diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index de76247..da47133 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1828,6 +1828,9 @@
   "incompatible operand types (%0 and %1)">;
 def err_typecheck_comparison_of_distinct_pointers : Error<
   "comparison of distinct pointer types (%0 and %1)">;
+def ext_typecheck_comparison_of_distinct_pointers_nonstandard : ExtWarn<
+  "comparison of distinct pointer types (%0 and %1) uses non-standard "
+  "composite pointer type %2">;
 def err_typecheck_vector_comparison : Error<
   "comparison of vector types (%0 and %1) not supported yet">;
 def err_typecheck_assign_const : Error<"read-only variable is not assignable">;
@@ -2252,6 +2255,9 @@
   "operand of type %0 where arithmetic or pointer type is required">;
 def err_typecheck_cond_incompatible_operands : Error<
   "incompatible operand types (%0 and %1)">;
+def ext_typecheck_cond_incompatible_operands_nonstandard : ExtWarn<
+  "incompatible operand types (%0 and %1) use non-standard composite pointer "
+  "type %2">;
 def err_cast_selector_expr : Error<
   "cannot type cast @selector expression">;
 def warn_typecheck_cond_incompatible_pointers : ExtWarn<
diff --git a/lib/Sema/Sema.h b/lib/Sema/Sema.h
index 2ed4bf6..42f9838 100644
--- a/lib/Sema/Sema.h
+++ b/lib/Sema/Sema.h
@@ -3947,7 +3947,8 @@
     Expr *&cond, Expr *&lhs, Expr *&rhs, SourceLocation questionLoc);
   QualType CXXCheckConditionalOperands( // C++ 5.16
     Expr *&cond, Expr *&lhs, Expr *&rhs, SourceLocation questionLoc);
-  QualType FindCompositePointerType(Expr *&E1, Expr *&E2); // C++ 5.9
+  QualType FindCompositePointerType(Expr *&E1, Expr *&E2,
+                                    bool *NonStandardCompositeType = 0);
 
   QualType FindCompositeObjCPointerType(Expr *&LHS, Expr *&RHS,
                                         SourceLocation questionLoc);
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index b074381..fc89b15 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -5324,11 +5324,18 @@
       //
       // C++ [expr.eq]p1 uses the same notion for (in)equality
       // comparisons of pointers.
-      QualType T = FindCompositePointerType(lex, rex);
+      bool NonStandardCompositeType = false;
+      QualType T = FindCompositePointerType(lex, rex,
+                              isSFINAEContext()? 0 : &NonStandardCompositeType);
       if (T.isNull()) {
         Diag(Loc, diag::err_typecheck_comparison_of_distinct_pointers)
           << lType << rType << lex->getSourceRange() << rex->getSourceRange();
         return QualType();
+      } else if (NonStandardCompositeType) {
+        Diag(Loc, 
+             diag::ext_typecheck_comparison_of_distinct_pointers_nonstandard)
+          << lType << rType << T 
+          << lex->getSourceRange() << rex->getSourceRange();
       }
 
       ImpCastExprToType(lex, T, CastExpr::CK_BitCast);
@@ -5390,11 +5397,18 @@
       //   of one of the operands, with a cv-qualification signature (4.4)
       //   that is the union of the cv-qualification signatures of the operand
       //   types.
-      QualType T = FindCompositePointerType(lex, rex);
+      bool NonStandardCompositeType = false;
+      QualType T = FindCompositePointerType(lex, rex,
+                              isSFINAEContext()? 0 : &NonStandardCompositeType);
       if (T.isNull()) {
         Diag(Loc, diag::err_typecheck_comparison_of_distinct_pointers)
-        << lType << rType << lex->getSourceRange() << rex->getSourceRange();
+          << lType << rType << lex->getSourceRange() << rex->getSourceRange();
         return QualType();
+      } else if (NonStandardCompositeType) {
+        Diag(Loc, 
+             diag::ext_typecheck_comparison_of_distinct_pointers_nonstandard)
+          << lType << rType << T 
+          << lex->getSourceRange() << rex->getSourceRange();
       }
 
       ImpCastExprToType(lex, T, CastExpr::CK_BitCast);
diff --git a/lib/Sema/SemaExprCXX.cpp b/lib/Sema/SemaExprCXX.cpp
index 1a6fdd7..df9442c 100644
--- a/lib/Sema/SemaExprCXX.cpp
+++ b/lib/Sema/SemaExprCXX.cpp
@@ -2080,9 +2080,18 @@
   //      performed to bring them to a common type, whose cv-qualification
   //      shall match the cv-qualification of either the second or the third
   //      operand. The result is of the common type.
-  QualType Composite = FindCompositePointerType(LHS, RHS);
-  if (!Composite.isNull())
+  bool NonStandardCompositeType = false;
+  QualType Composite = FindCompositePointerType(LHS, RHS,
+                              isSFINAEContext()? 0 : &NonStandardCompositeType);
+  if (!Composite.isNull()) {
+    if (NonStandardCompositeType)
+      Diag(QuestionLoc, 
+           diag::ext_typecheck_cond_incompatible_operands_nonstandard)
+        << LTy << RTy << Composite
+        << LHS->getSourceRange() << RHS->getSourceRange();
+      
     return Composite;
+  }
   
   // Similarly, attempt to find composite type of twp objective-c pointers.
   Composite = FindCompositeObjCPointerType(LHS, RHS, QuestionLoc);
@@ -2101,7 +2110,16 @@
 /// and @p E2 according to C++0x 5.9p2. It converts both expressions to this
 /// type and returns it.
 /// It does not emit diagnostics.
-QualType Sema::FindCompositePointerType(Expr *&E1, Expr *&E2) {
+///
+/// If \p NonStandardCompositeType is non-NULL, then we are permitted to find
+/// a non-standard (but still sane) composite type to which both expressions
+/// can be converted. When such a type is chosen, \c *NonStandardCompositeType
+/// will be set true.
+QualType Sema::FindCompositePointerType(Expr *&E1, Expr *&E2,
+                                        bool *NonStandardCompositeType) {
+  if (NonStandardCompositeType)
+    *NonStandardCompositeType = false;
+  
   assert(getLangOptions().CPlusPlus && "This function assumes C++");
   QualType T1 = E1->getType(), T2 = E2->getType();
 
@@ -2152,12 +2170,20 @@
   ContainingClassVector MemberOfClass;
   QualType Composite1 = Context.getCanonicalType(T1),
            Composite2 = Context.getCanonicalType(T2);
+  unsigned NeedConstBefore = 0;  
   do {
     const PointerType *Ptr1, *Ptr2;
     if ((Ptr1 = Composite1->getAs<PointerType>()) &&
         (Ptr2 = Composite2->getAs<PointerType>())) {
       Composite1 = Ptr1->getPointeeType();
       Composite2 = Ptr2->getPointeeType();
+      
+      // If we're allowed to create a non-standard composite type, keep track
+      // of where we need to fill in additional 'const' qualifiers. 
+      if (NonStandardCompositeType &&
+          Composite1.getCVRQualifiers() != Composite2.getCVRQualifiers())
+        NeedConstBefore = QualifierUnion.size();
+      
       QualifierUnion.push_back(
                  Composite1.getCVRQualifiers() | Composite2.getCVRQualifiers());
       MemberOfClass.push_back(std::make_pair((const Type *)0, (const Type *)0));
@@ -2169,6 +2195,13 @@
         (MemPtr2 = Composite2->getAs<MemberPointerType>())) {
       Composite1 = MemPtr1->getPointeeType();
       Composite2 = MemPtr2->getPointeeType();
+      
+      // If we're allowed to create a non-standard composite type, keep track
+      // of where we need to fill in additional 'const' qualifiers. 
+      if (NonStandardCompositeType &&
+          Composite1.getCVRQualifiers() != Composite2.getCVRQualifiers())
+        NeedConstBefore = QualifierUnion.size();
+      
       QualifierUnion.push_back(
                  Composite1.getCVRQualifiers() | Composite2.getCVRQualifiers());
       MemberOfClass.push_back(std::make_pair(MemPtr1->getClass(),
@@ -2182,6 +2215,18 @@
     break;
   } while (true);
 
+  if (NeedConstBefore && NonStandardCompositeType) {
+    // Extension: Add 'const' to qualifiers that come before the first qualifier
+    // mismatch, so that our (non-standard!) composite type meets the 
+    // requirements of C++ [conv.qual]p4 bullet 3.
+    for (unsigned I = 0; I != NeedConstBefore; ++I) {
+      if ((QualifierUnion[I] & Qualifiers::Const) == 0) {
+        QualifierUnion[I] = QualifierUnion[I] | Qualifiers::Const;
+        *NonStandardCompositeType = true;
+      }
+    }
+  }
+  
   // Rewrap the composites as pointers or member pointers with the union CVRs.
   ContainingClassVector::reverse_iterator MOC
     = MemberOfClass.rbegin();
diff --git a/test/SemaCXX/composite-pointer-type.cpp b/test/SemaCXX/composite-pointer-type.cpp
index fdf838f..e8b0920 100644
--- a/test/SemaCXX/composite-pointer-type.cpp
+++ b/test/SemaCXX/composite-pointer-type.cpp
@@ -50,3 +50,11 @@
 bool f(Matrix4 m1, const Matrix4 m2) {
   return m1 != m2;
 }
+
+// PR6346
+bool f1(bool b, void **p, const void **q) {
+  if (p == q) // expected-warning{{comparison of distinct pointer types ('void **' and 'void const **') uses non-standard composite pointer type 'void const *const *'}}
+    return false;
+
+  return b? p : q; // expected-warning{{incompatible operand types ('void **' and 'void const **') use non-standard composite pointer type 'void const *const *'}}
+}