[clang] [Clang] Warning as error for fold expressions over comparison operators (PR #136836)

[Younan Zhang via cfe-commits]

================
@@ -132,3 +132,30 @@ bool f();
 template <typename... T>
 void g(bool = (f<T>() || ...));
 }
+
+
+namespace comparison_warning {
+  struct S {
+    bool operator<(const S&) const;
+    bool operator<(int) const;
+    bool operator==(const S&) const;
+  };
+
+  template <typename...T>
+  void f(T... ts) {
+    (void)(ts == ...);
+    // expected-error at -1 2{{comparison in fold expression would evaluate to '(X == Y) == Z'}}
+    (void)(ts < ...);
+    // expected-error at -1 2{{comparison in fold expression would evaluate to '(X < Y) < Z'}}
+    (void)(... < ts);
+    // expected-error at -1 2{{comparison in fold expression would evaluate to '(X < Y) < Z'}}
+  }
+
+  void test() {
+    f(0, 1, 2); // expected-note{{in instantiation}}
+    f(0, 1); // expected-note{{in instantiation}}
----------------
zyn0217 wrote:

Hmmm. It looks strange to me if we warn for '0 < 1'

https://github.com/llvm/llvm-project/pull/136836