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

@@ -505,7 +505,8 @@ def __init__(self, data_dir: 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)
+            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 +357,14 @@ 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 --linecoverage-report report a.py
+[file a.py]
+empty = False
+[out]
+[out2]
+[outfile report/coverage.json]
+{"lines": {"$PWD/a.py": [1]}}