Use union of current context and left side for right side narrowing of binary ops (#19249)

Fixes #12001. Fixes #6898. Fixes #15368. Improves #17790 and #11508.

When encountering `a {and,or} b`, we used to use `a` as primary context
for `b` inference. This results in weird errors when `a` and `b` are
completely unrelated, and in many cases return type/assignment type
context can do much better. Inferring to union should be harmless in
most cases, so use union of `a` and current context instead.

Author: sterliakov

mypy/checkexpr.py

@@ -4288,7 +4288,9 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
         ):
             self.msg.unreachable_right_operand(e.op, e.right)
 
-        right_type = self.analyze_cond_branch(right_map, e.right, expanded_left_type)
+        right_type = self.analyze_cond_branch(
+            right_map, e.right, self._combined_context(expanded_left_type)
+        )
 
         if left_map is None and right_map is None:
             return UninhabitedType()
@@ -5919,6 +5921,16 @@ def analyze_cond_branch(
             self.chk.push_type_map(map)
             return self.accept(node, type_context=context, allow_none_return=allow_none_return)
 
+    def _combined_context(self, ty: Type | None) -> Type | None:
+        ctx_items = []
+        if ty is not None:
+            ctx_items.append(ty)
+        if self.type_context and self.type_context[-1] is not None:
+            ctx_items.append(self.type_context[-1])
+        if ctx_items:
+            return make_simplified_union(ctx_items)
+        return None
+
     #
     # Helpers
     #

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

@@ -1510,3 +1510,24 @@ def mymin(
 def check(paths: Iterable[str], key: Callable[[str], int]) -> Union[str, None]:
     return mymin(paths, key=key, default=None)
 [builtins fixtures/tuple.pyi]
+
+[case testBinaryOpInferenceContext]
+from typing import Literal, TypeVar
+
+T = TypeVar("T")
+
+def identity(x: T) -> T:
+    return x
+
+def check1(use: bool, val: str) -> "str | Literal[True]":
+    return use or identity(val)
+
+def check2(use: bool, val: str) -> "str | bool":
+    return use or identity(val)
+
+def check3(use: bool, val: str) -> "str | Literal[False]":
+    return use and identity(val)
+
+def check4(use: bool, val: str) -> "str | bool":
+    return use and identity(val)
+[builtins fixtures/tuple.pyi]