What seems to be the problem?
When using the xcode4 generator, .icon bundle files aren't being flagged with the meta data lastKnownFileType = folder.iconcomposer.icon;. Instead they are lastKnownFileType = text;. This is causing Xcode 26.4.1 to not compile them which prevents it from creating the glass effect icon on macOS Tahoe and a standard icon on pre-Tahoe systems.

What did you expect to happen?
When the xcode4 generator is used, .icon files are set with the correct lastKnownFileType meta data so they are compiled by Xcode.

What have you tried so far?
Edited the pbxproj file directly to set the lastKnownFileType to the correct value to fix the issue.

How can we reproduce this?
It's a bit complicated to set up an example project to reproduce this. The gist is Include a .icon file at the root of the Xcode project and set the Xcode build flags to use it. The premake file goes something like this:

...
vpaths { [""] = "assets/macOS" }

files {
      "assets/macOS/Icon.icon"
}

 xcodebuildsettings {
        ["PRODUCT_BUNDLE_IDENTIFIER"] = "com.example.app",
	["ASSETCATALOG_COMPILER_APPICON_NAME"] = "Icon"
        }

 xcodebuildresources {
	"assets/macOS/Icon.icon",
        }
...

• XCode (xcode)

What version of Premake are you using?
Premake beta 7