Exclude tuples with starred expressions from the %-formatting optimisation.

Closes GH-2939.

Cython/Compiler/Optimize.py

@@ -4322,6 +4322,9 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                 warning(pos, "Too few arguments for format placeholders", level=1)
                 can_be_optimised = False
                 break
+            if arg.is_starred:
+                can_be_optimised = False
+                break
             if format_type in u'asrfdoxX':
                 format_spec = s[1:]
                 if format_type in u'doxX' and u'.' in format_spec:
@@ -4339,6 +4342,7 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
             else:
                 # keep it simple for now ...
                 can_be_optimised = False
+                break
 
         if not can_be_optimised:
             # Print all warnings we can find before finally giving up here.

tests/run/pep448_extended_unpacking.pyx

+# mode: run
+# tag: all_language_levels
 
 cimport cython
 
@@ -144,6 +146,22 @@ def unpack_tuple_keep_originals(a, b, c):
     return (*a, *b, 2, *c)
 
 
+def unpack_tuple_in_string_formatting(a, *args):
+    """
+    >>> print(unpack_tuple_in_string_formatting(1, 2))
+    1 2
+    >>> print(unpack_tuple_in_string_formatting(1, 'x'))
+    1 'x'
+    >>> unpack_tuple_in_string_formatting(1)  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: ...format...
+    >>> unpack_tuple_in_string_formatting(1, 2, 3)  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: ...format...
+    """
+    return "%s %r" % (a, *args)
+
+
 #### lists