[clang-tidy]Add new check readability-avoid-nested-conditional-operator (#78022)

Finds nested conditional operator.
Nested conditional operators lead code hard to understand, so they
should be splited as several statement and stored in temporary varibale.

clang-tools-extra/clang-tidy/readability/AvoidNestedConditionalOperatorCheck.cpp

@@ -0,0 +1,52 @@
+//===--- AvoidNestedConditionalOperatorCheck.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 "AvoidNestedConditionalOperatorCheck.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Basic/DiagnosticIDs.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+void AvoidNestedConditionalOperatorCheck::registerMatchers(
+    MatchFinder *Finder) {
+  Finder->addMatcher(
+      conditionalOperator(
+          anyOf(
+              hasCondition(ignoringParenCasts(
+                  conditionalOperator().bind("nested-conditional-operator"))),
+              hasTrueExpression(ignoringParenCasts(
+                  conditionalOperator().bind("nested-conditional-operator"))),
+              hasFalseExpression(ignoringParenCasts(
+                  conditionalOperator().bind("nested-conditional-operator")))))
+          .bind("conditional-operator"),
+      this);
+}
+
+void AvoidNestedConditionalOperatorCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *CO =
+      Result.Nodes.getNodeAs<ConditionalOperator>("conditional-operator");
+  const auto *NCO = Result.Nodes.getNodeAs<ConditionalOperator>(
+      "nested-conditional-operator");
+  assert(CO);
+  assert(NCO);
+
+  if (CO->getBeginLoc().isMacroID() || NCO->getBeginLoc().isMacroID())
+    return;
+
+  diag(NCO->getBeginLoc(),
+       "conditional operator is used as sub-expression of parent conditional "
+       "operator, refrain from using nested conditional operators");
+  diag(CO->getBeginLoc(), "parent conditional operator here",
+       DiagnosticIDs::Note);
+}
+
+} // namespace clang::tidy::readability

clang-tools-extra/clang-tidy/readability/AvoidNestedConditionalOperatorCheck.h

@@ -0,0 +1,33 @@
+//===--- AvoidNestedConditionalOperatorCheck.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_READABILITY_AVOID_NESTED_CONDITIONAL_OPERATOR_CHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_AVOID_NESTED_CONDITIONAL_OPERATOR_CHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::readability {
+
+/// Identifies instances of nested conditional operators in the code.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/readability/avoid-nested-conditional-operator.html
+class AvoidNestedConditionalOperatorCheck : public ClangTidyCheck {
+public:
+  AvoidNestedConditionalOperatorCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  std::optional<TraversalKind> getCheckTraversalKind() const override {
+    return TK_IgnoreUnlessSpelledInSource;
+  }
+};
+
+} // namespace clang::tidy::readability
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_AVOID_NESTED_CONDITIONAL_OPERATOR_CHECK_H

clang-tools-extra/clang-tidy/readability/CMakeLists.txt

@@ -5,6 +5,7 @@ set(LLVM_LINK_COMPONENTS
 
 add_clang_library(clangTidyReadabilityModule
   AvoidConstParamsInDecls.cpp
+  AvoidNestedConditionalOperatorCheck.cpp
   AvoidReturnWithVoidValueCheck.cpp
   AvoidUnconditionalPreprocessorIfCheck.cpp
   BracesAroundStatementsCheck.cpp

clang-tools-extra/clang-tidy/readability/ReadabilityTidyModule.cpp

@@ -10,6 +10,7 @@
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 #include "AvoidConstParamsInDecls.h"
+#include "AvoidNestedConditionalOperatorCheck.h"
 #include "AvoidReturnWithVoidValueCheck.h"
 #include "AvoidUnconditionalPreprocessorIfCheck.h"
 #include "BracesAroundStatementsCheck.h"
@@ -64,6 +65,8 @@ public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
     CheckFactories.registerCheck<AvoidConstParamsInDecls>(
         "readability-avoid-const-params-in-decls");
+    CheckFactories.registerCheck<AvoidNestedConditionalOperatorCheck>(
+        "readability-avoid-nested-conditional-operator");
     CheckFactories.registerCheck<AvoidReturnWithVoidValueCheck>(
         "readability-avoid-return-with-void-value");
     CheckFactories.registerCheck<AvoidUnconditionalPreprocessorIfCheck>(

clang-tools-extra/docs/ReleaseNotes.rst

@@ -224,6 +224,11 @@ New checks
   Recommends the smallest possible underlying type for an ``enum`` or ``enum``
   class based on the range of its enumerators.
 
+- New :doc:`readability-avoid-nested-conditional-operator
+  <clang-tidy/checks/readability/avoid-nested-conditional-operator>` check.
+
+  Identifies instances of nested conditional operators in the code.
+
 - New :doc:`readability-avoid-return-with-void-value
   <clang-tidy/checks/readability/avoid-return-with-void-value>` check.
 

clang-tools-extra/docs/clang-tidy/checks/list.rst

@@ -337,6 +337,7 @@ Clang-Tidy Checks
    :doc:`portability-simd-intrinsics <portability/simd-intrinsics>`,
    :doc:`portability-std-allocator-const <portability/std-allocator-const>`,
    :doc:`readability-avoid-const-params-in-decls <readability/avoid-const-params-in-decls>`, "Yes"
+   :doc:`readability-avoid-nested-conditional-operator <readability/avoid-nested-conditional-operator>`,
    :doc:`readability-avoid-return-with-void-value <readability/avoid-return-with-void-value>`,
    :doc:`readability-avoid-unconditional-preprocessor-if <readability/avoid-unconditional-preprocessor-if>`,
    :doc:`readability-braces-around-statements <readability/braces-around-statements>`, "Yes"

clang-tools-extra/docs/clang-tidy/checks/readability/avoid-nested-conditional-operator.rst

@@ -0,0 +1,21 @@
+.. title:: clang-tidy - readability-avoid-nested-conditional-operator
+
+readability-avoid-nested-conditional-operator
+=============================================
+
+Identifies instances of nested conditional operators in the code.
+
+Nested conditional operators, also known as ternary operators, can contribute
+to reduced code readability and comprehension. So they should be split as
+several statements and stored the intermediate results in temporary variable.
+
+Examples:
+
+.. code-block:: c++
+
+  int NestInConditional = (condition1 ? true1 : false1) ? true2 : false2;
+  int NestInTrue = condition1 ? (condition2 ? true1 : false1) : false2;
+  int NestInFalse = condition1 ? true1 : condition2 ? true2 : false1;
+
+This check implements part of `AUTOSAR C++14 Rule A5-16-1
+<https://www.autosar.org/fileadmin/standards/R22-11/AP/AUTOSAR_RS_CPP14Guidelines.pdf>`_.

clang-tools-extra/test/clang-tidy/checkers/readability/avoid-nested-conditional-operator.cpp

@@ -0,0 +1,24 @@
+// RUN: %check_clang_tidy %s readability-avoid-nested-conditional-operator %t
+
+int NestInConditional = (true ? true : false) ? 1 : 2;
+// CHECK-MESSAGES: :[[@LINE-1]]:26: warning: conditional operator is used as sub-expression of parent conditional operator, refrain from using nested conditional operators
+// CHECK-MESSAGES: :[[@LINE-2]]:25: note: parent conditional operator here
+
+int NestInTrue = true ? (true ? 1 : 2) : 2;
+// CHECK-MESSAGES: :[[@LINE-1]]:26: warning: conditional operator is used as sub-expression of parent conditional operator, refrain from using nested conditional operators
+// CHECK-MESSAGES: :[[@LINE-2]]:18: note: parent conditional operator here
+
+int NestInFalse = true ? 1 : true ? 1 : 2;
+// CHECK-MESSAGES: :[[@LINE-1]]:30: warning: conditional operator is used as sub-expression of parent conditional operator, refrain from using nested conditional operators
+// CHECK-MESSAGES: :[[@LINE-2]]:19: note: parent conditional operator here
+int NestInFalse2 = true ? 1 : (true ? 1 : 2);
+// CHECK-MESSAGES: :[[@LINE-1]]:32: warning: conditional operator is used as sub-expression of parent conditional operator, refrain from using nested conditional operators
+// CHECK-MESSAGES: :[[@LINE-2]]:20: note: parent conditional operator here
+
+int NestWithParensis = true ? 1 : ((((true ? 1 : 2))));
+// CHECK-MESSAGES: :[[@LINE-1]]:39: warning: conditional operator is used as sub-expression of parent conditional operator, refrain from using nested conditional operators
+// CHECK-MESSAGES: :[[@LINE-2]]:24: note: parent conditional operator here
+
+#define CONDITIONAL_EXPR (true ? 1 : 2)
+// not diag for macro since it will not reduce readability
+int NestWithMacro = true ? CONDITIONAL_EXPR : 2;