Allow unnecessary-value-param to match templated functions including lambdas with auto.

[goldvitaly]

Clang-Tidy unnecessary-value-param value param will be triggered for templated functions if at least one instantiontion with expensive to copy type is present in translation unit.

It is relatively common mistake to write lambda functions with auto arguments for expensive to copy types.

Example: Copy of the vectors will happen on every comparison.

void SortBySize(std::vector<std::vector<int>>& a) {
  std::sort(
      a.begin(), a.end(),
      [](auto x, auto y) { return a.size() < b.size()});
}

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[llvmbot]

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clang-tidy

Author: Vitaly Goldshteyn (goldvitaly)

Changes

Clang-Tidy unnecessary-value-param value param will be triggered for templated functions if at least one instantiontion with expensive to copy type is present in translation unit.

It is relatively common mistake to write lambda functions with auto arguments for expensive to copy types.

Example: Copy of the vectors will happen on every comparison.

void SortBySize(std::vector<std::vector<int>>& a) {
  std::sort(
      a.begin(), a.end(),
      [](auto x, auto y) { return a.size() < b.size()});
}

---

Full diff: https://github.com/llvm/llvm-project/pull/97767.diff

3 Files Affected:

• (modified) clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp (+2-5)
• (added) clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp (+92)
• (modified) clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param.cpp (-45)

diff --git a/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp b/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
index c507043c367a8..fe8cc11018e1e 100644
--- a/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
+++ b/clang-tools-extra/clang-tidy/performance/UnnecessaryValueParamCheck.cpp
@@ -75,7 +75,7 @@ void UnnecessaryValueParamCheck::registerMatchers(MatchFinder *Finder) {
           functionDecl(hasBody(stmt()), isDefinition(), unless(isImplicit()),
                        unless(cxxMethodDecl(anyOf(isOverride(), isFinal()))),
                        has(typeLoc(forEach(ExpensiveValueParamDecl))),
-                       unless(isInstantiated()), decl().bind("functionDecl"))),
+                       decl().bind("functionDecl"))),
       this);
 }
 
@@ -133,12 +133,9 @@ void UnnecessaryValueParamCheck::check(const MatchFinder::MatchResult &Result) {
   // 2. the function is virtual as it might break overrides
   // 3. the function is referenced outside of a call expression within the
   //    compilation unit as the signature change could introduce build errors.
-  // 4. the function is a primary template or an explicit template
-  // specialization.
   const auto *Method = llvm::dyn_cast<CXXMethodDecl>(Function);
   if (Param->getBeginLoc().isMacroID() || (Method && Method->isVirtual()) ||
-      isReferencedOutsideOfCallExpr(*Function, *Result.Context) ||
-      (Function->getTemplatedKind() != FunctionDecl::TK_NonTemplate))
+      isReferencedOutsideOfCallExpr(*Function, *Result.Context))
     return;
   for (const auto *FunctionDecl = Function; FunctionDecl != nullptr;
        FunctionDecl = FunctionDecl->getPreviousDecl()) {
diff --git a/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp
new file mode 100644
index 0000000000000..4e6ec401f17c8
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp
@@ -0,0 +1,92 @@
+// RUN: %check_clang_tidy  -std=c++14-or-later %s performance-unnecessary-value-param %t
+
+struct ExpensiveToCopyType {
+  virtual ~ExpensiveToCopyType();
+};
+
+template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType S, T V) {
+  // CHECK-MESSAGES: [[@LINE-1]]:90: warning: the const qualified parameter 'S'
+  // CHECK-MESSAGES: [[@LINE-2]]:95: warning: the parameter 'V'
+  // CHECK-FIXES: template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType& S, const T& V) {
+}
+
+void instantiatedWithExpensiveValue() {
+  templateWithNonTemplatizedParameter(
+      ExpensiveToCopyType(), ExpensiveToCopyType());
+  templateWithNonTemplatizedParameter(ExpensiveToCopyType(), 5);
+}
+
+template <typename T> void templateWithNonTemplatizedParameterCheapTemplate(const ExpensiveToCopyType S, T V) {
+  // CHECK-MESSAGES: [[@LINE-1]]:103: warning: the const qualified parameter 'S'
+  // CHECK-FIXES: template <typename T> void templateWithNonTemplatizedParameterCheapTemplate(const ExpensiveToCopyType& S, T V) {
+}
+
+void instantiatedWithCheapValue() {
+  templateWithNonTemplatizedParameterCheapTemplate(ExpensiveToCopyType(), 5);
+}
+
+template <typename T> void nonInstantiatedTemplateWithConstValue(const T S) {}
+template <typename T> void nonInstantiatedTemplateWithNonConstValue(T S) {}
+
+template <typename T> void instantiatedTemplateSpecialization(T NoSpecS) {}
+template <>
+void instantiatedTemplateSpecialization<ExpensiveToCopyType>(
+    ExpensiveToCopyType SpecS) {
+  // CHECK-MESSAGES: [[@LINE-1]]:25: warning: the parameter 'SpecS'
+  // CHECK-FIXES: const T& NoSpecS
+  // CHECK-FIXES: const ExpensiveToCopyType& SpecS
+}
+
+void instantiatedTemplateSpecialization() {
+  instantiatedTemplateSpecialization(ExpensiveToCopyType());
+}
+
+template <typename T> void instantiatedTemplateWithConstValue(const T S) {
+  // CHECK-MESSAGES: [[@LINE-1]]:71: warning: the const qualified parameter 'S'
+  // CHECK-FIXES: template <typename T> void instantiatedTemplateWithConstValue(const T& S) {
+}
+
+void instantiatedConstValue() {
+  instantiatedTemplateWithConstValue(ExpensiveToCopyType());
+}
+
+template <typename T> void instantiatedTemplateWithNonConstValue(T S) {
+  // CHECK-MESSAGES: [[@LINE-1]]:68: warning: the parameter 'S'
+  // CHECK-FIXES: template <typename T> void instantiatedTemplateWithNonConstValue(const T& S) {
+}
+
+void instantiatedNonConstValue() {
+  instantiatedTemplateWithNonConstValue(ExpensiveToCopyType());
+}
+
+void lambdaConstValue() {
+  auto fn = [](const ExpensiveToCopyType S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:42: warning: the const qualified parameter 'S'
+    // CHECK-FIXES: auto fn = [](const ExpensiveToCopyType& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaNonConstValue() {
+  auto fn = [](ExpensiveToCopyType S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:36: warning: the parameter 'S'
+    // CHECK-FIXES: auto fn = [](const ExpensiveToCopyType& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaConstAutoValue() {
+  auto fn = [](const auto S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:27: warning: the const qualified parameter 'S'
+    // CHECK-FIXES: auto fn = [](const auto& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaNonConstAutoValue() {
+  auto fn = [](auto S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:21: warning: the parameter 'S'
+    // CHECK-FIXES: auto fn = [](const auto& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param.cpp b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param.cpp
index d578eedd94a39..0dffaefa213a4 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param.cpp
@@ -107,19 +107,6 @@ struct PositiveConstValueConstructor {
   // CHECK-FIXES: PositiveConstValueConstructor(const ExpensiveToCopyType& ConstCopy) {}
 };
 
-template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType S, T V) {
-  // CHECK-MESSAGES: [[@LINE-1]]:90: warning: the const qualified parameter 'S'
-  // CHECK-FIXES-NOT: template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType& S, T V) {
-}
-
-void instantiated() {
-  templateWithNonTemplatizedParameter(ExpensiveToCopyType(), ExpensiveToCopyType());
-  templateWithNonTemplatizedParameter(ExpensiveToCopyType(), 5);
-}
-
-template <typename T> void negativeTemplateType(const T V) {
-}
-
 void negativeArray(const ExpensiveToCopyType[]) {
 }
 
@@ -370,35 +357,3 @@ void fun() {
   ExpensiveToCopyType E;
   NegativeUsingConstructor S(E);
 }
-
-template<typename T>
-void templateFunction(T) {
-}
-
-template<>
-void templateFunction<ExpensiveToCopyType>(ExpensiveToCopyType E) {
-  // CHECK-MESSAGES: [[@LINE-1]]:64: warning: the parameter 'E' is copied
-  // CHECK-FIXES: void templateFunction<ExpensiveToCopyType>(ExpensiveToCopyType E) {
-  E.constReference();
-}
-
-template <class T>
-T templateSpecializationFunction(ExpensiveToCopyType E) {
-  // CHECK-MESSAGES: [[@LINE-1]]:54: warning: the parameter 'E' is copied
-  // CHECK-FIXES-NOT: T templateSpecializationFunction(const ExpensiveToCopyType& E) {
-  return T();
-}
-
-template <>
-bool templateSpecializationFunction(ExpensiveToCopyType E) {
-  // CHECK-MESSAGES: [[@LINE-1]]:57: warning: the parameter 'E' is copied
-  // CHECK-FIXES-NOT: bool templateSpecializationFunction(const ExpensiveToCopyType& E) {
-  return true;
-}
-
-template <>
-int templateSpecializationFunction(ExpensiveToCopyType E) {
-  // CHECK-MESSAGES: [[@LINE-1]]:56: warning: the parameter 'E' is copied
-  // CHECK-FIXES-NOT: int templateSpecializationFunction(const ExpensiveToCopyType& E) {
-  return 0;
-}

[legrosbuffle]

There seems to be failing tests (see "buildkite" above).

Can you please update the release notes too (clang-tools-extra/docs/ReleaseNotes.rst) ?

[EugeneZelenko]

Please mention changes in Release Notes.

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