONTAINER_H
+
Index: clang-tidy/misc/SizeofContainerCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/misc/SizeofContainerCheck.cpp
@@ -0,0 +1,76 @@
+//===--- SizeofContainerCheck.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 "SizeofContainerCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+
+namespace {
+
+bool needsParens(const Expr *E) {
+  E = E->IgnoreImpCasts();
+  if (isa<BinaryOperator>(E) || isa<ConditionalOperator>(E))
+    return true;
+  if (const auto *Op = dyn_cast<CXXOperatorCallExpr>(E)) {
+    return Op->getNumArgs() == 2 && Op->getOperator() != OO_Call &&
+           Op->getOperator() != OO_Subscript;
+  }
+  return false;
+}
+
+} // anonymous namespace
+
+void SizeofContainerCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      expr(sizeOfExpr(
+               has(expr(hasType(hasCanonicalType(hasDeclaration(recordDecl(
+                   matchesName("std::(basic_string|vector)|::string")))))))))
+          .bind("sizeof"),
+      this);
+}
+
+void SizeofContainerCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *SizeOf =
+      Result.Nodes.getNodeAs<UnaryExprOrTypeTraitExpr>("sizeof");
+
+  SourceLocation SizeOfLoc = SizeOf->getLocStart();
+  auto Diag = diag(SizeOfLoc, "sizeof() doesn't return the size of the "
+                              "container. Did you mean .size()?");
+
+  // Don't generate fixes for macros.
+  if (SizeOfLoc.isMacroID())
+    return;
+
+  SourceLocation RParenLoc = SizeOf->getRParenLoc();
+
+  // sizeof argument is wrapped in a single ParenExpr.
+  const auto *Arg = cast<ParenExpr>(SizeOf->getArgumentExpr());
+
+  if (needsParens(Arg->getSubExpr())) {
+    Diag << FixItHint::CreateRemoval(
+                CharSourceRange::getTokenRange(SizeOfLoc, SizeOfLoc))
+         << FixItHint::CreateInsertion(RParenLoc.getLocWithOffset(1),
+                                       ".size()");
+  } else {
+    Diag << FixItHint::CreateRemoval(
+                CharSourceRange::getTokenRange(SizeOfLoc, Arg->getLParen()))
+         << FixItHint::CreateReplacement(
+                CharSourceRange::getTokenRange(RParenLoc, RParenLoc),
+                ".size()");
+  }
+}
+
+} // namespace tidy
+} // namespace clang
+
Index: docs/clang-tidy/checks/list.rst
===================================================================
--- docs/clang-tidy/checks/list.rst
+++ docs/clang-tidy/checks/list.rst
@@ -31,6 +31,7 @@
    misc-macro-repeated-side-effects
    misc-move-constructor-init
    misc-noexcept-move-constructor
+   misc-sizeof-container
    misc-static-assert
    misc-swapped-arguments
    misc-undelegated-constructor
@@ -54,4 +55,4 @@
    readability-named-parameter
    readability-redundant-smartptr-get
    readability-redundant-string-cstr
-   readability-simplify-boolean-expr
\ No newline at end of file
+   readability-simplify-boolean-expr
Index: docs/clang-tidy/checks/misc-sizeof-container.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/misc-sizeof-container.rst
@@ -0,0 +1,17 @@
+misc-sizeof-container
+=====================
+
+The check finds usages of ``sizeof`` on expressions of STL container types. Most
+likely the user wanted to use ``.size()`` instead.
+
+Currently only ``std::string`` and ``std::vector<T>`` are supported.
+
+Examples:
+
+.. code:: c++
+
+  std::string s;
+  int a = 47 + sizeof(s); // warning: sizeof() doesn't return the size of the container. Did you mean .size()?
+                          // The suggested fix is: int a = 47 + s.size();
+
+  int b = sizeof(std::string); // no warning, probably intended.
Index: test/clang-tidy/misc-sizeof-container.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/misc-sizeof-container.cpp
@@ -0,0 +1,42 @@
+// RUN: %python %S/check_clang_tidy.py %s misc-sizeof-container %t
+
+namespace std {
+template <typename T>
+class basic_string {};
+
+template <typename T>
+basic_string<T> operator+(const basic_string<T> &, const T *);
+
+typedef basic_string<char> string;
+
+template <typename T>
+class vector {};
+}
+
+class string {};
+
+void f() {
+  string s1;
+  std::string s2;
+  std::vector<int> v;
+
+  int a = 42 + sizeof(s1);
+// CHECK-MESSAGES: :[[@LINE-1]]:16: warning: sizeof() doesn't return the size of the container. Did you mean .size()? [misc-sizeof-container]
+// CHECK-FIXES: int a = 42 + s1.size();
+  a = 123 * sizeof(s2);
+// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: sizeof() doesn't return the size
+// CHECK-FIXES: a = 123 * s2.size();
+  a = 45 + sizeof(s2 + "asdf");
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: sizeof() doesn't return the size
+// CHECK-FIXES: a = 45 + (s2 + "asdf").size();
+  a = sizeof(v);
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: sizeof() doesn't return the size
+// CHECK-FIXES: a = v.size();
+
+  a = sizeof(a);
+  a = sizeof(int);
+  a = sizeof(std::string);
+  a = sizeof(std::vector<int>);
+
+  (void)a;
+}