[clang] [clang][Sema] Warn consecutive builtin comparisons in an expression (PR #92200)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Youngsuk Kim (JOE1994)

<details>
<summary>Changes</summary>

Made the following decisions for consistency with `gcc 14.1`:
* Add warning under -Wparentheses
* Set the warning to DefaultIgnore, although -Wparentheses is enabled by default
  * This warning is only issued when -Wparentheses is explicitly enabled (via -Wparentheses or -Wall)

Closes #<!-- -->20456

---
Full diff: https://github.com/llvm/llvm-project/pull/92200.diff


4 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3) 
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+4) 
- (modified) clang/lib/Sema/SemaExpr.cpp (+5) 
- (modified) clang/test/Sema/parentheses.cpp (+7) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ae699ebfc6038..13d58e69aeeb7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -487,6 +487,9 @@ Improvements to Clang's diagnostics
        }
      };
 
+- Clang emits a ``-Wparentheses`` warning for expressions with consecutive comparisons like ``x < y < z``.
+  It was made a ``-Wparentheses`` warning to be consistent with gcc.
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 6100fba510059..612043f410890 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6911,6 +6911,10 @@ def warn_precedence_bitwise_conditional : Warning<
 def note_precedence_conditional_first : Note<
   "place parentheses around the '?:' expression to evaluate it first">;
 
+def warn_consecutive_comparison : Warning<
+  "comparisons like 'X<=Y<=Z' don't have their mathematical meaning">,
+  InGroup<Parentheses>, DefaultIgnore;
+
 def warn_enum_constant_in_bool_context : Warning<
   "converting the enum constant to a boolean">,
   InGroup<IntInBoolContext>, DefaultIgnore;
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index ec84798e4ce60..fab34f4fa3e14 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -14878,6 +14878,11 @@ ExprResult Sema::CreateBuiltinBinOp(SourceLocation OpLoc,
   case BO_GT:
     ConvertHalfVec = true;
     ResultTy = CheckCompareOperands(LHS, RHS, OpLoc, Opc);
+
+    if (const auto *BI = dyn_cast<BinaryOperator>(LHSExpr))
+      if (BI->isComparisonOp())
+        Diag(OpLoc, diag::warn_consecutive_comparison);
+
     break;
   case BO_EQ:
   case BO_NE:
diff --git a/clang/test/Sema/parentheses.cpp b/clang/test/Sema/parentheses.cpp
index 324d9b5f1e414..8e546461fb643 100644
--- a/clang/test/Sema/parentheses.cpp
+++ b/clang/test/Sema/parentheses.cpp
@@ -215,3 +215,10 @@ namespace PR20735 {
     // fix-it:"{{.*}}":{[[@LINE-9]]:20-[[@LINE-9]]:20}:")"
   }
 }
+
+void consecutive_builtin_compare(int x, int y, int z) {
+  (void)(x < y < z);  // expected-warning {{comparisons like 'X<=Y<=Z' don't have their mathematical meaning}}
+  (void)(x < y > z);  // expected-warning {{comparisons like 'X<=Y<=Z' don't have their mathematical meaning}}
+  (void)(x < y <= z); // expected-warning {{comparisons like 'X<=Y<=Z' don't have their mathematical meaning}}
+  (void)(x <= y > z); // expected-warning {{comparisons like 'X<=Y<=Z' don't have their mathematical meaning}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/92200