Merge pull request swiftlang#40172 from keith/ks/driver-add-test-for-vfs-path-behavior-nfc

[Driver] Add test for vfs path behavior (NFC)

Author: kastiglione

test/Driver/vfs.swift

@@ -11,3 +11,13 @@
 // Verifies that input paths are not rejected prematurely when -vfsoverlay is present as they may exist on the vfs (which the frontend accounts for) even if they don't exist on the real file system.
 // RUN: not %swiftc_driver -driver-print-jobs -c %t/file-not-on-the-real-filesystem.swift
 // RUN: %swiftc_driver -driver-print-jobs -c -vfsoverlay overlay1.yaml %t/file-not-on-the-real-filesystem.swift
+
+// RUN: mkdir -p %t
+// RUN: cd %t
+// RUN: echo "foobar" > main.swift
+// RUN: not %swiftc_driver main.swift 2>&1 | %FileCheck --check-prefix=CHECK-ERROR %s
+// RUN: echo '{"roots": [],"version": 0}' > %t.yaml
+// RUN: not %swiftc_driver -vfsoverlay %t.yaml main.swift 2>&1 | %FileCheck --check-prefix=CHECK-ERROR %s
+// RUN: echo '{"version": 0,"roots":[{"type":"directory","name":"%/t","contents":[{"type":"file","name":"vfsname.swift", "external-contents":"main.swift"}]}]}' > %t.yaml
+// RUN: not %swiftc_driver -vfsoverlay %t.yaml vfsname.swift -v 2>&1 | %FileCheck --check-prefix=CHECK-ERROR %s
+// CHECK-ERROR: {{^}}main.swift:1:1: error: