[clang-tidy] Add new hicpp-multiway-paths-covered check for missing branches

[JonasToth]

Summary

This check searches for missing else branches in if-else if-chains and
missing default labels in switch statements, that use integers as condition.

It is very similar to -Wswitch, but concentrates on integers only, since enums are
already covered.

The option to warn for missing else branches is deactivated by default, since it is
very noise on larger code bases.

Running it on LLVM:

clang_default.txt24 KBDownload

for default configuration

clang_noisy.txt29 KBDownload

just for llvm/lib/Analysis/ScalarEvolution.cpp, the else-path checker is very noisy!

[JonasToth]

I added a clarifying comment for some outcommented code still in the patch. I would like to have a bit of discussion on it.
Maybe some parts of the check could be warning-area, i just implemented it here to have it on one place.

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

48

s/noise/noisy/

69–79

I originally tried to implement some logic, that will realize when all paths are covered explicitly.
It didn't work and i think it is irrelevant anyway, since manually covering all paths for integertypes is IMHO unrealistic.

Iam willing to remove the code i currently commented out, just wanted it to be there if there is explicit need for a 'is everything covered'-check.

docs/clang-tidy/checks/list.rst

42

remove

test/clang-tidy/hicpp-multiway-paths-covered.cpp

8

explicitly test, that the warnings for else do not occur here.

[JonasToth]

• fix the first issues i found

[JonasToth]

fixed first stuff, sorry for noise.

[Eugene.Zelenko]

I think will be good idea to extend -Wswitch diagnostics.

[JonasToth]

In D37808#869602, @Eugene.Zelenko wrote:

I think will be good idea to extend -Wswitch diagnostics.

Ok. But it will introduce new warnings to llvm codebase itself. I prepare some example output i found right now.

[Eugene.Zelenko]

In D37808#869612, @JonasToth wrote:

In D37808#869602, @Eugene.Zelenko wrote:

I think will be good idea to extend -Wswitch diagnostics.

Ok. But it will introduce new warnings to llvm codebase itself. I prepare some example output i found right now.

If number of them will not be huge, it'll be worth to fix before extended -Wswitch will be committed.

[lebedev.ri]

What about GNU extension case 1 ... 3: ?

[JonasToth]

In D37808#869823, @lebedev.ri wrote:

What about GNU extension case 1 ... 3: ?

Strictly speaking, the coding standard (which should be enforced by the patch) requires strict ISO C++11, therefore this extension is not considered directly.
Does clang support this extension? When this would be a warning, then that case should be added as well.

[lebedev.ri]

In D37808#869879, @JonasToth wrote:

In D37808#869823, @lebedev.ri wrote:

What about GNU extension case 1 ... 3: ?

Strictly speaking, the coding standard (which should be enforced by the patch) requires strict ISO C++11, therefore this extension is not considered directly.

Does clang support this extension?

It does:

test.cpp:3:12: warning: use of GNU case range extension [-Wgnu-case-range]
    case 1 ... 3:
           ^

When this would be a warning, then that case should be added as well.

[JonasToth]

In D37808#869810, @Eugene.Zelenko wrote:

In D37808#869612, @JonasToth wrote:

In D37808#869602, @Eugene.Zelenko wrote:

I think will be good idea to extend -Wswitch diagnostics.

Ok. But it will introduce new warnings to llvm codebase itself. I prepare some example output i found right now.

If number of them will not be huge, it'll be worth to fix before extended -Wswitch will be committed.

Where would be the correct place to further discuss extending -Wswitch ? Should i ask that on the mailing list?

[lebedev.ri]

In D37808#869810, @Eugene.Zelenko wrote:

In D37808#869612, @JonasToth wrote:

In D37808#869602, @Eugene.Zelenko wrote:

I think will be good idea to extend -Wswitch diagnostics.

Ok. But it will introduce new warnings to llvm codebase itself. I prepare some example output i found right now.

If number of them will not be huge, it'll be worth to fix before extended -Wswitch will be committed.

The other way around, all new warnings must to be fixed first, before committing the new diagnostic itself.

I might agree that the part of this check that is handling switch() might indeed be better off extending -Wswitch (as several new flags -Wswitch-missing-default, -Wswitch-empty, -Wswitch-prefer-if, or something like that)
But then a much greater thought shall be put into ensuring quality of the new diagnostic.

Oh, and i'd say the WarnOnMissingElse should *not* be moved to clang diagnostic. At least right now it will result in many false-positives.
E.g. what would it say about

void maybefalse(int i) {
  if (i > 0) {
    return;
  } else if (i <= 0) {
    return;
  }

[JonasToth]

I on my part think that both switch and else should stay here for now. Running it over llvm gave really a lot of warnings, and that a rather good codebase.

The check does not recognize logic, your example would be warned against. I don't see a way to implement something like that here, that would require condition solving and so on.

In general, this check is more about defensive programming and therefore clang-tidy a good place.

[JonasToth]

ping :)

[JonasToth]

ping. is there something obviously wrong with this check?

[JonasToth]

ping: I don't understand the lack of feedback. This check should not be a frontend warning,
since it warns for perfectly valid code. It is supposed to give stronger guarantees
for developers requiring this.

• rebased to master

[JDevlieghere]

I very much like this check. I only have a few minor comments, but maybe this encourages others to have a look too!

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

86

I'm not a big fan of the 'found', can we just omit it? The same goes for the other diags.

97

Why not a switch?

153

I'm aware that the message and fixme are different, but since the structure is so similar to the handling of the other switch case, I wonder if there is any chance we could extract the common parts?

[JonasToth]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

86

Agree. The messages are better without 'found'.

97

I intent to check if all cases are explicitly covered.

In the testcases there is one switch with all numbers explicitly written, meaning there is no need to add a default branch.

This would allow further

else if (CaseCount == MaximumPossibleCases) { /* No warning */ }

path which is not modable with switch.

153

I try to get something shorter.
Maybe

if(CaseCount == 1 && MatchedSwitch) {}
else if(CaseCount == 1 && MatchedElseIf) {}

?

[JDevlieghere]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

97

Sounds good

153

Wouldn't it be easier to have a function and pass as arguments the stuff that's different? Both are SwitchStmts so if you pass that + the diagnostic message you should be able to share the other logic.

[JonasToth]

major codechange

• • address @JDevlieghere review notes
• simplify structure of checking and extract functions
• handle bitfields as well

[JonasToth]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

97

Woops. I mixed codeplaces (since bad duplication from my side). I did merge the diagnostics better, making this a natural fit for a switch.

153

I kinda rewrote the whole checking part.
Updated diff comes in a sec. I found that bitfields are not handled as they should.

[JonasToth]

Improved check a lot. Hope reviewers now have an easier time reading it.

[JonasToth]

llvm_lib_target_x86_paths11 KBDownload

Example output for llvm/lib/Target/X86
Running it over the whole llvm/lib codebase generates a lot of warnings. Please note, that it seems to be common to write code like this:

int Val;
switch(Val) {
  case 1: // something
  case 2: // something else
  case 16: // magic
}
llvm_unreachable("reason");

In some cases it has been taken care that no fallthrough happens, but it's not so simple to spot. Using default: llvm_unreachable("reason"); would comply with the check.

[JDevlieghere]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

77

Add a comment describing what this function does. I'd move and rephrase the comment below.

99

Unless you expect a whole bunch of logic to be added here, I'd un-const and initialize BitCount to zero, then just have if-clause reassign it and get rid of the lambda. This will save you a few lines of code and complexity.

150

If there's only going to be two messages, you could use the ternary operator and save an instantiation of the SmallVector.

196

Let's move this to right after where you define CaseCount

[JonasToth]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

99

I dont know if i missed some integral types, that need special care, so this one was defensive.

But with the refactoring i made, I should change this, agreed.

[JonasToth]

• address review comments

[JonasToth]

• Improve docs, grammar

[JDevlieghere]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

81

Maybe merge this with the function comment, no point in having both imho.

83

LLVM style guide says no braces with single line if/else statements. No need for the else after a return. Alternatively you could use the ternary operator.

101

No need for the else here. See previous comment.

147

Braces

181

Braces

184

You can just use if because of the return.

187

Drop the else

199

Braces

[JonasToth]

• Merge 'master'
• address review comments

[JonasToth]

simplified the if-else stuff

[JonasToth]

• improved docs and comments
• remove some newlines
• use ternary operator instead of smallvec

[JonasToth]

• fix rebasing issues in docs

[JonasToth]

@aaron.ballman could you take a (i think) finishing look on the check? Most issues should be resolved and i think its ready for the finishing line :)

[JonasToth]

• remove double whitespace

[JonasToth]

@aaron.ballman and @alexfh ping.

[aaron.ballman]

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

14

Including <iostream> is forbidden by our coding standard. However, it doesn't appear to be used in the source file, either.

42

either bitfield or integer condition -> either a bit-field or an integer condition

44

since -> because

Also, drop the comma.

49

binded -> bound

55

Don't use doxygen comments here.

64

Our usual preference is to make these methods static rather than put them in an anonymous namespace (types go into anonymous namespaces, however).

81

This logic does not make sense to me -- the goal is to test for overflow, which can be done solely by looking at the value of Bits and the size of std::size_t. The current logic appears to return 1 if Bits == 0 and will only return the max value if the wraparound happens to precisely create the value 0 or 1. I'd replace with:

return Bits >= std::numeric_limits<std::size_t>::digits() ? std::numeric_limits<std::size_t>::max() : static_cast<size_t>(1) << Bits;

85

How about: Get the number of possible values that can be switched on for the type T

92–93

I don't think this comment adds value.

95

No need for the ul suffix -- the type will undergo implicit conversion with or without the suffix.

95

Why call toPow instead of just returning the value 1 directly?

103

potential uncovered codepath -> potentially uncovered code path
branch -> statement

132

No need to make this const (we only do that for reference and pointer types, not other local variable or parameter types). Here and elsewhere.

133

statementt -> statement

139

as if/else -> as an if/else

144–146

Remove all commas.

162

seperatly -> separately.

(period also)

177

use if -> use an if

178

potential uncovered codepath -> potentially uncovered code path
default case -> default label

clang-tidy/hicpp/MultiwayPathsCoveredCheck.h

22

Drop the comma.

42–44

How about:

This option can be configured to warn on missing 'else' branches in an 'if-else if' chain. The default is false because this option might be noisy on some code bases.

docs/ReleaseNotes.rst

164

Perhaps: Checks on switch and if-else if constructs that do not cover all possible code paths.

docs/clang-tidy/checks/hicpp-multiway-paths-covered.rst

7

Perhaps: This check discovers situations where code paths are not fully-covered.

8

gets clearer with it -> will be more clear

17

Perhaps: This warning can be noisy on some code bases, so it is disabled by default.

33

, that -> which

63

, that are not default -> other than a `default` label

64

Otherwise should have a comma after it

I'd remove "common" and go with "with an `if` statement."

65

catched -> caught

69

if() -> an `if` statement:

87

Completly -> A completely
warned -> diagnosed

drop the comma.

test/clang-tidy/hicpp-multiway-paths-covered.cpp

446

For fun, can you add a switch over a value of type bool?

void f(bool b) {
  switch (b) {
  case true:
  }

  switch (b) {
  default:
  }

  switch (b) {
  case true:
  case false:
  }
}

[JonasToth]

• rebase to master
• address review comments from aaron
• handle bools correctly

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

14

Yes, forgot it from debugging.

92–93

i dropped all of it. The function doc should be clear enough, is it?

95

True!

test/clang-tidy/hicpp-multiway-paths-covered.cpp

446

There is already a warning in the frontend (-Wswitch-bool) that will complain about boolean conditions in switch.

I will implement the logic anyway. I think its more userfriendly to get this warning. One might have code style that allows switch for bools.

[JonasToth]

• address more comments, especially doc

[JonasToth]

• - fix ReleaseNotes

[aaron.ballman]

Aside from some minor commenting nits, LGTM

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

42

bitfield -> bit-field

92–93

Yes, I think so.

169

bitfield -> bit-field

[JonasToth]

• fix nits

[JonasToth]

finalize

clang-tidy/hicpp/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS support)
 
 add_clang_library(clangTidyHICPPModule
   ExceptionBaseclassCheck.cpp
+  MultiwayPathsCoveredCheck.cpp
   NoAssemblerCheck.cpp
   HICPPTidyModule.cpp
   SignedBitwiseCheck.cpp
 
   LINK_LIBS
   clangAST
   clangASTMatchers
   clangBasic

clang-tidy/hicpp/HICPPTidyModule.cpp

 #include "../modernize/UseEqualsDeleteCheck.h"
 #include "../modernize/UseNoexceptCheck.h"
 #include "../modernize/UseNullptrCheck.h"
 #include "../modernize/UseOverrideCheck.h"
 #include "../readability/BracesAroundStatementsCheck.h"
 #include "../readability/FunctionSizeCheck.h"
 #include "../readability/IdentifierNamingCheck.h"
 #include "ExceptionBaseclassCheck.h"
+#include "MultiwayPathsCoveredCheck.h"
 #include "NoAssemblerCheck.h"
 #include "SignedBitwiseCheck.h"
 
 namespace clang {
 namespace tidy {
 namespace hicpp {
 
 class HICPPModule : public ClangTidyModule {
 public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
     CheckFactories.registerCheck<readability::BracesAroundStatementsCheck>(
         "hicpp-braces-around-statements");
     CheckFactories.registerCheck<modernize::DeprecatedHeadersCheck>(
         "hicpp-deprecated-headers");
     CheckFactories.registerCheck<ExceptionBaseclassCheck>(
         "hicpp-exception-baseclass");
     CheckFactories.registerCheck<SignedBitwiseCheck>(
         "hicpp-signed-bitwise");
+    CheckFactories.registerCheck<MultiwayPathsCoveredCheck>(
+        "hicpp-multiway-paths-covered");
     CheckFactories.registerCheck<google::ExplicitConstructorCheck>(
         "hicpp-explicit-conversions");
     CheckFactories.registerCheck<readability::FunctionSizeCheck>(
         "hicpp-function-size");
     CheckFactories.registerCheck<readability::IdentifierNamingCheck>(
         "hicpp-named-parameter");
     CheckFactories.registerCheck<misc::UseAfterMoveCheck>(
         "hicpp-invalid-access-moved");

clang-tidy/hicpp/MultiwayPathsCoveredCheck.h

+//===--- MultiwayPathsCoveredCheck.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_HICPP_MULTIWAY_PATHS_COVERED_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_HICPP_MULTIWAY_PATHS_COVERED_H
+
+#include "../ClangTidy.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include <iostream>
+
+namespace clang {
+namespace tidy {
+namespace hicpp {
+
+/// Find occasions where not all codepaths are explicitly covered in code.
+/// This includes 'switch' without a 'default'-branch and 'if'-'else if'-chains
+/// without a final 'else'-branch.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/hicpp-multiway-paths-covered.html
+class MultiwayPathsCoveredCheck : public ClangTidyCheck {
+public:
+  MultiwayPathsCoveredCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context),
+        WarnOnMissingElse(Options.get("WarnOnMissingElse", 0)) {}
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  void handleSwitchWithDefault(const SwitchStmt *Switch);
+  void handleSwitchWithoutDefault(
+      const SwitchStmt *Switch,
+      const ast_matchers::MatchFinder::MatchResult &Result);
+  /// This option can be configured to warn on missing 'else' branches in an
+  /// 'if-else if' chain. The default is false because this option might be
+  /// noisy on some code bases.
+  const bool WarnOnMissingElse;
+};
+
+} // namespace hicpp
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_HICPP_MULTIWAY_PATHS_COVERED_H

clang-tidy/hicpp/MultiwayPathsCoveredCheck.cpp

+//===--- MultiwayPathsCoveredCheck.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 "MultiwayPathsCoveredCheck.h"
+#include "clang/AST/ASTContext.h"
+
+#include <limits>
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace hicpp {
+
+void MultiwayPathsCoveredCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "WarnOnMissingElse", WarnOnMissingElse);
+}
+
+void MultiwayPathsCoveredCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      stmt(allOf(
+          anyOf(switchStmt(forEachSwitchCase(defaultStmt()))
+                    .bind("switch-default"),
+                switchStmt(unless(hasDescendant(switchCase())))
+                    .bind("degenerate-switch"),
+                // This matcher must be the last one of the three
+                // 'switchStmt' options.
+                // Otherwise the cases 'switch-default' and
+                // 'degenerate-switch' are not found correctly.
+                switchStmt(forEachSwitchCase(unless(defaultStmt())))
+                    .bind("switch-no-default")),
+          switchStmt(hasCondition(allOf(
+              // Match on switch statements that have either a bitfield or an
+              // integer condition. The ordering in 'anyOf()' is important
+              // because the last condition is the most general.
+              anyOf(ignoringImpCasts(memberExpr(hasDeclaration(
+                        fieldDecl(isBitField()).bind("bitfield")))),
+                    hasDescendant(declRefExpr().bind("non-enum-condition"))),
+              // 'unless()' must be the last match here and must be bound,
+              // otherwise the matcher does not work correctly.
+              unless(hasDescendant(declRefExpr(hasType(enumType()))
+                                       .bind("enum-condition")))))))),
+      this);
+
+  // This option is noisy, therefore matching is configurable.
+  if (WarnOnMissingElse) {
+    Finder->addMatcher(
+        ifStmt(allOf(hasParent(ifStmt()), unless(hasElse(anything()))))
+            .bind("else-if"),
+        this);
+  }
+}
+
+static unsigned countCaseLabels(const SwitchStmt *Switch) {
+  unsigned CaseCount = 0;
+
+  const SwitchCase *CurrentCase = Switch->getSwitchCaseList();
+  while (CurrentCase) {
+    ++CaseCount;
+    CurrentCase = CurrentCase->getNextSwitchCase();
+  }
+
+  return CaseCount;
+}
+/// This function calculate 2 ** Bits and returns
+/// numeric_limits<std::size_t>::max() if an overflow occured.
+static std::size_t twoPow(std::size_t Bits) {
+  return Bits >= std::numeric_limits<std::size_t>::digits
+             ? std::numeric_limits<std::size_t>::max()
+             : static_cast<size_t>(1) << Bits;
+}
+/// Get the number of possible values that can be switched on for the type T.
+///
+/// \return - 0 if bitcount could not be determined
+///         - numeric_limits<std::size_t>::max() when overflow appeared due to
+///           more then 64 bits type size.
+static std::size_t getNumberOfPossibleValues(QualType T,
+                                             const ASTContext &Context) {
+  // `isBooleanType` must come first because `bool` is an integral type as well
+  // and would not return 2 as result.
+  if (T->isBooleanType())
+    return 2;
+  else if (T->isIntegralType(Context))
+    return twoPow(Context.getTypeSize(T));
+  else
+    return 1;
+}
+
+void MultiwayPathsCoveredCheck::check(const MatchFinder::MatchResult &Result) {
+  if (const auto *ElseIfWithoutElse =
+          Result.Nodes.getNodeAs<IfStmt>("else-if")) {
+    diag(ElseIfWithoutElse->getLocStart(),
+         "potentially uncovered codepath; add an ending else statement");
+    return;
+  }
+  // Checks the sanity of 'switch' statements that actually do define
+  // a default branch but might be degenerated by having no or only one case.
+  if (const auto *SwitchWithDefault =
+          Result.Nodes.getNodeAs<SwitchStmt>("switch-default")) {
+    handleSwitchWithDefault(SwitchWithDefault);
+    return;
+  }
+  // Checks all 'switch' statements that do not define a default label.
+  // Here the heavy lifting happens.
+  if (const auto *SwitchWithoutDefault =
+          Result.Nodes.getNodeAs<SwitchStmt>("switch-no-default")) {
+    handleSwitchWithoutDefault(SwitchWithoutDefault, Result);
+    return;
+  }
+  // Warns for degenerated 'switch' statements that neither define a case nor
+  // a default label.
+  if (const auto *DegenerateSwitch =
+          Result.Nodes.getNodeAs<SwitchStmt>("degenerate-switch")) {
+    diag(DegenerateSwitch->getLocStart(), "degenerated switch without labels");
+    return;
+  }
+  llvm_unreachable("matched a case, that was not explicitly handled");
+}
+
+void MultiwayPathsCoveredCheck::handleSwitchWithDefault(
+    const SwitchStmt *Switch) {
+  unsigned CaseCount = countCaseLabels(Switch);
+  assert(CaseCount > 0 && "Switch statement with supposedly one default "
+                          "branch did not contain any case labels");
+  if (CaseCount == 1 || CaseCount == 2)
+    diag(Switch->getLocStart(),
+         CaseCount == 1
+             ? "degenerated switch with default label only"
+             : "switch could be better written as an if/else statement");
+}
+
+void MultiwayPathsCoveredCheck::handleSwitchWithoutDefault(
+    const SwitchStmt *Switch, const MatchFinder::MatchResult &Result) {
+  // The matcher only works because some nodes are explicitly matched and
+  // bound but ignored. This is necessary to build the excluding logic for
+  // enums and 'switch' statements without a 'default' branch.
+  assert(!Result.Nodes.getNodeAs<DeclRefExpr>("enum-condition") &&
+         "switch over enum is handled by warnings already, explicitly ignoring "
+         "them");
+  assert(!Result.Nodes.getNodeAs<SwitchStmt>("switch-default") &&
+         "switch stmts with default branch do cover all paths, explicitly "
+         "ignoring them");
+  // Determine the number of case labels. Since 'default' is not present
+  // and duplicating case labels is not allowed this number represents
+  // the number of codepaths. It can be directly compared to 'MaxPathsPossible'
+  // to see if some cases are missing.
+  unsigned CaseCount = countCaseLabels(Switch);
+  // CaseCount == 0 is caught in DegenerateSwitch. Necessary because the
+  // matcher used for here does not match on degenerate 'switch'.
+  assert(CaseCount > 0 && "Switch statement without any case found. This case "
+                          "should be excluded by the matcher and is handled "
+                          "separatly.");
+  std::size_t MaxPathsPossible = [&]() {
+    if (const auto *GeneralCondition =
+            Result.Nodes.getNodeAs<DeclRefExpr>("non-enum-condition"))
+      return getNumberOfPossibleValues(GeneralCondition->getType(),
+                                       *Result.Context);
+    if (const auto *BitfieldDecl =
+            Result.Nodes.getNodeAs<FieldDecl>("bitfield"))
+      return twoPow(BitfieldDecl->getBitWidthValue(*Result.Context));
+    llvm_unreachable("either bitfield or non-enum must be condition");
+  }();
+
+  // FIXME: Transform the 'switch' into an 'if' for CaseCount == 1.
+  if (CaseCount < MaxPathsPossible)
+    diag(Switch->getLocStart(),
+         CaseCount == 1 ? "switch with only one case; use an if statement"
+                        : "potential uncovered code path; add a default label");
+}
+} // namespace hicpp
+} // namespace tidy
+} // namespace clang

docs/ReleaseNotes.rst

- New `cppcoreguidelines-owning-memory <http://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines-owning-memory.html>`_ check
   flow analysis.
 
 - New `hicpp-exception-baseclass
   <http://clang.llvm.org/extra/clang-tidy/checks/hicpp-exception-baseclass.html>`_ check
 
   Ensures that all exception will be instances of ``std::exception`` and classes 
   that are derived from it.
 
+- New `hicpp-multiway-paths-covered
+  <http://clang.llvm.org/extra/clang-tidy/checks/hicpp-multiway-paths-covered.html>`_ check
+
+  Checks on various possible constellations where ``switch`` and ``if-else if`` statements
+  do not cover all possible codepaths.
+
 - New `hicpp-signed-bitwise
   <http://clang.llvm.org/extra/clang-tidy/checks/hicpp-signed-bitwise.html>`_ check
 
   Finds uses of bitwise operations on signed integer types, which may lead to 
   undefined or implementation defined behaviour.
 
 - New `android-cloexec-inotify-init1
   <http://clang.llvm.org/extra/clang-tidy/checks/android-cloexec-inotify-init1.html>`_ check

docs/clang-tidy/checks/hicpp-multiway-paths-covered.rst

+.. title:: clang-tidy - hicpp-multiway-paths-covered
+
+hicpp-multiway-paths-covered
+============================
+
+This check discovers situations where code paths are not fully-covered.
+It furthermore suggests using ``if`` instead of ``switch`` if the code will be more clear.
+The `rule 6.1.2 <http://www.codingstandard.com/rule/6-1-2-explicitly-cover-all-paths-through-multi-way-selection-statements/>`_
+and `rule 6.1.4 <http://www.codingstandard.com/rule/6-1-4-ensure-that-a-switch-statement-has-at-least-two-case-labels-distinct-from-the-default-label/>`_
+of the High Integrity C++ Coding Standard are enforced.
+
+``if-else if`` chains that miss a final ``else`` branch might lead to unexpected 
+program execution and be the result of a logical error.
+If the missing ``else`` branch is intended you can leave it empty with a clarifying
+comment.
+This warning can be noisy on some code bases, so it is disabled by default.
+
+.. code-block:: c++
+
+  void f1() {
+    int i = determineTheNumber();
+
+     if(i > 0) { 
+       // Some Calculation 
+     } else if (i < 0) { 
+       // Precondition violated or something else. 
+     }
+     // ...
+  }
+
+Similar arguments hold for ``switch`` statements which do not cover all possible code paths.
+
+.. code-block:: c++
+
+  // The missing default branch might be a logical error. It can be kept empty
+  // if there is nothing to do, making it explicit.
+  void f2(int i) {
+    switch (i) {
+    case 0: // something
+      break;
+    case 1: // something else
+      break;
+    }
+    // All other numbers?
+  }
+
+  // Violates this rule as well, but already emits a compiler warning (-Wswitch).
+  enum Color { Red, Green, Blue, Yellow };
+  void f3(enum Color c) {
+    switch (c) {
+    case Red: // We can't drive for now.
+      break;
+    case Green:  // We are allowed to drive.
+      break;
+    }
+    // Other cases missing
+  }
+
+
+The `rule 6.1.4 <http://www.codingstandard.com/rule/6-1-4-ensure-that-a-switch-statement-has-at-least-two-case-labels-distinct-from-the-default-label/>`_
+requires every ``switch`` statement to have at least two ``case`` labels other than a `default` label.
+Otherwise, the ``switch`` could be better expressed with an ``if`` statement.
+Degenerated ``switch`` statements without any labels are caught as well.
+
+.. code-block:: c++
+
+  // Degenerated switch that could be better written as `if`
+  int i = 42;
+  switch(i) {
+    case 1: // do something here
+    default: // do somethe else here
+  }
+
+  // Should rather be the following:
+  if (i == 1) { 
+    // do something here 
+  }
+  else { 
+    // do something here 
+  }
+
+
+.. code-block:: c++
+  
+  // A completly degenerated switch will be diagnosed.
+  int i = 42;
+  switch(i) {}
+
+
+Options
+-------
+
+.. option:: WarnOnMissingElse
+
+  Activates warning for missing``else`` branches. Default is `0`.

docs/clang-tidy/checks/list.rst

.. toctree::
    cert-err58-cpp
    cert-err60-cpp
    cert-err61-cpp (redirects to misc-throw-by-value-catch-by-reference) <cert-err61-cpp>
    cert-fio38-c (redirects to misc-non-copyable-objects) <cert-fio38-c>
    cert-flp30-c
    cert-msc30-c (redirects to cert-msc50-cpp) <cert-msc30-c>
    cert-msc50-cpp
    cert-oop11-cpp (redirects to misc-move-constructor-init) <cert-oop11-cpp>
    cppcoreguidelines-c-copy-assignment-signature (redirects to misc-unconventional-assign-operator) <cppcoreguidelines-c-copy-assignment-signature>
    cppcoreguidelines-interfaces-global-init
    cppcoreguidelines-no-malloc
    cppcoreguidelines-owning-memory
    cppcoreguidelines-pro-bounds-array-to-pointer-decay
    cppcoreguidelines-pro-bounds-constant-array-index
    cppcoreguidelines-pro-bounds-pointer-arithmetic
    cppcoreguidelines-pro-type-const-cast
    cppcoreguidelines-pro-type-cstyle-cast
.. toctree::
    hicpp-braces-around-statements (redirects to readability-braces-around-statements) <hicpp-braces-around-statements>
    hicpp-deprecated-headers (redirects to modernize-deprecated-headers) <hicpp-deprecated-headers>
    hicpp-exception-baseclass
    hicpp-explicit-conversions (redirects to google-explicit-constructor) <hicpp-explicit-conversions>
    hicpp-function-size (redirects to readability-function-size) <hicpp-function-size>
    hicpp-invalid-access-moved (redirects to misc-use-after-move) <hicpp-invalid-access-moved>
    hicpp-member-init (redirects to cppcoreguidelines-pro-type-member-init) <hicpp-member-init>
    hicpp-move-const-arg (redirects to misc-move-const-arg) <hicpp-move-const-arg>
+   hicpp-multiway-paths-covered
    hicpp-named-parameter (redirects to readability-named-parameter) <hicpp-named-parameter>
    hicpp-new-delete-operators (redirects to misc-new-delete-overloads) <hicpp-new-delete-operators>
    hicpp-no-array-decay (redirects to cppcoreguidelines-pro-bounds-array-to-pointer-decay) <hicpp-no-array-decay>
    hicpp-no-assembler
    hicpp-no-malloc (redirects to cppcoreguidelines-no-malloc) <hicpp-no-malloc>
    hicpp-noexcept-move (redirects to misc-noexcept-moveconstructor) <hicpp-noexcept-move>
    hicpp-signed-bitwise
    hicpp-special-member-functions (redirects to cppcoreguidelines-special-member-functions) <hicpp-special-member-functions>

test/clang-tidy/hicpp-multiway-paths-covered-else.cpp

+// RUN: %check_clang_tidy %s hicpp-multiway-paths-covered %t \
+// RUN: -config='{CheckOptions: \
+// RUN:  [{key: hicpp-multiway-paths-covered.WarnOnMissingElse, value: 1}]}'\
+// RUN: --
+
+enum OS { Mac,
+          Windows,
+          Linux };
+
+void problematic_if(int i, enum OS os) {
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    // CHECK-MESSAGES: [[@LINE-1]]:10: warning: potentially uncovered codepath; add an ending else statement
+    return;
+  }
+
+  // Could be considered as false positive because all paths are covered logically.
+  // I still think this is valid since the possibility of a final 'everything else'
+  // codepath is expected from if-else if.
+  if (i > 0) {
+    return;
+  } else if (i <= 0) {
+    // CHECK-MESSAGES: [[@LINE-1]]:10: warning: potentially uncovered codepath; add an ending else statement
+    return;
+  }
+
+  // Test if nesting of if-else chains does get caught as well.
+  if (os == Mac) {
+    return;
+  } else if (os == Linux) {
+    // These checks are kind of degenerated, but the check will not try to solve
+    // if logically all paths are covered, which is more the area of the static analyzer.
+    if (true) {
+      return;
+    } else if (false) {
+      // CHECK-MESSAGES: [[@LINE-1]]:12: warning: potentially uncovered codepath; add an ending else statement
+      return;
+    }
+    return;
+  } else {
+    /* unreachable */
+    if (true) // check if the parent would match here as well
+      return;
+    // No warning for simple if statements, since it is common to just test one condition
+    // and ignore the opposite.
+  }
+
+  // Ok, because all paths are covered
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    return;
+  } else {
+    /* error, maybe precondition failed */
+  }
+}

test/clang-tidy/hicpp-multiway-paths-covered.cpp

+// RUN: %check_clang_tidy %s hicpp-multiway-paths-covered %t
+
+enum OS { Mac,
+          Windows,
+          Linux };
+
+struct Bitfields {
+  unsigned UInt : 3;
+  int SInt : 1;
+};
+
+int return_integer() { return 42; }
+
+void bad_switch(int i) {
+  switch (i) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: switch with only one case; use an if statement
+  case 0:
+    break;
+  }
+  // No default in this switch
+  switch (i) {
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 0:
+    break;
+  case 1:
+    break;
+  case 2:
+    break;
+  }
+
+  // degenerate, maybe even warning
+  switch (i) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: degenerated switch without labels
+  }
+
+  switch (int j = return_integer()) {
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 0:
+  case 1:
+  case 2:
+    break;
+  }
+
+  // Degenerated, only default case.
+  switch (i) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: degenerated switch with default label only
+  default:
+    break;
+  }
+
+  // Degenerated, only one case label and default case -> Better as if-stmt.
+  switch (i) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: switch could be better written as an if/else statement
+  case 0:
+    break;
+  default:
+    break;
+  }
+
+  unsigned long long BigNumber = 0;
+  switch (BigNumber) {
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 0:
+  case 1:
+    break;
+  }
+
+  const int &IntRef = i;
+  switch (IntRef) {
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 0:
+  case 1:
+    break;
+  }
+
+  char C = 'A';
+  switch (C) {
+  // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 'A':
+    break;
+  case 'B':
+    break;
+  }
+
+  Bitfields Bf;
+  // UInt has 3 bits size.
+  switch (Bf.UInt) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: potential uncovered code path; add a default label
+  case 0:
+  case 1:
+    break;
+  }
+  // All paths explicitly covered.
+  switch (Bf.UInt) {
+  case 0:
+  case 1:
+  case 2:
+  case 3:
+  case 4:
+  case 5:
+  case 6:
+  case 7:
+    break;
+  }
+  // SInt has 1 bit size, so this is somewhat degenerated.
+  switch (Bf.SInt) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: switch with only one case; use an if statement
+  case 0:
+    break;
+  }
+  // All paths explicitly covered.
+  switch (Bf.SInt) {
+  case 0:
+  case 1:
+    break;
+  }
+
+  bool Flag = false;
+  switch (Flag) {
+    // CHECK-MESSAGES:[[@LINE-1]]:3: warning: switch with only one case; use an if statement
+  case true:
+    break;
+  }
+
+  switch (Flag) {
+    // CHECK-MESSAGES: [[@LINE-1]]:3: warning: degenerated switch with default label only
+  default:
+    break;
+  }
+
+  // This `switch` will create a frontend warning from '-Wswitch-bool' but is
+  // ok for this check.
+  switch (Flag) {
+  case true:
+    break;
+  case false:
+    break;
+  }
+}
+
+void unproblematic_switch(unsigned char c) {
+  switch (c) {
+  case 0:
+  case 1:
+  case 2:
+  case 3:
+  case 4:
+  case 5:
+  case 6:
+  case 7:
+  case 8:
+  case 9:
+  case 10:
+  case 11:
+  case 12:
+  case 13:
+  case 14:
+  case 15:
+  case 16:
+  case 17:
+  case 18:
+  case 19:
+  case 20:
+  case 21:
+  case 22:
+  case 23:
+  case 24:
+  case 25:
+  case 26:
+  case 27:
+  case 28:
+  case 29:
+  case 30:
+  case 31:
+  case 32:
+  case 33:
+  case 34:
+  case 35:
+  case 36:
+  case 37:
+  case 38:
+  case 39:
+  case 40:
+  case 41:
+  case 42:
+  case 43:
+  case 44:
+  case 45:
+  case 46:
+  case 47:
+  case 48:
+  case 49:
+  case 50:
+  case 51:
+  case 52:
+  case 53:
+  case 54:
+  case 55:
+  case 56:
+  case 57:
+  case 58:
+  case 59:
+  case 60:
+  case 61:
+  case 62:
+  case 63:
+  case 64:
+  case 65:
+  case 66:
+  case 67:
+  case 68:
+  case 69:
+  case 70:
+  case 71:
+  case 72:
+  case 73:
+  case 74:
+  case 75:
+  case 76:
+  case 77:
+  case 78:
+  case 79:
+  case 80:
+  case 81:
+  case 82:
+  case 83:
+  case 84:
+  case 85:
+  case 86:
+  case 87:
+  case 88:
+  case 89:
+  case 90:
+  case 91:
+  case 92:
+  case 93:
+  case 94:
+  case 95:
+  case 96:
+  case 97:
+  case 98:
+  case 99:
+  case 100:
+  case 101:
+  case 102:
+  case 103:
+  case 104:
+  case 105:
+  case 106:
+  case 107:
+  case 108:
+  case 109:
+  case 110:
+  case 111:
+  case 112:
+  case 113:
+  case 114:
+  case 115:
+  case 116:
+  case 117:
+  case 118:
+  case 119:
+  case 120:
+  case 121:
+  case 122:
+  case 123:
+  case 124:
+  case 125:
+  case 126:
+  case 127:
+  case 128:
+  case 129:
+  case 130:
+  case 131:
+  case 132:
+  case 133:
+  case 134:
+  case 135:
+  case 136:
+  case 137:
+  case 138:
+  case 139:
+  case 140:
+  case 141:
+  case 142:
+  case 143:
+  case 144:
+  case 145:
+  case 146:
+  case 147:
+  case 148:
+  case 149:
+  case 150:
+  case 151:
+  case 152:
+  case 153:
+  case 154:
+  case 155:
+  case 156:
+  case 157:
+  case 158:
+  case 159:
+  case 160:
+  case 161:
+  case 162:
+  case 163:
+  case 164:
+  case 165:
+  case 166:
+  case 167:
+  case 168:
+  case 169:
+  case 170:
+  case 171:
+  case 172:
+  case 173:
+  case 174:
+  case 175:
+  case 176:
+  case 177:
+  case 178:
+  case 179:
+  case 180:
+  case 181:
+  case 182:
+  case 183:
+  case 184:
+  case 185:
+  case 186:
+  case 187:
+  case 188:
+  case 189:
+  case 190:
+  case 191:
+  case 192:
+  case 193:
+  case 194:
+  case 195:
+  case 196:
+  case 197:
+  case 198:
+  case 199:
+  case 200:
+  case 201:
+  case 202:
+  case 203:
+  case 204:
+  case 205:
+  case 206:
+  case 207:
+  case 208:
+  case 209:
+  case 210:
+  case 211:
+  case 212:
+  case 213:
+  case 214:
+  case 215:
+  case 216:
+  case 217:
+  case 218:
+  case 219:
+  case 220:
+  case 221:
+  case 222:
+  case 223:
+  case 224:
+  case 225:
+  case 226:
+  case 227:
+  case 228:
+  case 229:
+  case 230:
+  case 231:
+  case 232:
+  case 233:
+  case 234:
+  case 235:
+  case 236:
+  case 237:
+  case 238:
+  case 239:
+  case 240:
+  case 241:
+  case 242:
+  case 243:
+  case 244:
+  case 245:
+  case 246:
+  case 247:
+  case 248:
+  case 249:
+  case 250:
+  case 251:
+  case 252:
+  case 253:
+  case 254:
+  case 255:
+    break;
+  }
+
+  // Some paths are covered by the switch and a default case is present.
+  switch (c) {
+  case 1:
+  case 2:
+  case 3:
+  default:
+    break;
+  }
+}
+
+OS return_enumerator() {
+  return Linux;
+}
+
+// Enumpaths are already covered by a warning, this is just to ensure, that there is
+// no interference or false positives.
+// -Wswitch warns about uncovered enum paths and each here described case is already
+// covered.
+void switch_enums(OS os) {
+  switch (os) {
+  case Linux:
+    break;
+  }
+
+  switch (OS another_os = return_enumerator()) {
+  case Linux:
+    break;
+  }
+
+  switch (os) {
+  }
+}
+
+/// All of these cases will not emit a warning per default, but with explicit activation.
+/// Covered in extra test file.
+void problematic_if(int i, enum OS os) {
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    return;
+  }
+
+  if (os == Mac) {
+    return;
+  } else if (os == Linux) {
+    if (true) {
+      return;
+    } else if (false) {
+      return;
+    }
+    return;
+  } else {
+    /* unreachable */
+    if (true) // check if the parent would match here as well
+      return;
+  }
+
+  // Ok, because all paths are covered
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    return;
+  } else {
+    /* error, maybe precondition failed */
+  }
+}