[clang] [clang-tools-extra] [clang-tidy] fix misc-const-correctnes false-positive for fold expressions (PR #78320)

[Julian Schmidt via cfe-commits]

https://github.com/5chmidti created https://github.com/llvm/llvm-project/pull/78320

The check no longer emits a diagnostic for variables used as the
initializer of C++17 fold expressions.
The operator used is type-dependent because of the parameter pack
and can therefore not be guaranteed to not mutate the variable.

Fixes: #70323


>From d26970b63831d2554d5e9c6c73d41c147220d68d Mon Sep 17 00:00:00 2001
From: Julian Schmidt <44101708+5chmidti at users.noreply.github.com>
Date: Sat, 28 Oct 2023 18:39:18 +0200
Subject: [PATCH] [clang-tidy] fix misc-const-correctnes false-positive for
 fold expressions

The check no longer emits a diagnostic for variables used as the
initializer of C++17 fold expressions.
The operator used is type-dependent because of the parameter pack
and can therefore not be guaranteed to not mutate the variable.

Fixes: #70323
---
 clang-tools-extra/docs/ReleaseNotes.rst         | 10 +++-------
 .../misc/const-correctness-templates.cpp        | 17 +++++++++++++++++
 clang/lib/Analysis/ExprMutationAnalyzer.cpp     |  4 ++++
 .../Analysis/ExprMutationAnalyzerTest.cpp       | 15 +++++++++++++++
 4 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index d77267588db915..c975b99c5786be 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -382,7 +382,8 @@ Changes in existing checks
   using pointer to member function. Additionally, the check no longer emits
   a diagnostic when a variable that is not type-dependent is an operand of a
   type-dependent binary operator. Improved performance of the check through
-  optimizations.
+  optimizations. The check no longer emits a diagnostic for
+  variables used as the initializer of C++17 fold expressions.
 
 - Improved :doc:`misc-include-cleaner
   <clang-tidy/checks/misc/include-cleaner>` check by adding option
@@ -502,12 +503,7 @@ Changes in existing checks
   <clang-tidy/checks/readability/implicit-bool-conversion>` check to take
   do-while loops into account for the `AllowIntegerConditions` and
   `AllowPointerConditions` options. It also now provides more consistent
-  suggestions when parentheses are added to the return value or expressions. 
-  It also ignores false-positives for comparison containing bool bitfield.
-
-- Improved :doc:`readability-misleading-indentation
-  <clang-tidy/checks/readability/misleading-indentation>` check to ignore
-  false-positives for line started with empty macro.
+  suggestions when parentheses are added to the return value.
 
 - Improved :doc:`readability-non-const-parameter
   <clang-tidy/checks/readability/non-const-parameter>` check to ignore
diff --git a/clang-tools-extra/test/clang-tidy/checkers/misc/const-correctness-templates.cpp b/clang-tools-extra/test/clang-tidy/checkers/misc/const-correctness-templates.cpp
index 794578ceeeba8f..a4be41d20eae13 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/misc/const-correctness-templates.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/misc/const-correctness-templates.cpp
@@ -30,3 +30,20 @@ namespace gh57297{
 struct Stream { };
 template <typename T> void f() { T t; Stream x; x << t; }
 } // namespace gh57297
+
+namespace gh70323{
+// A fold expression may contain the checked variable as it's initializer.
+// We don't know if the operator modifies that variable because the
+// operator is type dependent due to the parameter pack.
+
+struct Stream {};
+template <typename T>
+Stream& operator<<(Stream&, T);
+template <typename... Args>
+void concatenate(Args... args)
+{
+    Stream stream;
+    (stream << ... << args);
+    (args << ... << stream);
+}
+} // namespace gh70323
diff --git a/clang/lib/Analysis/ExprMutationAnalyzer.cpp b/clang/lib/Analysis/ExprMutationAnalyzer.cpp
index 9af818be0415f3..bb042760d297a7 100644
--- a/clang/lib/Analysis/ExprMutationAnalyzer.cpp
+++ b/clang/lib/Analysis/ExprMutationAnalyzer.cpp
@@ -343,6 +343,10 @@ const Stmt *ExprMutationAnalyzer::findDirectMutation(const Expr *Exp) {
       // in different instantiations of the template.
       binaryOperator(isTypeDependent(),
                      hasEitherOperand(ignoringImpCasts(canResolveToExpr(Exp)))),
+      // A fold expression may contain `Exp` as it's initializer.
+      // We don't know if the operator modifies `Exp` because the
+      // operator is type dependent due to the parameter pack.
+      cxxFoldExpr(hasFoldInit(ignoringImpCasts(canResolveToExpr(Exp)))),
       // Within class templates and member functions the member expression might
       // not be resolved. In that case, the `callExpr` is considered to be a
       // modification.
diff --git a/clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp b/clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp
index a94f857720b035..50d0399ed4b015 100644
--- a/clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp
+++ b/clang/unittests/Analysis/ExprMutationAnalyzerTest.cpp
@@ -359,6 +359,21 @@ TEST(ExprMutationAnalyzerTest, DependentOperatorWithNonDependentOperand) {
   EXPECT_THAT(mutatedBy(Results, AST.get()), ElementsAre("x << t"));
 }
 
+TEST(ExprMutationAnalyzerTest, FoldExpression) {
+  // gh70323
+  // A fold expression may contain `Exp` as it's initializer.
+  // We don't know if the operator modifies `Exp` because the
+  // operator is type dependent due to the parameter pack.
+  const auto AST = buildASTFromCode(
+      "struct Stream {};"
+      "template <typename T> Stream& operator<<(Stream&, T); "
+      "template <typename... Args> void concatenate(Args... args) "
+      "{ Stream x; (x << ... << args); }");
+  const auto Results =
+      match(withEnclosingCompound(declRefTo("x")), AST->getASTContext());
+  EXPECT_THAT(mutatedBy(Results, AST.get()), ElementsAre("(x << ... << args)"));
+}
+
 // Section: expression as call argument
 
 TEST(ExprMutationAnalyzerTest, ByValueArgument) {