[clang] [clang] Reject array comparisons in C++26 for unknown/dependent types in templates (PR #191101)

[via cfe-commits]

================
@@ -12860,6 +12837,33 @@ QualType Sema::CheckCompareOperands(ExprResult &LHS, ExprResult &RHS,
     return Ty->isPointerType() || Ty->isMemberPointerType();
   };
 
+  // C++1a [array.comp]:
+  //   Equality and relational comparisons ([expr.eq], [expr.rel]) between two
+  //   operands of array type.
+  // C++2a [depr.array.comp]:
+  //   Equality and relational comparisons ([expr.eq], [expr.rel]) between two
+  //   operands of array type are deprecated.
+  Expr *LHSStripped = LHS.get()->IgnoreParenImpCasts();
+  Expr *RHSStripped = RHS.get()->IgnoreParenImpCasts();
+  if (getLangOpts().CPlusPlus && LHSStripped->getType()->isArrayType() &&
+      RHSStripped->getType()->isArrayType()) {
+
+    // In an SFINAE context, don't emit an error.
+    // Just return the operands so the caller can return false.
+    if (isSFINAEContext()) {
+      return InvalidOperands(Loc, LHS, RHS);
+    }
+    auto IsDeprArrayComparionIgnored =
+        getDiagnostics().isIgnored(diag::warn_depr_array_comparison, Loc);
+    auto DiagID = getLangOpts().CPlusPlus26 ? diag::warn_array_comparison_cxx26
+                  : !getLangOpts().CPlusPlus20 || IsDeprArrayComparionIgnored
+                      ? diag::warn_array_comparison
+                      : diag::warn_depr_array_comparison;
+    this->Diag(Loc, DiagID)
----------------
Sirraide wrote:

```suggestion
    Diag(Loc, DiagID)
```

https://github.com/llvm/llvm-project/pull/191101