[clang-tidy] Ignore expresions in unevaluated context in bugprone-inc-dec-in-conditions

[PiotrZSL]

Skip checking for references to variable in unevaluated context, like decltype, static_assert and so on.

Closes #85838

[llvmbot]

@llvm/pr-subscribers-clang-tidy

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

Author: Piotr Zegar (PiotrZSL)

Changes

Skip checking for references to variable in unevaluated context, like decltype, static_assert and so on.

Closes #85838

---

Full diff: https://github.com/llvm/llvm-project/pull/85849.diff

3 Files Affected:

• (modified) clang-tools-extra/clang-tidy/bugprone/IncDecInConditionsCheck.cpp (+6-1)
• (modified) clang-tools-extra/docs/ReleaseNotes.rst (+4)
• (modified) clang-tools-extra/test/clang-tidy/checkers/bugprone/inc-dec-in-conditions.cpp (+9)

diff --git a/clang-tools-extra/clang-tidy/bugprone/IncDecInConditionsCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/IncDecInConditionsCheck.cpp
index 16f43128d55e87..c9990cf6d10fd5 100644
--- a/clang-tools-extra/clang-tidy/bugprone/IncDecInConditionsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/IncDecInConditionsCheck.cpp
@@ -31,6 +31,10 @@ void IncDecInConditionsCheck::registerMatchers(MatchFinder *Finder) {
       anyOf(binaryOperator(anyOf(isComparisonOperator(), isLogicalOperator())),
             cxxOperatorCallExpr(isComparisonOperator())));
 
+  auto IsInUnevaluatedContext =
+      expr(anyOf(hasAncestor(expr(matchers::hasUnevaluatedContext())),
+                 hasAncestor(typeLoc())));
+
   Finder->addMatcher(
       expr(
           OperatorMatcher, unless(isExpansionInSystemHeader()),
@@ -47,7 +51,8 @@ void IncDecInConditionsCheck::registerMatchers(MatchFinder *Finder) {
                             hasDescendant(
                                 expr(unless(equalsBoundNode("operand")),
                                      matchers::isStatementIdenticalToBoundNode(
-                                         "operand"))
+                                         "operand"),
+                                     unless(IsInUnevaluatedContext))
                                     .bind("second")))))
                   .bind("operator"))),
       this);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 44680f79de6f54..8803be56d4fd1a 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -132,6 +132,10 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/assert-side-effect>` check by detecting side
   effect from calling a method with non-const reference parameters.
 
+- Improved :doc:`bugprone-inc-dec-in-conditions
+  <clang-tidy/checks/bugprone/inc-dec-in-conditions>` check to ignore code
+  within unevaluated contexts, such as ``decltype``.
+
 - Improved :doc:`bugprone-non-zero-enum-to-bool-conversion
   <clang-tidy/checks/bugprone/non-zero-enum-to-bool-conversion>` check by
   eliminating false positives resulting from direct usage of bitwise operators
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/inc-dec-in-conditions.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/inc-dec-in-conditions.cpp
index 82af039973c3bb..a9cb27a47f35b5 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/bugprone/inc-dec-in-conditions.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/inc-dec-in-conditions.cpp
@@ -68,3 +68,12 @@ bool doubleCheck(Container<int> x) {
   // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: decrementing and referencing a variable in a complex condition can cause unintended side-effects due to C++'s order of evaluation, consider moving the modification outside of the condition to avoid misunderstandings [bugprone-inc-dec-in-conditions]
   // CHECK-MESSAGES: :[[@LINE-2]]:31: warning: incrementing and referencing a variable in a complex condition can cause unintended side-effects due to C++'s order of evaluation, consider moving the modification outside of the condition to avoid misunderstandings [bugprone-inc-dec-in-conditions]
 }
+
+namespace PR85838 {
+  void test()
+  {
+    auto foo = 0;
+    auto bar = 0;
+    if (++foo < static_cast<decltype(foo)>(bar)) {}
+  }
+}

[5chmidti]

Looks good, but the operand node may also be in an unevaluated context, you only guard against the non ++/-- expr being in an unevaluated context here.
Fix: add unless(IsInUnevaluatedContext) to both expr().bind("operand").
Example: if (foo < static_cast<decltype(foo++)>(bar)) {}

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