Document issue with preferKeyPath and compactMap

README.md

@@ -723,6 +723,8 @@ Known issues
 
 * When running a version of SwiftFormat built using Xcode 10.2 on macOS 10.14.3 or earlier, you may experience a crash with the error "dyld: Library not loaded: @rpath/libswiftCore.dylib". To fix this, you need to install the [Swift 5 Runtime Support for Command Line Tools](https://support.apple.com/kb/DL1998). These tools are included by default in macOS 10.14.4 and later.
 
+* When using the `preferKeyPath` rule, conversion of `compactMap { $0.foo }` to `compactMap(\.foo)` will result in code that fails to compile if `foo` is not an `Optional` property. This is due to a difference in the way that Swift handles type inference for closures vs keyPaths, as discussed [here](https://bugs.swift.org/browse/SR-13347). The recommended workaround is to replace `compactMap()` with `map()` in these cases, which will not change the behavior of the code.
+
 * When using the `--self remove` option, the `redundantSelf` rule will remove references to `self` in autoclosure arguments, which may change the meaning of the code, or cause it not to compile. To work around this issue, use the `--selfrequired` option to provide a comma-delimited list of methods to be excluded from the rule. The `expect()` function from the popular [Nimble](https://github.com/Quick/Nimble) unit testing framework is already excluded by default. If you are using the `--self insert` option then this is not an issue.
 
 * If you assign `SomeClass.self` to a variable and then instantiate an instance of the class using that variable, Swift requires that you use an explicit `.init()`, however, the `redundantInit` rule is not currently capable of detecting this situation and will remove the `.init`. To work around this issue, use the `// swiftformat:disable:next redundantInit` comment directive to disable the rule for any affected lines of code (or just disable the `redundantInit` rule completely).
@@ -735,10 +737,6 @@ Known issues
 
 * If a file begins with a comment, the `stripHeaders` rule will remove it if it is followed by a blank line. To avoid this, make sure that the first comment is directly followed by a line of code.
 
-* The formatted file cache is based on a hash of the file contents, so it's possible (though unlikely) that an edited file will have the exact same hash as the previously formatted version, causing SwiftFormat to incorrectly identify it as not having changed, and fail to update it.
-
-    To fix this, you can use the command-line option `--cache ignore` to force SwiftFormat to ignore the cache for this run, or just type an extra space in the file (which SwiftFormat will then remove again when it applies the correct formatting).
-
 * When running on Linux, the `--symlinks` option has no effect, and some of the `fileHeader` placeholders are not supported.