option is always present, and emits diagnostic error if used on unsupported platform.

Author: sstadelman

Package.swift

@@ -20,7 +20,7 @@ let macOSPlatform: SupportedPlatform
 if let deploymentTarget = ProcessInfo.processInfo.environment["SWIFTPM_MACOS_DEPLOYMENT_TARGET"] {
     macOSPlatform = .macOS(deploymentTarget)
 } else {
-    macOSPlatform = .macOS(.v10_13)
+    macOSPlatform = .macOS(.v10_10)
 }
 
 let package = Package(

Sources/Commands/Options.swift

@@ -112,10 +112,8 @@ public class ToolOptions {
         archs.count > 1 ? .xcode : _buildSystem
     }
 
-    #if os(macOS)
     /// The location of the netrc file which should be use for authentication when downloading binary target artifacts
     public var netrcFilePath: AbsolutePath?
-    #endif
 
     public var _buildSystem: BuildSystemKind = .native
 

Sources/Commands/SwiftTool.swift

@@ -474,16 +474,11 @@ public class SwiftTool<Options: ToolOptions> {
             option: parser.add(option: "--build-system", kind: BuildSystemKind.self, usage: nil),
             to: { $0._buildSystem = $1 })
 
-        #if os(macOS)
-        if #available(macOS 10.13, *) {
-            // implementation of the netrc reader depends upon macOS 10.13
-            binder.bind(
-                option: parser.add(
-                    option: "--netrc-file", kind: PathArgument.self,
-                    usage: "Absolute path to netrc file for downloading binary target artifacts"),
-                to: { $0.netrcFilePath = $1.path })
-        }
-        #endif
+        binder.bind(
+            option: parser.add(
+                option: "--netrc-file", kind: PathArgument.self,
+                usage: "Absolute path to netrc file for downloading binary target artifacts (macOS >=10.13)"),
+            to: { $0.netrcFilePath = $1.path })
 
         // Let subclasses bind arguments.
         type(of: self).defineArguments(parser: parser, binder: binder)
@@ -574,6 +569,19 @@ public class SwiftTool<Options: ToolOptions> {
         if result.exists(arg: "--arch") && result.exists(arg: "--triple") {
             diagnostics.emit(.mutuallyExclusiveArgumentsError(arguments: ["--arch", "--triple"]))
         }
+        
+        if result.exists(arg: "--netrc-file") {
+            // --netrc-file option only supported on macOS >=10.13
+            #if os(macOS)
+            if #available(macOS 10.13, *) {
+             // ok, check succeeds
+                diagnostics.emit(error: "--netrc-file option is only supported on macOS >=10.13")
+            } else {
+            }
+            #else
+            diagnostics.emit(error: "--netrc-file option is only supported on macOS >=10.13")
+            #endif
+        }
     }
 
     class func defineArguments(parser: ArgumentParser, binder: ArgumentBinder<Options>) {