[clang-tidy] Add new check bugprone-unintended-char-ostream-output (llvm#127720)

It wants to find unintended character output from `uint8_t` and `int8_t`
to an ostream.
e.g.
```c++
uint8_t v = 9;
std::cout << v;
```

---------

Co-authored-by: whisperity <[email protected]>

Author: HerrCai0907

clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp

@@ -90,6 +90,7 @@
 #include "UndelegatedConstructorCheck.h"
 #include "UnhandledExceptionAtNewCheck.h"
 #include "UnhandledSelfAssignmentCheck.h"
+#include "UnintendedCharOstreamOutputCheck.h"
 #include "UniquePtrArrayMismatchCheck.h"
 #include "UnsafeFunctionsCheck.h"
 #include "UnusedLocalNonTrivialVariableCheck.h"
@@ -147,6 +148,8 @@ class BugproneModule : public ClangTidyModule {
         "bugprone-incorrect-enable-if");
     CheckFactories.registerCheck<IncorrectEnableSharedFromThisCheck>(
         "bugprone-incorrect-enable-shared-from-this");
+    CheckFactories.registerCheck<UnintendedCharOstreamOutputCheck>(
+        "bugprone-unintended-char-ostream-output");
     CheckFactories.registerCheck<ReturnConstRefFromParameterCheck>(
         "bugprone-return-const-ref-from-parameter");
     CheckFactories.registerCheck<SwitchMissingDefaultCaseCheck>(

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

@@ -29,6 +29,7 @@ add_clang_library(clangTidyBugproneModule STATIC
   InaccurateEraseCheck.cpp
   IncorrectEnableIfCheck.cpp
   IncorrectEnableSharedFromThisCheck.cpp
+  UnintendedCharOstreamOutputCheck.cpp
   ReturnConstRefFromParameterCheck.cpp
   SuspiciousStringviewDataUsageCheck.cpp
   SwitchMissingDefaultCaseCheck.cpp

clang-tools-extra/clang-tidy/bugprone/UnintendedCharOstreamOutputCheck.cpp

@@ -0,0 +1,91 @@
+//===--- UnintendedCharOstreamOutputCheck.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 "UnintendedCharOstreamOutputCheck.h"
+#include "clang/AST/Type.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Tooling/FixIt.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+namespace {
+
+// check if the type is unsigned char or signed char
+AST_MATCHER(Type, isNumericChar) {
+  return Node.isSpecificBuiltinType(BuiltinType::SChar) ||
+         Node.isSpecificBuiltinType(BuiltinType::UChar);
+}
+
+// check if the type is char
+AST_MATCHER(Type, isChar) {
+  return Node.isSpecificBuiltinType(BuiltinType::Char_S) ||
+         Node.isSpecificBuiltinType(BuiltinType::Char_U);
+}
+
+} // namespace
+
+UnintendedCharOstreamOutputCheck::UnintendedCharOstreamOutputCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context), CastTypeName(Options.get("CastTypeName")) {
+}
+void UnintendedCharOstreamOutputCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  if (CastTypeName.has_value())
+    Options.store(Opts, "CastTypeName", CastTypeName.value());
+}
+
+void UnintendedCharOstreamOutputCheck::registerMatchers(MatchFinder *Finder) {
+  auto BasicOstream =
+      cxxRecordDecl(hasName("::std::basic_ostream"),
+                    // only basic_ostream<char, Traits> has overload operator<<
+                    // with char / unsigned char / signed char
+                    classTemplateSpecializationDecl(
+                        hasTemplateArgument(0, refersToType(isChar()))));
+  Finder->addMatcher(
+      cxxOperatorCallExpr(
+          hasOverloadedOperatorName("<<"),
+          hasLHS(hasType(hasUnqualifiedDesugaredType(
+              recordType(hasDeclaration(cxxRecordDecl(
+                  anyOf(BasicOstream, isDerivedFrom(BasicOstream)))))))),
+          hasRHS(hasType(hasUnqualifiedDesugaredType(isNumericChar()))))
+          .bind("x"),
+      this);
+}
+
+void UnintendedCharOstreamOutputCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *Call = Result.Nodes.getNodeAs<CXXOperatorCallExpr>("x");
+  const Expr *Value = Call->getArg(1);
+  const SourceRange SourceRange = Value->getSourceRange();
+
+  DiagnosticBuilder Builder =
+      diag(Call->getOperatorLoc(),
+           "%0 passed to 'operator<<' outputs as character instead of integer. "
+           "cast to 'unsigned int' to print numeric value or cast to 'char' to "
+           "print as character")
+      << Value->getType() << SourceRange;
+
+  QualType T = Value->getType();
+  const Type *UnqualifiedDesugaredType = T->getUnqualifiedDesugaredType();
+
+  llvm::StringRef CastType = CastTypeName.value_or(
+      UnqualifiedDesugaredType->isSpecificBuiltinType(BuiltinType::SChar)
+          ? "int"
+          : "unsigned int");
+
+  Builder << FixItHint::CreateReplacement(
+      SourceRange, ("static_cast<" + CastType + ">(" +
+                    tooling::fixit::getText(*Value, *Result.Context) + ")")
+                       .str());
+}
+
+} // namespace clang::tidy::bugprone

clang-tools-extra/clang-tidy/bugprone/UnintendedCharOstreamOutputCheck.h

@@ -0,0 +1,38 @@
+//===--- UnintendedCharOstreamOutputCheck.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_BUGPRONE_UNINTENDEDCHAROSTREAMOUTPUTCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_UNINTENDEDCHAROSTREAMOUTPUTCHECK_H
+
+#include "../ClangTidyCheck.h"
+#include <optional>
+
+namespace clang::tidy::bugprone {
+
+/// Finds unintended character output from `unsigned char` and `signed char` to
+/// an ostream.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/bugprone/unintended-char-ostream-output.html
+class UnintendedCharOstreamOutputCheck : public ClangTidyCheck {
+public:
+  UnintendedCharOstreamOutputCheck(StringRef Name, ClangTidyContext *Context);
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+
+private:
+  const std::optional<StringRef> CastTypeName;
+};
+
+} // namespace clang::tidy::bugprone
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_UNINTENDEDCHAROSTREAMOUTPUTCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -91,6 +91,12 @@ Improvements to clang-tidy
 New checks
 ^^^^^^^^^^
 
+- New :doc:`bugprone-unintended-char-ostream-output
+  <clang-tidy/checks/bugprone/unintended-char-ostream-output>` check.
+
+  Finds unintended character output from ``unsigned char`` and ``signed char`` to an
+  ``ostream``.
+
 New check aliases
 ^^^^^^^^^^^^^^^^^
 

clang-tools-extra/docs/clang-tidy/checks/bugprone/unintended-char-ostream-output.rst

@@ -0,0 +1,45 @@
+.. title:: clang-tidy - bugprone-unintended-char-ostream-output
+
+bugprone-unintended-char-ostream-output
+=======================================
+
+Finds unintended character output from ``unsigned char`` and ``signed char`` to an
+``ostream``.
+
+Normally, when ``unsigned char (uint8_t)`` or ``signed char (int8_t)`` is used, it
+is more likely a number than a character. However, when it is passed directly to
+``std::ostream``'s ``operator<<``, the result is the character output instead
+of the numeric value. This often contradicts the developer's intent to print
+integer values.
+
+.. code-block:: c++
+
+  uint8_t v = 65;
+  std::cout << v; // output 'A' instead of '65'
+
+The check will suggest casting the value to an appropriate type to indicate the
+intent, by default, it will cast to ``unsigned int`` for ``unsigned char`` and
+``int`` for ``signed char``.
+
+.. code-block:: c++
+
+  std::cout << static_cast<unsigned int>(v); // when v is unsigned char
+  std::cout << static_cast<int>(v); // when v is signed char
+
+To avoid lengthy cast statements, add prefix ``+`` to the variable can also
+suppress warnings because unary expression will promote the value to an ``int``.
+
+.. code-block:: c++
+
+  std::cout << +v;
+
+Or cast to char to explicitly indicate that output should be a character.
+
+.. code-block:: c++
+
+  std::cout << static_cast<char>(v);
+
+.. option:: CastTypeName
+
+  When `CastTypeName` is specified, the fix-it will use `CastTypeName` as the
+  cast target type. Otherwise, fix-it will automatically infer the type.

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

@@ -158,6 +158,7 @@ Clang-Tidy Checks
    :doc:`bugprone-undelegated-constructor <bugprone/undelegated-constructor>`,
    :doc:`bugprone-unhandled-exception-at-new <bugprone/unhandled-exception-at-new>`,
    :doc:`bugprone-unhandled-self-assignment <bugprone/unhandled-self-assignment>`,
+   :doc:`bugprone-unintended-char-ostream-output <bugprone/unintended-char-ostream-output>`, "Yes"
    :doc:`bugprone-unique-ptr-array-mismatch <bugprone/unique-ptr-array-mismatch>`, "Yes"
    :doc:`bugprone-unsafe-functions <bugprone/unsafe-functions>`,
    :doc:`bugprone-unused-local-non-trivial-variable <bugprone/unused-local-non-trivial-variable>`,

clang-tools-extra/test/clang-tidy/checkers/bugprone/unintended-char-ostream-output-cast-type.cpp

@@ -0,0 +1,45 @@
+// RUN: %check_clang_tidy %s bugprone-unintended-char-ostream-output %t -- \
+// RUN:   -config="{CheckOptions: \
+// RUN:             {bugprone-unintended-char-ostream-output.CastTypeName: "uint8_t"}}"
+
+namespace std {
+
+template <class _CharT, class _Traits = void> class basic_ostream {
+public:
+  basic_ostream &operator<<(int);
+  basic_ostream &operator<<(unsigned int);
+};
+
+template <class CharT, class Traits>
+basic_ostream<CharT, Traits> &operator<<(basic_ostream<CharT, Traits> &, CharT);
+template <class CharT, class Traits>
+basic_ostream<CharT, Traits> &operator<<(basic_ostream<CharT, Traits> &, char);
+template <class _Traits>
+basic_ostream<char, _Traits> &operator<<(basic_ostream<char, _Traits> &, char);
+template <class _Traits>
+basic_ostream<char, _Traits> &operator<<(basic_ostream<char, _Traits> &,
+                                          signed char);
+template <class _Traits>
+basic_ostream<char, _Traits> &operator<<(basic_ostream<char, _Traits> &,
+                                          unsigned char);
+
+using ostream = basic_ostream<char>;
+
+} // namespace std
+
+class A : public std::ostream {};
+
+void origin_ostream(std::ostream &os) {
+  unsigned char unsigned_value = 9;
+  os << unsigned_value;
+  // CHECK-MESSAGES: [[@LINE-1]]:6: warning: 'unsigned char' passed to 'operator<<' outputs as character instead of integer
+  // CHECK-FIXES: os << static_cast<uint8_t>(unsigned_value);
+
+  signed char signed_value = 9;
+  os << signed_value;
+  // CHECK-MESSAGES: [[@LINE-1]]:6: warning: 'signed char' passed to 'operator<<' outputs as character instead of integer
+  // CHECK-FIXES: os << static_cast<uint8_t>(signed_value);
+
+  char char_value = 9;
+  os << char_value;
+}