Don't structurally resolve during method ambiguity in probe

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -12,6 +12,7 @@ use rustc_hir::def::DefKind;
 use rustc_hir_analysis::autoderef::{self, Autoderef};
 use rustc_infer::infer::canonical::OriginalQueryValues;
 use rustc_infer::infer::canonical::{Canonical, QueryResponse};
+use rustc_infer::infer::error_reporting::TypeAnnotationNeeded::E0282;
 use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
 use rustc_middle::middle::stability;
@@ -448,15 +449,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     );
                 }
             } else {
-                // Encountered a real ambiguity, so abort the lookup. If `ty` is not
-                // an `Err`, report the right "type annotations needed" error pointing
-                // to it.
+                // Ended up encountering a type variable when doing autoderef,
+                // but it may not be a type variable after processing obligations
+                // in our local `FnCtxt`, so don't call `structurally_resolved_type`.
                 let ty = &bad_ty.ty;
                 let ty = self
                     .probe_instantiate_query_response(span, &orig_values, ty)
                     .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
-                let ty = self.structurally_resolved_type(span, ty.value);
-                assert!(matches!(ty.kind(), ty::Error(_)));
+                let ty = self.resolve_vars_if_possible(ty.value);
+                let guar = match *ty.kind() {
+                    ty::Infer(ty::TyVar(_)) => self
+                        .err_ctxt()
+                        .emit_inference_failure_err(self.body_id, span, ty.into(), E0282, true)
+                        .emit(),
+                    ty::Error(guar) => guar,
+                    _ => bug!("unexpected bad final type in method autoderef"),
+                };
+                self.demand_eqtype(span, ty, self.tcx.ty_error(guar));
                 return Err(MethodError::NoMatch(NoMatchData {
                     static_candidates: Vec::new(),
                     unsatisfied_predicates: Vec::new(),

tests/ui/autoref-autoderef/deref-ambiguity-becomes-nonambiguous.rs

@@ -0,0 +1,40 @@
+use std::ops::Deref;
+use std::rc::Rc;
+
+struct Value<T>(T);
+
+pub trait Wrap<T> {
+    fn wrap() -> Self;
+}
+
+impl<R, A1, A2> Wrap<fn(A1, A2) -> R> for Value<fn(A1, A2) -> R> {
+    fn wrap() -> Self {
+        todo!()
+    }
+}
+
+impl<F, R, A1, A2> Wrap<F> for Value<Rc<dyn Fn(A1, A2) -> R>> {
+    fn wrap() -> Self {
+        todo!()
+    }
+}
+
+impl<F> Deref for Value<Rc<F>> {
+    type Target = F;
+
+    fn deref(&self) -> &Self::Target {
+        &*self.0
+    }
+}
+
+fn main() {
+    let var_fn = Value::wrap();
+    //~^ ERROR type annotations needed for `Value<Rc<_>>`
+
+    // The combination of `Value: Wrap` obligation plus the autoderef steps
+    // (caused by the `Deref` impl above) actually means that the self type
+    // of the method fn below is constrained to be `Value<Rc<dyn Fn(?0, ?1) -> ?2>>`.
+    // However, that's only known to us on the error path -- we still need
+    // to emit an ambiguity error, though.
+    let _ = var_fn.clone();
+}

tests/ui/autoref-autoderef/deref-ambiguity-becomes-nonambiguous.stderr

@@ -0,0 +1,17 @@
+error[E0282]: type annotations needed for `Value<Rc<_>>`
+  --> $DIR/deref-ambiguity-becomes-nonambiguous.rs:31:9
+   |
+LL |     let var_fn = Value::wrap();
+   |         ^^^^^^
+...
+LL |     let _ = var_fn.clone();
+   |                    ----- type must be known at this point
+   |
+help: consider giving `var_fn` an explicit type, where the placeholders `_` are specified
+   |
+LL |     let var_fn: Value<Rc<_>> = Value::wrap();
+   |               ++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.