Error helpfully if install-from-cache and offline are used together.

src/zc/buildout/buildout.py

@@ -335,8 +335,14 @@ class Buildout(DictMixin):
 
             zc.buildout.easy_install.download_cache(download_cache)
 
-        zc.buildout.easy_install.install_from_cache(
-            bool_option(options, 'install-from-cache'))
+        if bool_option(options, 'install-from-cache'):
+            if self.offline:
+                raise zc.buildout.UserError(
+                    "install-from-cache can't be used with offline mode.\n"
+                    "Nothing is installed, even fromn cache, in offline\n"
+                    "mode, which might better be called 'no-install mode'.\n"
+                    )
+            zc.buildout.easy_install.install_from_cache(True)
 
         # "Use" each of the defaults so they aren't reported as unused options.
         for name in _buildout_default_options:

src/zc/buildout/tests.py

@@ -2788,6 +2788,24 @@ def bootstrap_honors_relative_paths():
         sys.exit(zc.buildout.buildout.main())
     """
 
+def cant_use_install_from_cache_and_offline_together():
+    r"""
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts =
+    ... offline = true
+    ... install-from-cache = true
+    ... ''')
+    >>> print_(system(join('bin', 'buildout')), end='') # doctest: +ELLIPSIS
+    While:
+      Initializing.
+    Error: install-from-cache can't be used with offline mode.
+    Nothing is installed, even fromn cache, in offline
+    mode, which might better be called 'no-install mode'.
+    <BLANKLINE>
+    """
+
 
 ######################################################################