diff --git a/lit.cfg b/lit.cfg
index 464ad64..56375a4 100644
--- a/lit.cfg
+++ b/lit.cfg
@@ -171,6 +171,7 @@ if not os.path.exists(lldb_path):
     lit_config.fatal("lldb does not exist!")
 
 # Define our supported substitutions.
+config.substitutions.append( ('%{not}', os.path.join(srcroot, "not")) )
 config.substitutions.append( ('%{lldb}', lldb_path) )
 config.substitutions.append( ('%{swift}', swift_path) )
 config.substitutions.append( ('%{swiftc}', swiftc_path) )
diff --git a/litTest b/litTest
old mode 100644
new mode 100755
diff --git a/not b/not
new file mode 100755
index 0000000..825e0f7
--- /dev/null
+++ b/not
@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# Check that a command terminates with an error exit code.
+if "$@"; then
+    echo "error: unexpected successful exit: " "$@" 1>&2
+    exit 1
+fi
+exit 0
diff --git a/swift-update-safety.md b/swift-update-safety.md
new file mode 100644
index 0000000..dffc1a9
--- /dev/null
+++ b/swift-update-safety.md
@@ -0,0 +1,72 @@
+# Test the safety of `swift package update`.
+
+Establish our sandbox.
+
+```
+RUN: rm -rf %t.dir
+```
+
+Create a dummy package.
+
+```
+RUN: mkdir -p %t.dir/Dep
+RUN: %{swift} package -C %t.dir/Dep init=library
+RUN: git -C %t.dir/Dep init
+RUN: git -C %t.dir/Dep add -A
+RUN: git -C %t.dir/Dep commit -m "Initial commit."
+RUN: git -C %t.dir/Dep tag 1.0.0
+```
+
+Create the test package.
+
+```
+RUN: mkdir -p %t.dir/Cmd
+RUN: %{swift} package -C %t.dir/Cmd init=executable
+RUN: echo "import PackageDescription" >  %t.dir/Cmd/Package.swift
+RUN: echo "let package = Package(" >>  %t.dir/Cmd/Package.swift
+RUN: echo "  name: \"Cmd\"," >>  %t.dir/Cmd/Package.swift
+RUN: echo "  dependencies: [.Package(url: \"../Dep\", Version(1,0,0))]" >>  %t.dir/Cmd/Package.swift
+RUN: echo ")" >>  %t.dir/Cmd/Package.swift
+```
+
+Build the test package.
+
+```
+RUN: mkdir -p %t.dir/Cmd
+RUN: %{swift} build -C %t.dir/Cmd &> %t.build-log
+RUN: %{FileCheck} --check-prefix CHECK-BUILD-LOG --input-file %t.build-log %s
+
+CHECK-BUILD-LOG: Compile Swift Module 'Cmd'
+```
+
+Validate we can run `--update`.
+
+```
+RUN: %{swift} package -C %t.dir/Cmd update
+```
+
+Modify the sources of the dependency.
+
+```
+RUN: test -f %t.dir/Cmd/Packages/Dep-1.0.0/Sources/Dep.swift
+RUN: echo "INVALID CODE" >> %t.dir/Cmd/Packages/Dep-1.0.0/Sources/Dep.swift
+```
+
+Validate that `--update` errors out.
+
+```
+RUN: %{not} %{swift} package -C %t.dir/Cmd update
+```
+
+Stage the changes we made and verify it is still an error.
+
+```
+RUN: git -C %t.dir/Cmd/Packages/Dep-1.0.0 add Sources/Dep.swift
+RUN: %{not} %{swift} package -C %t.dir/Cmd update
+```
+
+Validate we still have our modified source.
+
+```
+RUN: grep "INVALID CODE" %t.dir/Cmd/Packages/Dep-1.0.0/Sources/Dep.swift
+```