Improve type inference special case that involves union with Any (#10887)

When trying to match `list[T]` and `list[str] | Any`, previously we
gave up, because the union items caused conflicting inferred values
for `T` (`str` and `Any`). Work around the issue by dropping `Any`
constraints if there are multiple sets of contraints, since we
prefer more precise types.

This fixes false positives resulting from python/typeshed#5557,
which changed some return types to contain unions with `Any` items.
See mypy primer results in #10881 for a real-world example of this
in sphinx.

Author: JukkaL

mypy/constraints.py

@@ -216,6 +216,16 @@ def any_constraints(options: List[Optional[List[Constraint]]], eager: bool) -> L
         # TODO: More generally, if a given (variable, direction) pair appears in
         #       every option, combine the bounds with meet/join.
         return valid_options[0]
+    elif len(valid_options) > 1:
+        # Drop constraints that only refer to "Any" and try again. This way Any types
+        # in unions don't interfere with type inference.
+        narrowed_options = [option
+                            for option in valid_options
+                            if not (option and
+                                    all(isinstance(get_proper_type(c.target), AnyType)
+                                        for c in option))]
+        if len(narrowed_options) < len(valid_options):
+            return any_constraints([option for option in narrowed_options], eager)
 
     # Otherwise, there are either no valid options or multiple, inconsistent valid
     # options. Give up and deduce nothing.

test-data/unit/check-inference-context.test

@@ -1375,3 +1375,11 @@ def f(x: Callable[..., T]) -> T:
 
 x: G[str] = f(G)
 [out]
+
+[case testConditionalExpressionWithEmptyListAndUnionWithAny]
+from typing import Union, List, Any
+
+def f(x: Union[List[str], Any]) -> None:
+    a = x if x else []
+    reveal_type(a)  # N: Revealed type is "Union[builtins.list[builtins.str], Any]"
+[builtins fixtures/list.pyi]