Fix %c string and bytes interpolation (#10869)

According to [PEP461](https://www.python.org/dev/peps/pep-0461/), conversion type %c performs differently for string and bytes. In addition, `b'%c’ % str` is not supported in Python3.

## Test Plan

CPython Python3: 
```py
ss = 'a'
bb = b'a'
ii = 97
ff = 97.0

print('%c' % ss)
# print('%c' % '') # TypeError: %c requires int or char
# print('%c' % 'aa') # TypeError: %c requires int or char
# print('%c' % bb) # TypeError: %c requires int or char
print('%c' % ii)
# print('%c' % ff) # TypeError: %c requires int or char

# print(b'%c' % ss) # TypeError: %c requires an integer in range(256) or a single byte
# print(b'%c' % '') # TypeError: %c requires an integer in range(256) or a single byte
# print(b'%c' % 'aa') # TypeError: %c requires an integer in range(256) or a single byte
print(b'%c' % bb)
print(b'%c' % ii)
# print(b'%c' % ff) # TypeError: %c requires an integer in range(256) or a single byte
```

CPython Python2: 
```py
print('%c' % ss)
# print('%c' % '') # TypeError: %c requires int or char
# print('%c' % 'aa') # TypeError: %c requires int or char
print('%c' % bb)
print('%c' % ii)
# print('%c' % ff) # TypeError: integer argument expected, got float

print(b'%c' % ss)
# print('%c' % '') # TypeError: %c requires int or char
# print('%c' % 'aa') # TypeError: %c requires int or char
print(b'%c' % bb)
print(b'%c' % ii)
# print(b'%c' % ff) # TypeError: integer argument expected, got float
```

mypy/checkstrformat.py

@@ -786,7 +786,7 @@ def build_replacement_checkers(self, specifiers: List[ConversionSpecifier],
     def replacement_checkers(self, specifier: ConversionSpecifier, context: Context,
                              expr: FormatStringExpr) -> Optional[List[Checkers]]:
         """Returns a list of tuples of two functions that check whether a replacement is
-        of the right type for the specifier. The first functions take a node and checks
+        of the right type for the specifier. The first function takes a node and checks
         its type in the right type context. The second function just checks a type.
         """
         checkers: List[Checkers] = []
@@ -874,11 +874,11 @@ def check_s_special_cases(self, expr: FormatStringExpr, typ: Type, context: Cont
 
     def checkers_for_c_type(self, type: str,
                             context: Context,
-                            expr: FormatStringExpr) -> Optional[Checkers]:
+                            format_expr: FormatStringExpr) -> Optional[Checkers]:
         """Returns a tuple of check functions that check whether, respectively,
         a node or a type is compatible with 'type' that is a character type.
         """
-        expected_type = self.conversion_type(type, context, expr)
+        expected_type = self.conversion_type(type, context, format_expr)
         if expected_type is None:
             return None
 
@@ -889,8 +889,12 @@ def check_type(type: Type) -> None:
         def check_expr(expr: Expression) -> None:
             """int, or str with length 1"""
             type = self.accept(expr, expected_type)
-            if isinstance(expr, (StrExpr, BytesExpr)) and len(cast(StrExpr, expr).value) != 1:
-                self.msg.requires_int_or_char(context)
+            # TODO: Use the same the error message when incompatible types match %c
+            # Python 3 doesn't support b'%c' % str
+            if not (self.chk.options.python_version >= (3, 0)
+                    and isinstance(format_expr, BytesExpr)):
+                if isinstance(expr, (StrExpr, BytesExpr)) and len(expr.value) != 1:
+                    self.msg.requires_int_or_char(context)
             check_type(type)
 
         return check_expr, check_type
@@ -939,9 +943,12 @@ def conversion_type(self, p: str, context: Context, expr: FormatStringExpr,
                         numeric_types.append(self.named_type('typing.SupportsInt'))
             return UnionType.make_union(numeric_types)
         elif p in ['c']:
-            return UnionType([self.named_type('builtins.int'),
-                              self.named_type('builtins.float'),
-                              self.named_type('builtins.str')])
+            if isinstance(expr, BytesExpr):
+                return UnionType([self.named_type('builtins.int'),
+                                  self.named_type('builtins.bytes')])
+            else:
+                return UnionType([self.named_type('builtins.int'),
+                                  self.named_type('builtins.str')])
         else:
             self.msg.unsupported_placeholder(p, context)
             return None

test-data/unit/check-expressions.test

@@ -1278,11 +1278,44 @@ b'%a' % 3
 [builtins fixtures/primitives.pyi]
 [typing fixtures/typing-medium.pyi]
 
+[case testStringInterPolationCPython2]
+# flags: --py2 --no-strict-optional
+'%c' % 1
+'%c' % 1.0  # E: Incompatible types in string interpolation (expression has type "float", placeholder has type "Union[int, str]")
+'%c' % 's'
+'%c' % ''  # E: "%c" requires int or char
+'%c' % 'ab'  # E: "%c" requires int or char
+'%c' % b'a'
+[builtins_py2 fixtures/python2.pyi]
+
 [case testStringInterpolationC]
+# flags: --python-version 3.6
 '%c' % 1
+'%c' % 1.0  # E: Incompatible types in string interpolation (expression has type "float", placeholder has type "Union[int, str]")
 '%c' % 's'
 '%c' % ''  # E: "%c" requires int or char
 '%c' % 'ab'  # E: "%c" requires int or char
+'%c' % b'a'  # E: Incompatible types in string interpolation (expression has type "bytes", placeholder has type "Union[int, str]")
+[builtins fixtures/primitives.pyi]
+
+[case testBytesInterPolationCPython2]
+# flags: --py2 --no-strict-optional
+b'%c' % 1
+b'%c' % 1.0  # E: Incompatible types in string interpolation (expression has type "float", placeholder has type "Union[int, str]")
+b'%c' % 's'
+b'%c' % ''  # E: "%c" requires int or char
+b'%c' % 'ab'  # E: "%c" requires int or char
+b'%c' % b'a'
+[builtins_py2 fixtures/python2.pyi]
+
+[case testBytesInterpolationC]
+# flags: --python-version 3.6
+b'%c' % 1
+b'%c' % 1.0   # E: Incompatible types in string interpolation (expression has type "float", placeholder has type "Union[int, bytes]")
+b'%c' % 's'   # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, bytes]")
+b'%c' % ''    # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, bytes]")
+b'%c' % 'ab'  # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, bytes]")
+b'%c' % b'a'
 [builtins fixtures/primitives.pyi]
 
 [case testStringInterpolationMappingTypes]
@@ -1540,7 +1573,7 @@ x: Union[Good, Bad]
 
 class C:
     ...
-'{:c}'.format(C())  # E: Incompatible types in string interpolation (expression has type "C", placeholder has type "Union[int, float, str]")
+'{:c}'.format(C())  # E: Incompatible types in string interpolation (expression has type "C", placeholder has type "Union[int, str]")
 x: str
 '{:c}'.format(x)
 [builtins fixtures/primitives.pyi]

test-data/unit/fixtures/python2.pyi

@@ -18,6 +18,8 @@ class unicode:
     def format(self, *args, **kwars) -> unicode: ...
 class bool(int): pass
 
+bytes = str
+
 T = TypeVar('T')
 S = TypeVar('S')
 class list(Iterable[T], Generic[T]):