Be aware of StarExpr in tuple when checking string conversions (#2866)

Fixes #2794.

Author: gvanrossum

mypy/checkstrformat.py

@@ -8,7 +8,7 @@
     Type, AnyType, TupleType, Instance, UnionType
 )
 from mypy.nodes import (
-    StrExpr, BytesExpr, UnicodeExpr, TupleExpr, DictExpr, Context, Expression
+    StrExpr, BytesExpr, UnicodeExpr, TupleExpr, DictExpr, Context, Expression, StarExpr
 )
 if False:
     # break import cycle only needed for mypy
@@ -140,7 +140,8 @@ def check_simple_str_interpolation(self, specifiers: List[ConversionSpecifier],
                     check_type(rhs_type.items[0])
                 else:
                     check_node(replacements)
-            elif isinstance(replacements, TupleExpr):
+            elif (isinstance(replacements, TupleExpr)
+                  and not any(isinstance(item, StarExpr) for item in replacements.items)):
                 for checks, rep_node in zip(checkers, replacements.items):
                     check_node, check_type = checks
                     check_node(rep_node)

test-data/unit/check-expressions.test

@@ -1149,6 +1149,10 @@ def foo(a: bytes, b: bytes):
     b'%s:%s' % (a, b)
 foo(b'a', b'b') == b'a:b'
 
+[case testStringInterpolationStarArgs]
+x = (1, 2)
+"%d%d" % (*x,)
+
 [case testBytePercentInterpolationSupported]
 b'%s' % (b'xyz',)
 b'%(name)s' % {'name': 'jane'}