Stop reporting an implicit Any the expectation already absorbed

pyrefly/lib/alt/expr.rs

@@ -465,7 +465,18 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     got.with_ty(want.clone())
                 }
             }
-            ExprExpectation::Check { .. } => self.expr_infer_impl(x, None, options.errors),
+            // `want` is `Any` here, so the check above is skipped: it always succeeds.
+            // Succeeding trivially never solves the placeholder an empty display leaves
+            // behind, which then survives into an answer and is reported as an implicit
+            // `Any` the expectation had already absorbed. Pin it instead. Containment
+            // keeps this to placeholders `x` minted itself, so a name whose type is
+            // still open keeps its own diagnostic.
+            ExprExpectation::Check { .. } => {
+                let got = self.expr_infer_impl(x, None, options.errors);
+                self.solver()
+                    .pin_contained_placeholders_within(got.ty(), x.range());
+                got
+            }
             ExprExpectation::Infer(hint) => self.expr_infer_impl(x, hint, options.errors),
         }
     }

pyrefly/lib/alt/solve.rs

@@ -5477,6 +5477,19 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
     ) -> (Type, Type) {
         let base = self.expr_infer(&subscript.value, errors);
         let slice_ty = self.expr_infer(&subscript.slice, errors);
+        // Whether an empty display assigned here can still be observed as an implicit
+        // `Any` is a property of the whole target. It absorbs only if its own type is
+        // known: not if it still holds an unpinned placeholder, and not if it holds an
+        // `Any` pyrefly inferred rather than the user declaring one, since a container
+        // pinned from `{}` absorbs nothing - it propagates the same uncertainty, and
+        // its contents deserve the diagnostic as much as it did. Computed before
+        // distributing because pinning is global, so a solved union arm would otherwise
+        // silence a placeholder that an unsolved arm genuinely leaks.
+        let target_absorbs = !base.any(|t| match t {
+            Type::Any(AnyStyle::Implicit) => true,
+            Type::Var(v) => self.solver().var_is_partial(*v),
+            _ => false,
+        });
         let assigned_ty = self.distribute_over_union(&base, |base| {
             self.distribute_over_union(&slice_ty, |key| {
                 match (base, key) {
@@ -5527,13 +5540,31 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                             ExprOrBinding::Expr(e) => {
                                 call_setitem(CallArg::expr(e));
                                 // We already emit errors for `e` during `call_method_or_error`
-                                self.expr_infer(
+                                let ty = self.expr_infer(
                                     e,
                                     &ErrorCollector::new(
                                         errors.module().clone(),
                                         ErrorStyle::Never,
                                     ),
-                                )
+                                );
+                                // That call already checked `e` against `__setitem__`'s value
+                                // parameter; this pass only produces the type the subscript
+                                // narrows to, and re-inferring mints a *fresh* placeholder for
+                                // every empty display in `e`. Once the target is fully solved
+                                // it has already absorbed the value, so those placeholders are
+                                // unobservable and are pinned rather than reported. A target
+                                // still holding a partial var is what the diagnostic exists
+                                // for - there the placeholder really does leak. Containment
+                                // keeps this to placeholders `e` minted itself; one belonging
+                                // to a name `e` merely mentions outlives the assignment and
+                                // keeps its own diagnostic. Note the target's *shape* is never
+                                // consulted, so `dict`, `list` and a hand-written
+                                // `__setitem__` all absorb alike.
+                                if target_absorbs {
+                                    self.solver()
+                                        .pin_contained_placeholders_within(&ty, e.range());
+                                }
+                                ty
                             }
                             ExprOrBinding::Binding(b) => {
                                 let binding_ty = self

pyrefly/lib/solver/solver.rs

@@ -1275,6 +1275,36 @@ impl Solver {
         self.sanitize_vars(ty.collect_all_vars(), pin_partial_types)
     }
 
+    /// Pin empty-container placeholders that were created *inside* `range`, silently.
+    ///
+    /// A `PartialContained` var records the literal that created it, so containment
+    /// distinguishes a placeholder the expression at `range` minted itself from one it
+    /// merely mentions by name. Only the former can be pinned without consequence: a
+    /// name's placeholder outlives the expression and keeps its own diagnostic.
+    pub fn pin_contained_placeholders_within(&self, ty: &Type, range: TextRange) {
+        let mut pending = ty.collect_all_vars();
+        let mut seen = SmallSet::new();
+        while let Some(var) = pending.pop() {
+            if !seen.insert(var) {
+                continue;
+            }
+            let variables = self.variables.lock();
+            let mut variable = variables.get_mut(var);
+            match &mut *variable {
+                Variable::PartialContained(literal) if range.contains_range(*literal) => {
+                    *variable = Variable::Answer(self.heap.mk_any_implicit());
+                }
+                // Traverse only through variables that already have an answer. `force_var`
+                // would pin an unanswered one as a side effect, which is precisely what this
+                // must not do - it would silence the placeholders the containment test is
+                // here to protect. Missing a var this way only costs a diagnostic we would
+                // have suppressed, never one we should have kept.
+                Variable::Answer(answer) => pending.extend(answer.collect_all_vars()),
+                _ => {}
+            }
+        }
+    }
+
     pub fn sanitize_vars(&self, mut pending: Vec<Var>, pin_partial_types: bool) -> Vec<PinError> {
         let mut seen = SmallSet::new();
         let mut errors = Vec::new();

pyrefly/lib/test/callable.rs

@@ -1668,6 +1668,176 @@ f: Callable[[int], None] = lambda x, y: None  # E: Type of lambda parameter `y`
 "#,
 );
 
+// Regression test for https://github.com/facebook/pyrefly/issues/4301
+testcase!(
+    test_implicit_any_empty_container_absorbed_by_any,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from typing import Any
+
+# An `Any` annotation absorbs the placeholder, so there is nothing to warn about.
+a: Any = {"t": None}
+b: dict[str, Any] = {"t": None}
+
+def takes_any(v: Any) -> None: ...
+takes_any({"t": None})
+
+def subscript(response: dict[str, Any]) -> None:
+    response["meta"] = {"t": None}
+
+# The container's own value type is still unsolved here, so the placeholder does
+# leak into what `x` is inferred as, and the diagnostic is warranted.
+x = {}
+x["y"] = {"t": None}  # E: Cannot infer type of empty container
+"#,
+);
+
+// Absorbing is a property of the expectation, not of the syntax that carries it, so
+// every position that checks a display against a declared `Any` absorbs alike.
+testcase!(
+    test_implicit_any_empty_container_absorbed_in_every_position,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from typing import Any, TypedDict
+
+class C:
+    attr: Any
+
+def attribute(c: C) -> None:
+    c.attr = {"t": None}
+
+def returns_any() -> Any:
+    return {"t": None}
+
+def default_arg(x: Any = {"t": None}) -> None: ...
+
+class TD(TypedDict):
+    k: Any
+
+def typed_dict(t: TD) -> None:
+    t["k"] = {"t": None}
+
+G: Any = {}
+
+def global_assign() -> None:
+    global G
+    G = {"t": None}
+"#,
+);
+
+// Unpacking is the one position left out, and deliberately so: `bind_unpacking`
+// records that "we never contextually type unpacks, we do the unpacking at type
+// level for simplicity (for now)". The target's `Any` never reaches the value, so
+// the placeholder is minted with no expectation to absorb it. Closing this belongs
+// to contextually typing unpacks, not here.
+testcase!(
+    bug = "unpacked subscript targets do not absorb the placeholder",
+    test_implicit_any_empty_container_unpacked_target,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from typing import Any
+
+def unpacked(d: dict[str, Any]) -> None:
+    d["a"], d["b"] = {"t": None}, {"u": None}  # E: Cannot infer type of empty container  # E: Cannot infer type of empty container
+"#,
+);
+
+// A name keeps its own diagnostic wherever it appears on the right-hand side: its
+// placeholder outlives the assignment, so an absorbing target must not silence the
+// warning that the name itself has no inferrable type. Only the placeholders the
+// display minted for itself are absorbed.
+testcase!(
+    test_implicit_any_empty_container_name_rhs_still_reported,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from typing import Any
+
+def name_rhs(d: dict[str, Any]) -> None:
+    y = []  # E: Cannot infer type of empty container
+    d["k"] = y
+
+def name_nested_in_display(d: dict[str, Any]) -> None:
+    y = []  # E: Cannot infer type of empty container
+    d["k"] = {"a": y}
+
+def name_nested_deeply(d: dict[str, Any]) -> None:
+    y = []  # E: Cannot infer type of empty container
+    d["k"] = {"a": [y]}
+
+def display_rhs(d: dict[str, Any]) -> None:
+    d["k"] = {"a": []}
+"#,
+);
+
+// A target only absorbs if its own type is known. One pinned from `{}` holds an
+// `Any` pyrefly inferred rather than one the user declared, so it propagates the
+// same uncertainty and its contents stay reportable. The union case is why this is
+// decided for the whole target rather than per arm: pinning is global, so a solved
+// arm must not silence what an unsolved arm leaks.
+testcase!(
+    test_implicit_any_empty_container_unknown_target_still_reported,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from typing import Any
+
+def inferred_target() -> None:
+    x = {}  # E: Cannot infer type of empty container
+    y = x
+    y["k"] = {"t": None}  # E: Cannot infer type of empty container
+
+def chained() -> None:
+    x = {}
+    x["a"] = {}  # E: Cannot infer type of empty container
+    x["a"]["b"] = {"t": None}  # E: Cannot infer type of empty container
+
+def union_with_unsolved_arm(flag: bool, declared: dict[str, Any]) -> None:
+    x = {}  # E: Cannot infer type of empty container
+    target = declared if flag else x
+    target["k"] = {"t": None}  # E: Cannot infer type of empty container
+"#,
+);
+
+// A fully-known target that cannot hold the value at all: `__setitem__` already
+// rejects it, and nothing can observe the placeholder inside a value being
+// rejected, so the second diagnostic is dropped as redundant.
+testcase!(
+    test_implicit_any_empty_container_narrow_target_reports_once,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+def narrow(d: dict[str, int]) -> None:
+    d["k"] = {"t": None}  # E: Cannot set item in `dict[str, int]`
+"#,
+);
+
+// Whether the placeholder can escape depends on the target, not on which container
+// the target happens to be, so every fully-solved target absorbs it alike.
+testcase!(
+    test_implicit_any_empty_container_absorbed_by_any_containers,
+    TestEnv::new().enable_implicit_any_error(),
+    r#"
+from collections import defaultdict
+from typing import Any, MutableMapping
+
+def sequence(xs: list[Any]) -> None:
+    xs[0] = {"t": None}
+
+def non_str_key(d: dict[int, Any]) -> None:
+    d[0] = {"t": None}
+
+def protocol(d: MutableMapping[str, Any]) -> None:
+    d["k"] = {"t": None}
+
+def subclass(d: defaultdict[str, Any]) -> None:
+    d["k"] = {"t": None}
+
+class Custom:
+    def __setitem__(self, key: str, value: Any) -> None: ...
+
+def custom(c: Custom) -> None:
+    c["k"] = {"t": None}
+"#,
+);
+
 testcase!(
     test_implicit_any_lambda_explicit_any_context,
     TestEnv::new().enable_implicit_any_lambda_error(),