fix syntax error reporting from stdin (PyCQA#357)

In b73a3d1, there was an assumption that text is None only if there was an
encoding error with the file. However this was the case for all pythons before
3.9 when reading code from stdin.

This takes care to correctly report as much context as possible, so errors
aren't silently dropped with the unhelpful "problem decoding source" message.

pyflakes/api.py

@@ -55,14 +55,7 @@ def check(codeString, filename, reporter=None):
                             text = None
             offset -= 1
 
-        # If there's an encoding problem with the file, the text is None.
-        if text is None:
-            # Avoid using msg, since for the only known case, it contains a
-            # bogus message that claims the encoding the file declared was
-            # unknown.
-            reporter.unexpectedError(filename, 'problem decoding source')
-        else:
-            reporter.syntaxError(filename, msg, lineno, offset, text)
+        reporter.syntaxError(filename, msg, lineno, offset, text)
         return 1
     except Exception:
         reporter.unexpectedError(filename, 'problem decoding source')

pyflakes/reporter.py

@@ -51,19 +51,28 @@ def syntaxError(self, filename, msg, lineno, offset, text):
         @param text: The source code containing the syntax error.
         @ptype text: C{unicode}
         """
-        line = text.splitlines()[-1]
+        if text is None:
+            line = None
+        else:
+            line = text.splitlines()[-1]
+
         if offset is not None:
-            if sys.version_info < (3, 8):
+            if sys.version_info < (3, 8) and text is not None:
                 offset = offset - (len(text) - len(line)) + 1
+            # some versions of python emit an offset of -1 for certain encoding errors
+            if offset < 0:
+                offset = 0
             self._stderr.write('%s:%d:%d: %s\n' %
                                (filename, lineno, offset, msg))
         else:
             self._stderr.write('%s:%d: %s\n' % (filename, lineno, msg))
-        self._stderr.write(line)
-        self._stderr.write('\n')
-        if offset is not None:
-            self._stderr.write(re.sub(r'\S', ' ', line[:offset - 1]) +
-                               "^\n")
+
+        if line is not None:
+            self._stderr.write(line)
+            self._stderr.write('\n')
+            if offset is not None:
+                self._stderr.write(re.sub(r'\S', ' ', line[:offset - 1]) +
+                                   "^\n")
 
     def flake(self, message):
         """

pyflakes/test/test_api.py

@@ -606,7 +606,11 @@ def test_misencodedFileUTF8(self):
 """ % SNOWMAN).encode('utf-8')
         with self.makeTempFile(source) as sourcePath:
             self.assertHasErrors(
-                sourcePath, [f"{sourcePath}: problem decoding source\n"])
+                sourcePath,
+                [f"""\
+{sourcePath}:0:0: 'ascii' codec can't decode byte 0xe2 in position 21: ordinal not in \
+range(128)
+"""])
 
     def test_misencodedFileUTF16(self):
         """