Index: cfe/trunk/include/clang/Basic/VersionTuple.h
===================================================================
--- cfe/trunk/include/clang/Basic/VersionTuple.h
+++ cfe/trunk/include/clang/Basic/VersionTuple.h
@@ -24,9 +24,7 @@
 
 /// Represents a version number in the form major[.minor[.subminor[.build]]].
 class VersionTuple {
-  unsigned Major : 31;
-
-  unsigned UsesUnderscores : 1;
+  unsigned Major : 32;
 
   unsigned Minor : 31;
   unsigned HasMinor : 1;
@@ -39,30 +37,25 @@
 
 public:
   VersionTuple()
-      : Major(0), UsesUnderscores(false), Minor(0), HasMinor(false),
-        Subminor(0), HasSubminor(false), Build(0), HasBuild(false) {}
+      : Major(0), Minor(0), HasMinor(false), Subminor(0), HasSubminor(false),
+        Build(0), HasBuild(false) {}
 
   explicit VersionTuple(unsigned Major)
-      : Major(Major), UsesUnderscores(false), Minor(0), HasMinor(false),
-        Subminor(0), HasSubminor(false), Build(0), HasBuild(false) {}
-
-  explicit VersionTuple(unsigned Major, unsigned Minor,
-                        bool UsesUnderscores = false)
-      : Major(Major), UsesUnderscores(UsesUnderscores), Minor(Minor),
-        HasMinor(true), Subminor(0), HasSubminor(false), Build(0),
-        HasBuild(false) {}
+      : Major(Major), Minor(0), HasMinor(false), Subminor(0),
+        HasSubminor(false), Build(0), HasBuild(false) {}
 
-  explicit VersionTuple(unsigned Major, unsigned Minor, unsigned Subminor,
-                        bool UsesUnderscores = false)
-      : Major(Major), UsesUnderscores(UsesUnderscores), Minor(Minor),
-        HasMinor(true), Subminor(Subminor), HasSubminor(true), Build(0),
-        HasBuild(false) {}
+  explicit VersionTuple(unsigned Major, unsigned Minor)
+      : Major(Major), Minor(Minor), HasMinor(true), Subminor(0),
+        HasSubminor(false), Build(0), HasBuild(false) {}
+
+  explicit VersionTuple(unsigned Major, unsigned Minor, unsigned Subminor)
+      : Major(Major), Minor(Minor), HasMinor(true), Subminor(Subminor),
+        HasSubminor(true), Build(0), HasBuild(false) {}
 
   explicit VersionTuple(unsigned Major, unsigned Minor, unsigned Subminor,
-                        unsigned Build, bool UsesUnderscores = false)
-      : Major(Major), UsesUnderscores(UsesUnderscores), Minor(Minor),
-        HasMinor(true), Subminor(Subminor), HasSubminor(true), Build(Build),
-        HasBuild(true) {}
+                        unsigned Build)
+      : Major(Major), Minor(Minor), HasMinor(true), Subminor(Subminor),
+        HasSubminor(true), Build(Build), HasBuild(true) {}
 
   /// Determine whether this version information is empty
   /// (e.g., all version components are zero).
@@ -93,14 +86,6 @@
       return None;
     return Build;
   }
-
-  bool usesUnderscores() const {
-    return UsesUnderscores;
-  }
-
-  void UseDotAsSeparator() {
-    UsesUnderscores = false;
-  }
   
   /// Determine if two version numbers are equivalent. If not
   /// provided, minor and subminor version numbers are considered to be zero.
Index: cfe/trunk/lib/AST/DeclBase.cpp
===================================================================
--- cfe/trunk/lib/AST/DeclBase.cpp
+++ cfe/trunk/lib/AST/DeclBase.cpp
@@ -550,7 +550,6 @@
       Message->clear();
       llvm::raw_string_ostream Out(*Message);
       VersionTuple VTI(A->getIntroduced());
-      VTI.UseDotAsSeparator();
       Out << "introduced in " << PrettyPlatformName << ' ' 
           << VTI << HintMessage;
     }
@@ -564,7 +563,6 @@
       Message->clear();
       llvm::raw_string_ostream Out(*Message);
       VersionTuple VTO(A->getObsoleted());
-      VTO.UseDotAsSeparator();
       Out << "obsoleted in " << PrettyPlatformName << ' ' 
           << VTO << HintMessage;
     }
@@ -578,7 +576,6 @@
       Message->clear();
       llvm::raw_string_ostream Out(*Message);
       VersionTuple VTD(A->getDeprecated());
-      VTD.UseDotAsSeparator();
       Out << "first deprecated in " << PrettyPlatformName << ' '
           << VTD << HintMessage;
     }
Index: cfe/trunk/lib/Basic/VersionTuple.cpp
===================================================================
--- cfe/trunk/lib/Basic/VersionTuple.cpp
+++ cfe/trunk/lib/Basic/VersionTuple.cpp
@@ -29,11 +29,11 @@
                                      const VersionTuple &V) {
   Out << V.getMajor();
   if (Optional<unsigned> Minor = V.getMinor())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Minor;
+    Out << '.' << *Minor;
   if (Optional<unsigned> Subminor = V.getSubminor())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Subminor;
+    Out << '.' << *Subminor;
   if (Optional<unsigned> Build = V.getBuild())
-    Out << (V.usesUnderscores() ? '_' : '.') << *Build;
+    Out << '.' << *Build;
   return Out;
 }
 
Index: cfe/trunk/lib/Parse/ParseDecl.cpp
===================================================================
--- cfe/trunk/lib/Parse/ParseDecl.cpp
+++ cfe/trunk/lib/Parse/ParseDecl.cpp
@@ -762,8 +762,10 @@
 ///
 /// version:
 ///   simple-integer
-///   simple-integer ',' simple-integer
-///   simple-integer ',' simple-integer ',' simple-integer
+///   simple-integer '.' simple-integer
+///   simple-integer '_' simple-integer
+///   simple-integer '.' simple-integer '.' simple-integer
+///   simple-integer '_' simple-integer '_' simple-integer
 VersionTuple Parser::ParseVersionTuple(SourceRange &Range) {
   Range = SourceRange(Tok.getLocation(), Tok.getEndLoc());
 
@@ -841,7 +843,7 @@
       return VersionTuple();
     }
 
-    return VersionTuple(Major, Minor, (AfterMajorSeparator == '_'));
+    return VersionTuple(Major, Minor);
   }
 
   const char AfterMinorSeparator = ThisTokBegin[AfterMinor];
@@ -872,7 +874,7 @@
     return VersionTuple();
   }
   ConsumeToken();
-  return VersionTuple(Major, Minor, Subminor, (AfterMajorSeparator == '_'));
+  return VersionTuple(Major, Minor, Subminor);
 }
 
 /// Parse the contents of the "availability" attribute.
Index: cfe/trunk/test/Misc/ast-print-objectivec.m
===================================================================
--- cfe/trunk/test/Misc/ast-print-objectivec.m
+++ cfe/trunk/test/Misc/ast-print-objectivec.m
@@ -30,7 +30,7 @@
 // CHECK: @end
 
 // CHECK: @interface I(CAT)
-// CHECK: - (void)MethCAT __attribute__((availability(macos, introduced=10_1_0, deprecated=10_2)));
+// CHECK: - (void)MethCAT __attribute__((availability(macos, introduced=10.1.0, deprecated=10.2)));
 // CHECK: @end
 
 // CHECK: @implementation I
Index: cfe/trunk/test/Sema/availability-guard-format.mm
===================================================================
--- cfe/trunk/test/Sema/availability-guard-format.mm
+++ cfe/trunk/test/Sema/availability-guard-format.mm
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macosx-10.11 -Wunguarded-availability -fdiagnostics-parseable-fixits -fsyntax-only -verify %s
+
+// Testing that even for source code using '_' as a delimiter in availability version tuple '.' is actually used in diagnostic output as a delimiter.
+
+@interface foo
+- (void) method_bar __attribute__((availability(macosx, introduced = 10_12))); // expected-note {{'method_bar' has been explicitly marked partial here}}
+@end
+
+int main() {
+    [foo method_bar]; // \
+    // expected-warning {{'method_bar' is only available on macOS 10.12 or newer}} \
+    // expected-note {{enclose 'method_bar' in an @available check to silence this warning}} \
+    // CHECK: "fix-it:.*if (@available(macOS 10.12, *))"
+    return 0;
+}
Index: cfe/trunk/test/SemaObjC/attr-availability-1.m
===================================================================
--- cfe/trunk/test/SemaObjC/attr-availability-1.m
+++ cfe/trunk/test/SemaObjC/attr-availability-1.m
@@ -25,10 +25,10 @@
 // rdar://11475360
 @interface B : A
 - (void)method; // NOTE: we expect 'method' to *not* inherit availability.
-- (void)overridden __attribute__((availability(macosx,introduced=10_4))); // expected-warning{{overriding method introduced after overridden method on macOS (10_4 vs. 10_3)}}
+- (void)overridden __attribute__((availability(macosx,introduced=10_4))); // expected-warning{{overriding method introduced after overridden method on macOS (10.4 vs. 10.3)}}
 - (void)overridden2 __attribute__((availability(macosx,introduced=10_2)));
 - (void)overridden3 __attribute__((availability(macosx,deprecated=10_4)));
-- (void)overridden4 __attribute__((availability(macosx,deprecated=10_2))); // expected-warning{{overriding method deprecated before overridden method on macOS (10_3 vs. 10_2)}}
+- (void)overridden4 __attribute__((availability(macosx,deprecated=10_2))); // expected-warning{{overriding method deprecated before overridden method on macOS (10.3 vs. 10.2)}}
 - (void)overridden5 __attribute__((availability(macosx,introduced=10_3)));
 - (void)overridden6 __attribute__((availability(macosx,unavailable))); // expected-warning{{overriding method cannot be unavailable on macOS when its overridden method is available}}
 @end