Disable cache when producing reports

If the cache is used, coverage information will only be reported for
modules that were not in the cache.

Fixes #5103

Author: mthuurne

mypy/build.py

@@ -504,8 +504,10 @@ def __init__(self, data_dir: str,
         self.stale_modules = set()  # type: Set[str]
         self.rechecked_modules = set()  # type: Set[str]
         self.flush_errors = flush_errors
-        self.cache_enabled = options.incremental and (
-            not options.fine_grained_incremental or options.use_fine_grained_cache)
+        self.cache_enabled = (options.incremental
+                              and (not options.fine_grained_incremental
+                                   or options.use_fine_grained_cache)
+                              and not reports.reporters)
         self.fscache = fscache
         self.find_module_cache = FindModuleCache(self.search_paths, self.fscache, self.options)
         if options.sqlite_cache:

test-data/unit/reports.test

@@ -357,3 +357,15 @@ class Foo:
 @attr.s
 class Z(object):
     pass
+
+[case testCoverageIgnoresCache]
+-- Performs two runs to verify that cached information does not prevent
+-- modules from being included in reports.
+# cmd: mypy --linecount-report report a.py
+[file a.py]
+empty = False
+[out]
+[out2]
+[outfile report/linecount.txt]
+      1       1      0      0 total
+      1       1      0      0 a