Add backcompat hack to support

```rust
fn foo() -> impl MyTrait {
    panic!();
    MyStruct
}

struct MyStruct;
trait MyTrait {}

impl MyTrait for MyStruct {}
```

compiler/rustc_middle/src/ty/context.rs

@@ -30,6 +30,7 @@ use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{self, Lock, Lrc, WorkerLocal};
+use rustc_data_structures::vec_map::VecMap;
 use rustc_errors::ErrorReported;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
@@ -464,9 +465,13 @@ pub struct TypeckResults<'tcx> {
     /// this field will be set to `Some(ErrorReported)`.
     pub tainted_by_errors: Option<ErrorReported>,
 
-    /// All the opaque types that are restricted to concrete types
+    /// All the opaque types that have hidden types set
-    /// by this function.
+    /// by this function. For return-position-impl-trait we also store the
-    pub concrete_opaque_types: FxHashSet<DefId>,
+    /// type here, so that mir-borrowck can figure out hidden types,
+    /// even if they are only set in dead code (which doesn't show up in MIR).
+    /// For type-alias-impl-trait, this map is only used to prevent query cycles,
+    /// so the hidden types are all `None`.
+    pub concrete_opaque_types: VecMap<DefId, Option<Ty<'tcx>>>,
 
     /// Tracks the minimum captures required for a closure;
     /// see `MinCaptureInformationMap` for more details.

compiler/rustc_typeck/src/check/writeback.rs

@@ -499,8 +499,14 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
     fn visit_opaque_types(&mut self) {
         let opaque_types =
             self.fcx.infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
-        for (opaque_type_key, _) in opaque_types {
+        for (opaque_type_key, decl) in opaque_types {
-            self.typeck_results.concrete_opaque_types.insert(opaque_type_key.def_id);
+            let hidden_type = match decl.origin {
+                hir::OpaqueTyOrigin::FnReturn(_) | hir::OpaqueTyOrigin::AsyncFn(_) => {
+                    Some(self.resolve(decl.hidden_type.ty, &decl.hidden_type.span))
+                }
+                hir::OpaqueTyOrigin::TyAlias => None,
+            };
+            self.typeck_results.concrete_opaque_types.insert(opaque_type_key.def_id, hidden_type);
         }
     }
 

compiler/rustc_typeck/src/collect/type_of.rs

@@ -389,21 +389,22 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                         .get_value_matching(|(key, _)| key.def_id == def_id.to_def_id())
                         .copied()
                         .unwrap_or_else(|| {
-                            if let Some(ErrorReported) =
+                            let table = tcx.typeck(owner);
-                                tcx.typeck(owner).tainted_by_errors
+                            if let Some(ErrorReported) = table.tainted_by_errors {
-                            {
                                 // Some error in the
                                 // owner fn prevented us from populating
                                 // the `concrete_opaque_types` table.
                                 tcx.ty_error()
                             } else {
-                                // We failed to resolve the opaque type or it
+                                table.concrete_opaque_types.get(&def_id.to_def_id()).copied().unwrap_or_else(|| {
-                                // resolves to itself. We interpret this as the
+                                    // We failed to resolve the opaque type or it
-                                // no values of the hidden type ever being constructed,
+                                    // resolves to itself. We interpret this as the
-                                // so we can just make the hidden type be `!`.
+                                    // no values of the hidden type ever being constructed,
-                                // For backwards compatibility reasons, we fall back to
+                                    // so we can just make the hidden type be `!`.
-                                // `()` until we the diverging default is changed.
+                                    // For backwards compatibility reasons, we fall back to
-                                tcx.mk_diverging_default()
+                                    // `()` until we the diverging default is changed.
+                                    Some(tcx.mk_diverging_default())
+                                }).expect("RPIT always have a hidden type from typeck")
                             }
                         });
                     debug!("concrete_ty = {:?}", concrete_ty);
@@ -597,7 +598,7 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
             }
             // Calling `mir_borrowck` can lead to cycle errors through
             // const-checking, avoid calling it if we don't have to.
-            if !self.tcx.typeck(def_id).concrete_opaque_types.contains(&self.def_id) {
+            if self.tcx.typeck(def_id).concrete_opaque_types.get(&self.def_id).is_none() {
                 debug!("no constraints in typeck results");
                 return;
             }