Re-apply "Paser: IfConfigDecl should reflect the underlying source. rdar://34315827"

include/swift/Basic/LangOptions.h

@@ -293,6 +293,9 @@ namespace swift {
     /// Returns the value for the given platform condition or an empty string.
     StringRef getPlatformConditionValue(PlatformConditionKind Kind) const;
 
+    /// Check whether the given platform condition matches the given value.
+    bool checkPlatformCondition(PlatformConditionKind Kind, StringRef Value) const;
+
     /// Explicit conditional compilation flags, initialized via the '-D'
     /// compiler flag.
     void addCustomConditionalCompilationFlag(StringRef Name) {

lib/Basic/LangOptions.cpp

@@ -116,6 +116,21 @@ LangOptions::getPlatformConditionValue(PlatformConditionKind Kind) const {
   return StringRef();
 }
 
+bool LangOptions::
+checkPlatformCondition(PlatformConditionKind Kind, StringRef Value) const {
+  // Check a special case that "macOS" is an alias of "OSX".
+  if (Kind == PlatformConditionKind::OS && Value == "macOS")
+    return checkPlatformCondition(Kind, "OSX");
+
+  for (auto &Opt : reversed(PlatformConditionValues)) {
+    if (Opt.first == Kind)
+      if (Opt.second == Value)
+        return true;
+  }
+
+  return false;
+}
+
 bool LangOptions::isCustomConditionalCompilationFlagSet(StringRef Name) const {
   return std::find(CustomConditionalCompilationFlags.begin(),
                    CustomConditionalCompilationFlags.end(), Name)

lib/Parse/ParseIfConfig.cpp

@@ -304,15 +304,6 @@ class ValidateIfConfigCondition :
       return nullptr;
     }
 
-    // FIXME: Perform the replacement macOS -> OSX elsewhere.
-    if (Kind == PlatformConditionKind::OS && *ArgStr == "macOS") {
-      *ArgStr = "OSX";
-      ArgP->setSubExpr(
-          new (Ctx) UnresolvedDeclRefExpr(Ctx.getIdentifier(*ArgStr),
-                                          DeclRefKind::Ordinary,
-                                          DeclNameLoc(Arg->getLoc())));
-    }
-
     std::vector<StringRef> suggestions;
     if (!LangOptions::checkPlatformConditionSupported(*Kind, *ArgStr,
                                                       suggestions)) {
@@ -460,8 +451,7 @@ class EvaluateIfConfigCondition :
 
     auto Val = getDeclRefStr(Arg);
     auto Kind = getPlatformConditionKind(KindName).getValue();
-    auto Target = Ctx.LangOpts.getPlatformConditionValue(Kind);
-    return Target == Val;
+    return Ctx.LangOpts.checkPlatformCondition(Kind, Val);
   }
 
   bool visitPrefixUnaryExpr(PrefixUnaryExpr *E) {

test/IDE/coloring.swift

@@ -608,6 +608,10 @@ func keywordInCaseAndLocalArgLabel(_ for: Int, for in: Int, class _: Int) {
   }
 }
 
+#if os(macOS)
+#endif
+// CHECK: <#kw>#if</#kw> <#id>os</#id>(<#id>macOS</#id>)
+
 // Keep this as the last test
 /**
   Trailing off ...