Fix compilation with swift build (#7588)

Adding a resource to a target makes `swift build` try to force-include
Foundation.h when building source files in that target, which breaks if any of
the source files aren't obj-c. To work around this, the resources need to be in
a separate target from the source files.

xcodebuild breaks if a target has zero source files, so add an empty obj-c file
to build as part of this new resource target.

Author: tgoyne

CHANGELOG.md

@@ -11,6 +11,7 @@
 * SyncUser::all_sessions() included sessions in every state *except* for waiting for access token, which was weirdly inconsistent. It now includes all sessions. ([PR #7300](https://github.com/realm/realm-core/pull/7300)).
 * App::all_users() included logged out users only if they were logged out while the App instance existed. It now always includes all logged out users. ([PR #7300](https://github.com/realm/realm-core/pull/7300)).
 * Deleting the active user left the active user unset rather than selecting another logged-in user as the active user like logging out and removing users did. ([PR #7300](https://github.com/realm/realm-core/pull/7300)).
+* Fix compilation errors when using command-line `swift build` ([#7587](https://github.com/realm/realm-core/pull/7587), since v14.5.1).
 
 ### Breaking changes
 * The following things have been renamed or moved as part of moving all of the App Services functionality to the app namespace:

Package.swift

@@ -364,7 +364,7 @@ let package = Package(
     products: [
         .library(
             name: "RealmCore",
-            targets: ["RealmCore"]),
+            targets: ["RealmCoreResources"]),
         .library(
             name: "RealmQueryParser",
             targets: ["RealmQueryParser"]),
@@ -417,12 +417,10 @@ let package = Package(
                 "realm/tools",
                 "realm/util/config.h.in",
                 "realm/version_numbers.hpp.in",
+                "spm",
                 "swift",
                 "win32",
             ] + syncExcludes + syncServerSources) as [String],
-            resources: [
-                .copy("realm/PrivacyInfo.xcprivacy")
-            ],
             publicHeadersPath: ".",
             cxxSettings: ([
                 .headerSearchPath("external"),
@@ -433,6 +431,20 @@ let package = Package(
                 .linkedFramework("Foundation", .when(platforms: [.macOS, .iOS, .tvOS, .watchOS, .macCatalyst])),
                 .linkedFramework("Security", .when(platforms: [.macOS, .iOS, .tvOS, .watchOS, .macCatalyst])),
             ]),
+        // Adding resources to a target makes command line swift build (but not
+        // xcodebuild) force-include Foundation.h without properly excluding
+        // non-objc c++ files. Adding the resources in a separate target which
+        // depends on RealmCore works around this. This target needs a single
+        // source file (dummy.mm) to work around a different bug in xcodebuild
+        // (but not swift build) that makes empty targets not work.
+        .target(
+            name: "RealmCoreResources",
+            dependencies: ["RealmCore"],
+            path: "src/spm",
+            resources: [
+                .copy("PrivacyInfo.xcprivacy")
+            ],
+            publicHeadersPath: "."),
         .target(
             name: "RealmQueryParser",
             dependencies: ["RealmCore"],