Index: lib/Parse/ParseDecl.cpp
===================================================================
--- lib/Parse/ParseDecl.cpp
+++ lib/Parse/ParseDecl.cpp
@@ -2724,10 +2724,9 @@
     case tok::identifier: {
       // In C++, check to see if this is a scope specifier like foo::bar::, if
       // so handle it as such.  This is important for ctor parsing.
-      if (getLangOpts().CPlusPlus) {
+      if (getLangOpts().CPlusPlus && !DS.hasTypeSpecifier()) {
         if (TryAnnotateCXXScopeToken(EnteringContext)) {
-          if (!DS.hasTypeSpecifier())
-            DS.SetTypeSpecError();
+          DS.SetTypeSpecError();
           goto DoneWithDeclSpec;
         }
         if (!Tok.is(tok::identifier))
@@ -4526,8 +4525,10 @@
   // Member pointers get special handling, since there's no place for the
   // scope spec in the generic path below.
   if (getLangOpts().CPlusPlus &&
-      (Tok.is(tok::coloncolon) || Tok.is(tok::identifier) ||
-       Tok.is(tok::annot_cxxscope))) {
+      (Tok.is(tok::coloncolon) ||
+      (Tok.is(tok::identifier) && (NextToken().is(tok::coloncolon) ||
+                                   NextToken().is(tok::less))) ||
+      Tok.is(tok::annot_cxxscope))) {
     bool EnteringContext = D.getContext() == Declarator::FileContext ||
                            D.getContext() == Declarator::MemberContext;
     CXXScopeSpec SS;
@@ -4719,6 +4720,11 @@
   DeclaratorScopeObj DeclScopeObj(*this, D.getCXXScopeSpec());
 
   if (getLangOpts().CPlusPlus && D.mayHaveIdentifier()) {
+    // Don't parse FOO:BAR as if it were a typo for FOO::BAR, in this context it
+    // is a bitfield.
+    ColonProtectionRAIIObject X(*this,
+                                D.getContext() == Declarator::MemberContext);
+
     // ParseDeclaratorInternal might already have parsed the scope.
     if (D.getCXXScopeSpec().isEmpty()) {
       bool EnteringContext = D.getContext() == Declarator::FileContext ||
Index: lib/Parse/ParseDeclCXX.cpp
===================================================================
--- lib/Parse/ParseDeclCXX.cpp
+++ lib/Parse/ParseDeclCXX.cpp
@@ -1229,7 +1229,8 @@
   // Parse the (optional) nested-name-specifier.
   CXXScopeSpec &SS = DS.getTypeSpecScope();
   if (getLangOpts().CPlusPlus) {
-    // "FOO : BAR" is not a potential typo for "FOO::BAR".
+    // "FOO : BAR" is not a potential typo for "FOO::BAR".  In this context it
+    // is base-specifier-list.
     ColonProtectionRAIIObject X(*this);
 
     if (ParseOptionalCXXScopeSpecifier(SS, ParsedType(), EnteringContext))
@@ -1916,14 +1917,8 @@
   //   declarator pure-specifier[opt]
   //   declarator brace-or-equal-initializer[opt]
   //   identifier[opt] ':' constant-expression
-  if (Tok.isNot(tok::colon)) {
-    // Don't parse FOO:BAR as if it were a typo for FOO::BAR, in this context it
-    // is a bitfield.
-    // FIXME: This should only apply when parsing the id-expression (see
-    // PR18587).
-    ColonProtectionRAIIObject X(*this);
+  if (Tok.isNot(tok::colon))
     ParseDeclarator(DeclaratorInfo);
-  }
 
   if (!DeclaratorInfo.isFunctionDeclarator() && TryConsumeToken(tok::colon)) {
     BitfieldSize = ParseConstantExpression();
@@ -2118,13 +2113,8 @@
   if (MalformedTypeSpec)
     DS.SetTypeSpecError();
 
-  {
-    // Don't parse FOO:BAR as if it were a typo for FOO::BAR, in this context it
-    // is a bitfield.
-    ColonProtectionRAIIObject X(*this);
-    ParseDeclarationSpecifiers(DS, TemplateInfo, AS, DSC_class,
-                               &CommonLateParsedAttrs);
-  }
+  ParseDeclarationSpecifiers(DS, TemplateInfo, AS, DSC_class,
+                             &CommonLateParsedAttrs);
 
   // If we had a free-standing type definition with a missing semicolon, we
   // may get this far before the problem becomes obvious.
Index: test/SemaCXX/nested-name-spec.cpp
===================================================================
--- test/SemaCXX/nested-name-spec.cpp
+++ test/SemaCXX/nested-name-spec.cpp
@@ -311,3 +311,128 @@
 
 namespace TypedefNamespace { typedef int F; };
 TypedefNamespace::F::NonexistentName BadNNSWithCXXScopeSpec; // expected-error {{'F' (aka 'int') is not a class, namespace, or scoped enumeration}}
+
+namespace PR18587 {
+
+struct C1 {
+  int a, b, c;
+  typedef int C2;
+  struct B1 {
+    struct B2 {
+      int a, b, c;
+    };
+  };
+};
+struct C2 { static const unsigned N1 = 1; };
+struct B1 {
+  enum E1 { B2 = 2 };
+  static const int B3 = 3;
+};
+const int N1 = 2;
+
+// Function declarators
+struct S1a { int f(C1::C2); };
+struct S1b { int f(C1:C2); };  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+
+struct S2a {
+  C1::C2 f(C1::C2);
+};
+struct S2b {
+  C1:C2 f(C1::C2);  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S2c {
+  C1::C2 f(C1:C2);  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+struct S3a {
+  int f(C1::C2), C2 : N1;
+  int g : B1::B2;
+};
+struct S3b {
+  int g : B1:B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+// Inside square brackets
+struct S4a {
+  int f[C2::N1];
+};
+struct S4b {
+  int f[C2:N1];  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+struct S5a {
+  int f(int xx[B1::B3 ? C2::N1 : B1::B2]);
+};
+struct S5b {
+  int f(int xx[B1::B3 ? C2::N1 : B1:B2]);  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S5c {
+  int f(int xx[B1:B3 ? C2::N1 : B1::B2]);  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+// Bit fields
+struct S6a {
+  C1::C2 m1 : B1::B2;
+};
+struct S6b {
+  C1:C2 m1 : B1::B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S6c {
+  C1::C2 m1 : B1:B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S6d {
+  int C2:N1;
+};
+struct S6e {
+  static const int N = 3;
+  B1::E1 : N;
+};
+struct S6f {
+  C1:C2 : B1::B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+  B1:E1 : B1::B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S6g {
+  C1::C2 : B1:B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+  B1::E1 : B1:B2;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+// Template parameters
+template <int N> struct T1 {
+  int a,b,c;
+  static const unsigned N1 = N;
+  typedef unsigned C1;
+};
+T1<C2::N1> var_1a;
+T1<C2:N1> var_1b;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+template<int N> int F() {}
+int (*X1)() = (B1::B2 ? F<1> : F<2>);
+int (*X2)() = (B1:B2 ? F<1> : F<2>);  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+
+// Bit fields + templates
+struct S7a {
+  T1<B1::B2>::C1 m1 : T1<B1::B2>::N1;
+};
+struct S7b {
+  T1<B1:B2>::C1 m1 : T1<B1::B2>::N1;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+struct S7c {
+  T1<B1::B2>::C1 m1 : T1<B1:B2>::N1;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+// Member pointers
+struct S8a {
+  C1::C2 C1::*m1;
+};
+struct S8b {
+  C1:C2 C1::*m1;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+// nested types
+struct S9a {
+  typedef C1::C2 t1;
+};
+struct S9b {
+  typedef C1:C2 t1;  // expected-error{{unexpected ':' in nested name specifier; did you mean '::'?}}
+};
+
+}