[mypyc] fix generator regression with empty tuple (#20371)

BobTheBuidler · hauntsaninja · commit 8a6eff478416 · 2025-12-13T14:27:51.000-08:00
This PR fixes #20341
diff --git a/mypyc/irbuild/builder.py b/mypyc/irbuild/builder.py
@@ -990,8 +990,10 @@ def get_sequence_type_from_type(self, target_type: Type) -> RType:
         elif isinstance(target_type, TypeVarLikeType):
             return self.get_sequence_type_from_type(target_type.upper_bound)
         elif isinstance(target_type, TupleType):
+            items = target_type.items
+            assert items, "This function does not support empty tuples"
             # Tuple might have elements of different types.
-            rtypes = {self.mapper.type_to_rtype(item) for item in target_type.items}
+            rtypes = set(map(self.mapper.type_to_rtype, items))
             if len(rtypes) == 1:
                 return rtypes.pop()
             else:
diff --git a/mypyc/irbuild/for_helpers.py b/mypyc/irbuild/for_helpers.py
@@ -7,7 +7,7 @@
 
 from __future__ import annotations
 
-from typing import Callable, ClassVar
+from typing import Callable, ClassVar, cast
 
 from mypy.nodes import (
     ARG_POS,
@@ -241,25 +241,45 @@ def sequence_from_generator_preallocate_helper(
         rtype = builder.node_type(sequence_expr)
         if not (is_sequence_rprimitive(rtype) or isinstance(rtype, RTuple)):
             return None
-        sequence = builder.accept(sequence_expr)
-        length = get_expr_length_value(builder, sequence_expr, sequence, line, use_pyssize_t=True)
+
         if isinstance(rtype, RTuple):
             # If input is RTuple, box it to tuple_rprimitive for generic iteration
             # TODO: this can be optimized a bit better with an unrolled ForRTuple helper
             proper_type = get_proper_type(builder.types[sequence_expr])
             assert isinstance(proper_type, TupleType), proper_type
 
-            get_item_ops = [
-                (
-                    LoadLiteral(typ.value, object_rprimitive)
-                    if isinstance(typ, LiteralType)
-                    else TupleGet(sequence, i, line)
-                )
-                for i, typ in enumerate(get_proper_types(proper_type.items))
-            ]
+            # the for_loop_helper_with_index crashes for empty tuples, bail out
+            if not proper_type.items:
+                return None
+
+            proper_types = get_proper_types(proper_type.items)
+
+            get_item_ops: list[LoadLiteral | TupleGet]
+            if all(isinstance(typ, LiteralType) for typ in proper_types):
+                get_item_ops = [
+                    LoadLiteral(cast(LiteralType, typ).value, object_rprimitive)
+                    for typ in proper_types
+                ]
+
+            else:
+                sequence = builder.accept(sequence_expr)
+                get_item_ops = [
+                    (
+                        LoadLiteral(typ.value, object_rprimitive)
+                        if isinstance(typ, LiteralType)
+                        else TupleGet(sequence, i, line)
+                    )
+                    for i, typ in enumerate(proper_types)
+                ]
+
             items = list(map(builder.add, get_item_ops))
             sequence = builder.new_tuple(items, line)
 
+        else:
+            sequence = builder.accept(sequence_expr)
+
+        length = get_expr_length_value(builder, sequence_expr, sequence, line, use_pyssize_t=True)
+
         target_op = empty_op_llbuilder(length, line)
 
         def set_item(item_index: Value) -> None:
diff --git a/mypyc/test-data/run-generators.test b/mypyc/test-data/run-generators.test
@@ -936,3 +936,11 @@ def test_generator_override() -> None:
      assert base1_foo(Base1()) == [1]
      assert base1_foo(Derived1()) == [2, 3]
      assert derived1_foo(Derived1()) == [2, 3]
+
+[case testGeneratorEmptyTuple]
+from collections.abc import Generator
+from typing import Optional, Union
+
+def test_compiledGeneratorEmptyTuple() -> None:
+    jobs: Generator[Optional[str], None, None] = (_ for _ in ())
+    assert list(jobs) == []
diff --git a/mypyc/test-data/run-loops.test b/mypyc/test-data/run-loops.test
@@ -1,7 +1,7 @@
 # Test cases for "range" objects, "for" and "while" loops (compile and run)
 
 [case testFor]
-from typing import List, Tuple
+from typing import Any, List, Tuple
 def count(n: int) -> None:
     for i in range(n):
         print(i)
@@ -21,6 +21,10 @@ def list_iter(l: List[int]) -> None:
 def tuple_iter(l: Tuple[int, ...]) -> None:
     for i in l:
         print(i)
+def empty_tuple_iter(l: Tuple[()]) -> None:
+    i: Any
+    for i in l:
+        print(i)
 def str_iter(l: str) -> None:
     for i in l:
         print(i)
@@ -39,7 +43,7 @@ def count_down_short() -> None:
 [file driver.py]
 from native import (
     count, list_iter, list_rev_iter, list_rev_iter_lol, count_between, count_down, count_double,
-    count_down_short, tuple_iter, str_iter,
+    count_down_short, tuple_iter, empty_tuple_iter, str_iter,
 )
 count(5)
 list_iter(list(reversed(range(5))))
@@ -52,6 +56,7 @@ count_down_short()
 print('==')
 list_rev_iter_lol(list(reversed(range(5))))
 tuple_iter((1, 2, 3))
+empty_tuple_iter(())
 str_iter("abc")
 [out]
 0
