[clang] Fix pointer comparisons between pointers to constexpr-unknown (#147663)

A constexpr-unknown reference can be equal to an arbitrary value, except
values allocated during constant evaluation. Fix the handling.

The standard is unclear exactly which pointer comparisons count as
"unknown" in this context; for example, in some cases we could use
alignment to prove two constexpr-unknown references are not equal. I
decided to ignore all the cases involving variables not allocated during
constant evaluation.

While looking at this, I also spotted that there might be issues with
lifetimes, but I didn't try to address it.

(cherry picked from commit 20c8e3c)

Author: efriedma-quic

clang/docs/ReleaseNotes.rst

@@ -967,6 +967,7 @@ Bug Fixes to C++ Support
 - Fix a crash with NTTP when instantiating local class.
 - Fixed a crash involving list-initialization of an empty class with a
   non-empty initializer list. (#GH147949)
+- Fixed constant evaluation of equality comparisons of constexpr-unknown references. (#GH147663)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -14478,12 +14478,6 @@ 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)) {
@@ -14545,6 +14539,10 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
           (LHSValue.Base && isZeroSized(RHSValue)))
         return DiagComparison(
             diag::note_constexpr_pointer_comparison_zero_sized);
+      if (LHSValue.AllowConstexprUnknown || RHSValue.AllowConstexprUnknown)
+        return DiagComparison(
+            diag::note_constexpr_pointer_comparison_unspecified);
+      // FIXME: Verify both variables are live.
       return Success(CmpResult::Unequal, E);
     }
 

clang/test/SemaCXX/constant-expression-cxx14.cpp

@@ -1321,3 +1321,18 @@ constexpr bool check = different_in_loop();
   // expected-error@-1 {{}} expected-note@-1 {{in call}}
 
 }
+
+namespace comparison_dead_variable {
+  constexpr bool f() {
+    int *p1 = 0, *p2 = 0;
+    {
+        int x = 0; p1 = &x;
+    }
+    {
+        int x = 0; p2 = &x;
+    }
+    return p1 != p2;
+  }
+  // FIXME: This should fail.
+  static_assert(f(),"");
+}

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 +330,30 @@ 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}}
+    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; // nointerpreter-note {{comparison between pointers to unrelated objects '&x' and '&extern_n' has unspecified value}} \
+                            // interpreter-note {{initializer of 'extern_n' is unknown}}
+  }
+  static_assert(!f3()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                        // expected-note {{in call to 'f3()'}}
+  constexpr int f4() {
+    int *p = new int;
+    bool b = p == &extern_n; // nointerpreter-note {{comparison between pointers to unrelated objects '&{*new int#0}' and '&extern_n' has unspecified value}} \
+                             // interpreter-note {{initializer of 'extern_n' is unknown}}
+    delete p;
+    return b;
+  }
+  static_assert(!f4()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                        // expected-note {{in call to 'f4()'}}
 }