[clang] Fix pointer comparisons between pointers to constexpr-unknown

clang/lib/AST/ExprConstant.cpp

@@ -14477,12 +14477,6 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     if (!EvaluatePointer(E->getRHS(), RHSValue, Info) || !LHSOK)
       return false;
 
-    // If we have Unknown pointers we should fail if they are not global values.
-    if (!(IsGlobalLValue(LHSValue.getLValueBase()) &&
-          IsGlobalLValue(RHSValue.getLValueBase())) &&
-        (LHSValue.AllowConstexprUnknown || RHSValue.AllowConstexprUnknown))
-      return false;
-
     // Reject differing bases from the normal codepath; we special-case
     // comparisons to null.
     if (!HasSameBase(LHSValue, RHSValue)) {
@@ -14544,6 +14538,23 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
           (LHSValue.Base && isZeroSized(RHSValue)))
         return DiagComparison(
             diag::note_constexpr_pointer_comparison_zero_sized);
+      // A constexpr-unknown reference can be equal to any other lvalue, except
+      // for variables allocated during constant evaluation. (The "lifetime
+      // [...] includes the entire constant evaluation", so it has to be
+      // distinct from anything allocated during constant evaluation.)
+      //
+      // Theoretically we could handle other cases, but the standard doesn't say
+      // what other cases we need to handle; it just says an "equality
+      // operator where the result is unspecified" isn't a constant expression.
+      auto AllocatedDuringEval = [](LValue &Value) {
+        return Value.Base.is<DynamicAllocLValue>() ||
+               Value.getLValueCallIndex();
+      };
+      if ((LHSValue.AllowConstexprUnknown && !AllocatedDuringEval(RHSValue)) ||
+          (RHSValue.AllowConstexprUnknown && !AllocatedDuringEval(LHSValue)))
+        return DiagComparison(
+            diag::note_constexpr_pointer_comparison_unspecified);
+      // FIXME: Verify both variables are live.
       return Success(CmpResult::Unequal, E);
     }
 

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -319,7 +319,7 @@ namespace casting {
 }
 
 namespace pointer_comparisons {
-  extern int &extern_n; // interpreter-note 2 {{declared here}}
+  extern int &extern_n; // interpreter-note 4 {{declared here}}
   extern int &extern_n2;
   constexpr int f1(bool b, int& n) {
     if (b) {
@@ -330,14 +330,29 @@ namespace pointer_comparisons {
   // FIXME: interpreter incorrectly rejects; both sides are the same constexpr-unknown value.
   static_assert(f1(false, extern_n)); // interpreter-error {{static assertion expression is not an integral constant expression}} \
                                       // interpreter-note {{initializer of 'extern_n' is unknown}}
-  // FIXME: We should diagnose this: we don't know if the references bind
-  // to the same object.
-  static_assert(&extern_n != &extern_n2); // interpreter-error {{static assertion expression is not an integral constant expression}} \
+  static_assert(&extern_n != &extern_n2); // expected-error {{static assertion expression is not an integral constant expression}} \
+                                          // nointerpreter-note {{comparison between pointers to unrelated objects '&extern_n' and '&extern_n2' has unspecified value}} \
                                           // interpreter-note {{initializer of 'extern_n' is unknown}}
   void f2(const int &n) {
-    // FIXME: We should not diagnose this: the two objects provably have
-    // different addresses because the lifetime of "n" extends across
-    // the initialization.
-    constexpr int x = &x == &n; // nointerpreter-error {{must be initialized by a constant expression}}
+    // We can prove these two aren't equal, but for now we don't try.
+    constexpr int x = &x == &n; // nointerpreter-error {{must be initialized by a constant expression}} \
+                                // nointerpreter-note {{comparison between pointers to unrelated objects '&x' and '&n' has unspecified value}}
+    // Distinct variables are not equal, even if they're local variables.
+    constexpr int y = &x == &y;
+    static_assert(!y);
   }
+  constexpr int f3() {
+    int x;
+    return &x == &extern_n; // interpreter-note {{initializer of 'extern_n' is unknown}}
+  }
+  static_assert(!f3()); // interpreter-error {{static assertion expression is not an integral constant expression}} \
+                        // interpreter-note {{in call to 'f3()'}}
+  constexpr int f4() {
+    int *p = new int;
+    bool b = p == &extern_n; // interpreter-note {{initializer of 'extern_n' is unknown}}
+    delete p;
+    return b;
+  }
+  static_assert(!f4()); // interpreter-error {{static assertion expression is not an integral constant expression}} \
+                        // interpreter-note {{in call to 'f4()'}}
 }