[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

Author: 5chmidti

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
@@ -502,7 +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. 
+  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-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

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.

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) {