Index: cfe/trunk/include/clang/AST/DeclObjC.h
===================================================================
--- cfe/trunk/include/clang/AST/DeclObjC.h
+++ cfe/trunk/include/clang/AST/DeclObjC.h
@@ -1039,10 +1039,9 @@
   typedef llvm::DenseMap<std::pair<IdentifierInfo*,
                                    unsigned/*isClassProperty*/>,
                          ObjCPropertyDecl*> PropertyMap;
-  
-  typedef llvm::DenseMap<const ObjCProtocolDecl *, ObjCPropertyDecl*>
-            ProtocolPropertyMap;
-  
+
+  typedef llvm::SmallDenseSet<const ObjCProtocolDecl *, 8> ProtocolPropertySet;
+
   typedef llvm::SmallVector<ObjCPropertyDecl*, 8> PropertyDeclOrder;
   
   /// This routine collects list of properties to be implemented in the class.
@@ -2159,7 +2158,8 @@
                                     PropertyDeclOrder &PO) const override;
 
   void collectInheritedProtocolProperties(const ObjCPropertyDecl *Property,
-                                          ProtocolPropertyMap &PM) const;
+                                          ProtocolPropertySet &PS,
+                                          PropertyDeclOrder &PO) const;
 
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == ObjCProtocol; }
Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -808,8 +808,10 @@
   "property type %0 is incompatible with type %1 inherited from %2">,
   InGroup<DiagGroup<"incompatible-property-type">>;
 def warn_protocol_property_mismatch : Warning<
-  "property of type %0 was selected for synthesis">,
+  "property %select{of type %1|with attribute '%1'|without attribute '%1'|with "
+  "getter %1|with setter %1}0 was selected for synthesis">,
   InGroup<DiagGroup<"protocol-property-synthesis-ambiguity">>;
+def err_protocol_property_mismatch: Error<warn_protocol_property_mismatch.Text>;
 def err_undef_interface : Error<"cannot find interface declaration for %0">;
 def err_category_forward_interface : Error<
   "cannot define %select{category|class extension}0 for undefined class %1">;
@@ -1088,7 +1090,9 @@
 def note_property_declare : Note<
   "property declared here">;
 def note_protocol_property_declare : Note<
-  "it could also be property of type %0 declared here">;
+  "it could also be property "
+  "%select{of type %1|without attribute '%1'|with attribute '%1'|with getter "
+  "%1|with setter %1}0 declared here">;
 def note_property_synthesize : Note<
   "property synthesized here">;
 def err_synthesize_category_decl : Error<
Index: cfe/trunk/lib/AST/DeclObjC.cpp
===================================================================
--- cfe/trunk/lib/AST/DeclObjC.cpp
+++ cfe/trunk/lib/AST/DeclObjC.cpp
@@ -1889,25 +1889,23 @@
   }
 }
 
-    
 void ObjCProtocolDecl::collectInheritedProtocolProperties(
-                                                const ObjCPropertyDecl *Property,
-                                                ProtocolPropertyMap &PM) const {
+    const ObjCPropertyDecl *Property, ProtocolPropertySet &PS,
+    PropertyDeclOrder &PO) const {
   if (const ObjCProtocolDecl *PDecl = getDefinition()) {
-    bool MatchFound = false;
+    if (!PS.insert(PDecl).second)
+      return;
     for (auto *Prop : PDecl->properties()) {
       if (Prop == Property)
         continue;
       if (Prop->getIdentifier() == Property->getIdentifier()) {
-        PM[PDecl] = Prop;
-        MatchFound = true;
-        break;
+        PO.push_back(Prop);
+        return;
       }
     }
     // Scan through protocol's protocols which did not have a matching property.
-    if (!MatchFound)
-      for (const auto *PI : PDecl->protocols())
-        PI->collectInheritedProtocolProperties(Property, PM);
+    for (const auto *PI : PDecl->protocols())
+      PI->collectInheritedProtocolProperties(Property, PS, PO);
   }
 }
 
Index: cfe/trunk/lib/Sema/SemaObjCProperty.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaObjCProperty.cpp
+++ cfe/trunk/lib/Sema/SemaObjCProperty.cpp
@@ -814,53 +814,185 @@
     property->setPropertyAttributes(ObjCPropertyDecl::OBJC_PR_weak);
 }
 
-/// DiagnosePropertyMismatchDeclInProtocols - diagnose properties declared
-/// in inherited protocols with mismatched types. Since any of them can
-/// be candidate for synthesis.
-static void
-DiagnosePropertyMismatchDeclInProtocols(Sema &S, SourceLocation AtLoc,
+static bool
+isIncompatiblePropertyAttribute(unsigned Attr1, unsigned Attr2,
+                                ObjCPropertyDecl::PropertyAttributeKind Kind) {
+  return (Attr1 & Kind) != (Attr2 & Kind);
+}
+
+static bool areIncompatiblePropertyAttributes(unsigned Attr1, unsigned Attr2,
+                                              unsigned Kinds) {
+  return ((Attr1 & Kinds) != 0) != ((Attr2 & Kinds) != 0);
+}
+
+/// SelectPropertyForSynthesisFromProtocols - Finds the most appropriate
+/// property declaration that should be synthesised in all of the inherited
+/// protocols. It also diagnoses properties declared in inherited protocols with
+/// mismatched types or attributes, since any of them can be candidate for
+/// synthesis.
+static ObjCPropertyDecl *
+SelectPropertyForSynthesisFromProtocols(Sema &S, SourceLocation AtLoc,
                                         ObjCInterfaceDecl *ClassDecl,
                                         ObjCPropertyDecl *Property) {
-  ObjCInterfaceDecl::ProtocolPropertyMap PropMap;
+  assert(isa<ObjCProtocolDecl>(Property->getDeclContext()) &&
+         "Expected a property from a protocol");
+  ObjCInterfaceDecl::ProtocolPropertySet ProtocolSet;
+  ObjCInterfaceDecl::PropertyDeclOrder Properties;
   for (const auto *PI : ClassDecl->all_referenced_protocols()) {
     if (const ObjCProtocolDecl *PDecl = PI->getDefinition())
-      PDecl->collectInheritedProtocolProperties(Property, PropMap);
+      PDecl->collectInheritedProtocolProperties(Property, ProtocolSet,
+                                                Properties);
   }
-  if (ObjCInterfaceDecl *SDecl = ClassDecl->getSuperClass())
+  if (ObjCInterfaceDecl *SDecl = ClassDecl->getSuperClass()) {
     while (SDecl) {
       for (const auto *PI : SDecl->all_referenced_protocols()) {
         if (const ObjCProtocolDecl *PDecl = PI->getDefinition())
-          PDecl->collectInheritedProtocolProperties(Property, PropMap);
+          PDecl->collectInheritedProtocolProperties(Property, ProtocolSet,
+                                                    Properties);
       }
       SDecl = SDecl->getSuperClass();
     }
-  
-  if (PropMap.empty())
-    return;
-  
+  }
+
+  if (Properties.empty())
+    return Property;
+
+  ObjCPropertyDecl *OriginalProperty = Property;
+  size_t SelectedIndex = 0;
+  for (const auto &Prop : llvm::enumerate(Properties)) {
+    // Select the 'readwrite' property if such property exists.
+    if (Property->isReadOnly() && !Prop.value()->isReadOnly()) {
+      Property = Prop.value();
+      SelectedIndex = Prop.index();
+    }
+  }
+  if (Property != OriginalProperty) {
+    // Check that the old property is compatible with the new one.
+    Properties[SelectedIndex] = OriginalProperty;
+  }
+
   QualType RHSType = S.Context.getCanonicalType(Property->getType());
-  bool FirsTime = true;
-  for (ObjCInterfaceDecl::ProtocolPropertyMap::iterator
-       I = PropMap.begin(), E = PropMap.end(); I != E; I++) {
-    ObjCPropertyDecl *Prop = I->second;
+  unsigned OriginalAttributes = Property->getPropertyAttributes();
+  enum MismatchKind {
+    IncompatibleType = 0,
+    HasNoExpectedAttribute,
+    HasUnexpectedAttribute,
+    DifferentGetter,
+    DifferentSetter
+  };
+  // Represents a property from another protocol that conflicts with the
+  // selected declaration.
+  struct MismatchingProperty {
+    const ObjCPropertyDecl *Prop;
+    MismatchKind Kind;
+    StringRef AttributeName;
+  };
+  SmallVector<MismatchingProperty, 4> Mismatches;
+  for (ObjCPropertyDecl *Prop : Properties) {
+    // Verify the property attributes.
+    unsigned Attr = Prop->getPropertyAttributes();
+    if (Attr != OriginalAttributes) {
+      auto Diag = [&](bool OriginalHasAttribute, StringRef AttributeName) {
+        MismatchKind Kind = OriginalHasAttribute ? HasNoExpectedAttribute
+                                                 : HasUnexpectedAttribute;
+        Mismatches.push_back({Prop, Kind, AttributeName});
+      };
+      if (isIncompatiblePropertyAttribute(OriginalAttributes, Attr,
+                                          ObjCPropertyDecl::OBJC_PR_copy)) {
+        Diag(OriginalAttributes & ObjCPropertyDecl::OBJC_PR_copy, "copy");
+        continue;
+      }
+      if (areIncompatiblePropertyAttributes(
+              OriginalAttributes, Attr, ObjCPropertyDecl::OBJC_PR_retain |
+                                            ObjCPropertyDecl::OBJC_PR_strong)) {
+        Diag(OriginalAttributes & (ObjCPropertyDecl::OBJC_PR_retain |
+                                   ObjCPropertyDecl::OBJC_PR_strong),
+             "retain (or strong)");
+        continue;
+      }
+      if (isIncompatiblePropertyAttribute(OriginalAttributes, Attr,
+                                          ObjCPropertyDecl::OBJC_PR_atomic)) {
+        Diag(OriginalAttributes & ObjCPropertyDecl::OBJC_PR_atomic, "atomic");
+        continue;
+      }
+    }
+    if (Property->getGetterName() != Prop->getGetterName()) {
+      Mismatches.push_back({Prop, DifferentGetter, ""});
+      continue;
+    }
+    if (!Property->isReadOnly() && !Prop->isReadOnly() &&
+        Property->getSetterName() != Prop->getSetterName()) {
+      Mismatches.push_back({Prop, DifferentSetter, ""});
+      continue;
+    }
     QualType LHSType = S.Context.getCanonicalType(Prop->getType());
     if (!S.Context.propertyTypesAreCompatible(LHSType, RHSType)) {
       bool IncompatibleObjC = false;
       QualType ConvertedType;
       if (!S.isObjCPointerConversion(RHSType, LHSType, ConvertedType, IncompatibleObjC)
           || IncompatibleObjC) {
-        if (FirsTime) {
-          S.Diag(Property->getLocation(), diag::warn_protocol_property_mismatch)
-            << Property->getType();
-          FirsTime = false;
-        }
-        S.Diag(Prop->getLocation(), diag::note_protocol_property_declare)
-          << Prop->getType();
+        Mismatches.push_back({Prop, IncompatibleType, ""});
+        continue;
       }
     }
   }
-  if (!FirsTime && AtLoc.isValid())
+
+  if (Mismatches.empty())
+    return Property;
+
+  // Diagnose incompability.
+  {
+    bool HasIncompatibleAttributes = false;
+    for (const auto &Note : Mismatches)
+      HasIncompatibleAttributes =
+          Note.Kind != IncompatibleType ? true : HasIncompatibleAttributes;
+    // Promote the warning to an error if there are incompatible attributes or
+    // incompatible types together with readwrite/readonly incompatibility.
+    auto Diag = S.Diag(Property->getLocation(),
+                       Property != OriginalProperty || HasIncompatibleAttributes
+                           ? diag::err_protocol_property_mismatch
+                           : diag::warn_protocol_property_mismatch);
+    Diag << Mismatches[0].Kind;
+    switch (Mismatches[0].Kind) {
+    case IncompatibleType:
+      Diag << Property->getType();
+      break;
+    case HasNoExpectedAttribute:
+    case HasUnexpectedAttribute:
+      Diag << Mismatches[0].AttributeName;
+      break;
+    case DifferentGetter:
+      Diag << Property->getGetterName();
+      break;
+    case DifferentSetter:
+      Diag << Property->getSetterName();
+      break;
+    }
+  }
+  for (const auto &Note : Mismatches) {
+    auto Diag =
+        S.Diag(Note.Prop->getLocation(), diag::note_protocol_property_declare)
+        << Note.Kind;
+    switch (Note.Kind) {
+    case IncompatibleType:
+      Diag << Note.Prop->getType();
+      break;
+    case HasNoExpectedAttribute:
+    case HasUnexpectedAttribute:
+      Diag << Note.AttributeName;
+      break;
+    case DifferentGetter:
+      Diag << Note.Prop->getGetterName();
+      break;
+    case DifferentSetter:
+      Diag << Note.Prop->getSetterName();
+      break;
+    }
+  }
+  if (AtLoc.isValid())
     S.Diag(AtLoc, diag::note_property_synthesize);
+
+  return Property;
 }
 
 /// Determine whether any storage attributes were written on the property.
@@ -996,8 +1128,9 @@
       }
     }
     if (Synthesize && isa<ObjCProtocolDecl>(property->getDeclContext()))
-      DiagnosePropertyMismatchDeclInProtocols(*this, AtLoc, IDecl, property);
-        
+      property = SelectPropertyForSynthesisFromProtocols(*this, AtLoc, IDecl,
+                                                         property);
+
   } else if ((CatImplClass = dyn_cast<ObjCCategoryImplDecl>(ClassImpDecl))) {
     if (Synthesize) {
       Diag(AtLoc, diag::err_synthesize_category_decl);
Index: cfe/trunk/test/CodeGenObjC/arc-property.m
===================================================================
--- cfe/trunk/test/CodeGenObjC/arc-property.m
+++ cfe/trunk/test/CodeGenObjC/arc-property.m
@@ -131,4 +131,24 @@
 - (void) setCopyMachine: (id) x {}
 @end
 
+// rdar://31579994
+// When synthesizing a property that's declared in multiple protocols, ensure
+// that the setter is emitted if any of these declarations is readwrite.
+@protocol ABC
+@property (copy, nonatomic,  readonly) Test3 *someId;
+@end
+@protocol ABC__Mutable <ABC>
+@property (copy, nonatomic, readwrite) Test3 *someId;
+@end
+
+@interface ABC_Class <ABC, ABC__Mutable>
+@end
+
+@implementation ABC_Class
+@synthesize someId = _someId;
+// CHECK:  define internal %{{.*}}* @"\01-[ABC_Class someId]"
+// CHECK:  define internal void @"\01-[ABC_Class setSomeId:]"(
+@end
+
+
 // CHECK: attributes [[NUW]] = { nounwind }
Index: cfe/trunk/test/SemaObjC/arc-property-decl-attrs.m
===================================================================
--- cfe/trunk/test/SemaObjC/arc-property-decl-attrs.m
+++ cfe/trunk/test/SemaObjC/arc-property-decl-attrs.m
@@ -121,3 +121,107 @@
 @implementation I2
 @synthesize prop;
 @end
+
+// rdar://31579994
+// Verify that the all of the property declarations in inherited protocols are
+// compatible when synthesing a property from a protocol.
+
+@protocol CopyVsAssign1
+@property (copy, nonatomic,  readonly) id prop; // expected-error {{property with attribute 'copy' was selected for synthesis}}
+@end
+@protocol CopyVsAssign2
+@property (assign, nonatomic, readonly) id prop; // expected-note {{it could also be property without attribute 'copy' declared here}}
+@end
+
+@interface CopyVsAssign: Foo <CopyVsAssign1, CopyVsAssign2>
+@end
+@implementation CopyVsAssign
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol RetainVsNonRetain1
+@property (readonly) id prop; // expected-error {{property without attribute 'retain (or strong)' was selected for synthesis}}
+@end
+@protocol RetainVsNonRetain2
+@property (retain, readonly) id prop; // expected-note {{it could also be property with attribute 'retain (or strong)' declared here}}
+@end
+
+@interface RetainVsNonRetain: Foo <RetainVsNonRetain1, RetainVsNonRetain2>
+@end
+@implementation RetainVsNonRetain
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol AtomicVsNonatomic1
+@property (copy, nonatomic, readonly) id prop; // expected-error {{property without attribute 'atomic' was selected for synthesis}}
+@end
+@protocol AtomicVsNonatomic2
+@property (copy, atomic, readonly) id prop; // expected-note {{it could also be property with attribute 'atomic' declared here}}
+@end
+
+@interface AtomicVsNonAtomic: Foo <AtomicVsNonatomic1, AtomicVsNonatomic2>
+@end
+@implementation AtomicVsNonAtomic
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol Getter1
+@property (copy, readonly) id prop; // expected-error {{property with getter 'prop' was selected for synthesis}}
+@end
+@protocol Getter2
+@property (copy, getter=x, readonly) id prop; // expected-note {{it could also be property with getter 'x' declared here}}
+@end
+
+@interface GetterVsGetter: Foo <Getter1, Getter2>
+@end
+@implementation GetterVsGetter
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol Setter1
+@property (copy, readonly) id prop;
+@end
+@protocol Setter2
+@property (copy, setter=setp:, readwrite) id prop; // expected-error {{property with setter 'setp:' was selected for synthesis}}
+@end
+@protocol Setter3
+@property (copy, readwrite) id prop; // expected-note {{it could also be property with setter 'setProp:' declared here}}
+@end
+
+@interface SetterVsSetter: Foo <Setter1, Setter2, Setter3>
+@end
+@implementation SetterVsSetter
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol TypeVsAttribute1
+@property (assign, atomic, readonly) int prop; // expected-error {{property of type 'int' was selected for synthesis}}
+@end
+@protocol TypeVsAttribute2
+@property (assign, atomic, readonly) id prop; // expected-note {{it could also be property of type 'id' declared here}}
+@end
+@protocol TypeVsAttribute3
+@property (copy, readonly) id prop; // expected-note {{it could also be property with attribute 'copy' declared here}}
+@end
+
+@interface TypeVsAttribute: Foo <TypeVsAttribute1, TypeVsAttribute2, TypeVsAttribute3>
+@end
+@implementation TypeVsAttribute
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
+
+@protocol TypeVsSetter1
+@property (assign, nonatomic, readonly) int prop; // expected-note {{it could also be property of type 'int' declared here}}
+@end
+@protocol TypeVsSetter2
+@property (assign, nonatomic, readonly) id prop; // ok
+@end
+@protocol TypeVsSetter3
+@property (assign, nonatomic, readwrite) id prop; // expected-error {{property of type 'id' was selected for synthesis}}
+@end
+
+@interface TypeVsSetter: Foo <TypeVsSetter1, TypeVsSetter2, TypeVsSetter3>
+@end
+@implementation TypeVsSetter
+@synthesize prop; // expected-note {{property synthesized here}}
+@end
Index: cfe/trunk/test/SemaObjC/property-ambiguous-synthesis.m
===================================================================
--- cfe/trunk/test/SemaObjC/property-ambiguous-synthesis.m
+++ cfe/trunk/test/SemaObjC/property-ambiguous-synthesis.m
@@ -2,7 +2,7 @@
 // rdar://13075400
 
 @protocol FooAsID
-@property (copy) id foo; // expected-note 2 {{it could also be property of type 'id' declared here}} \\
+@property (assign) id foo; // expected-note 2 {{it could also be property of type 'id' declared here}} \\
 			 // expected-warning {{property of type 'id' was selected for synthesis}}
 @end