Fix crash in relative imports (#4259)

Fixes #4111

This corrects a previous wrong --strict-optional change (and adds various tests).

Author: ilevkivskyi

mypy/fastparse.py

@@ -651,8 +651,8 @@ def visit_Import(self, n: ast3.Import) -> Import:
     def visit_ImportFrom(self, n: ast3.ImportFrom) -> ImportBase:
         assert n.level is not None
         if len(n.names) == 1 and n.names[0].name == '*':
-            assert n.module is not None
-            i = ImportAll(n.module, n.level)  # type: ImportBase
+            mod = n.module if n.module is not None else ''
+            i = ImportAll(mod, n.level)  # type: ImportBase
         else:
             i = ImportFrom(self.translate_module_id(n.module) if n.module is not None else '',
                            n.level,

mypy/fastparse2.py

@@ -647,8 +647,8 @@ def visit_Import(self, n: ast27.Import) -> Import:
     def visit_ImportFrom(self, n: ast27.ImportFrom) -> ImportBase:
         assert n.level is not None
         if len(n.names) == 1 and n.names[0].name == '*':
-            assert n.module is not None
-            i = ImportAll(n.module, n.level)  # type: ImportBase
+            mod = n.module if n.module is not None else ''
+            i = ImportAll(mod, n.level)  # type: ImportBase
         else:
             i = ImportFrom(self.translate_module_id(n.module) if n.module is not None else '',
                            n.level,

test-data/unit/check-fastparse.test

@@ -364,3 +364,32 @@ def m(x, ((x, y), z)):  # E: Duplicate argument 'x' in function definition
   pass
 
 lambda x, (y, x): None  # E: Duplicate argument 'x' in function definition
+
+[case testNoCrashOnImportFromStar]
+from pack import *
+[file pack/__init__.py]
+from . import *
+
+[case testNoCrashOnImportFromStarNested]
+import blamodule
+[file blamodule/__init__.py]
+from . import command
+from . import backends
+
+[file blamodule/backends/__init__.py]
+from .Bla import Bla
+reveal_type(Bla().method())  # E: Revealed type is 'builtins.str'
+
+[file blamodule/backends/Bla.py]
+from .. import *
+
+class Bla:
+    def method(self) -> str:
+        return command.call()
+
+[file blamodule/command.py]
+def call() -> str: pass
+
+[case testNoCrashOnImportFromStarPython2]
+# flags: --py2
+from . import * # E: No parent module -- cannot perform relative import