From ebb8ff9edfe0a908be55fc8c7852f291cc57f62a Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Mon, 23 Nov 2020 07:42:06 -0500
Subject: [PATCH] remove diverging type variables from fn check

The comment seems incorrect. Testing revealed that the examples in
question still work (as well as some variants) even without the
special casing here.
---
 .../rustc_infer/src/infer/type_variable.rs    |  6 +++-
 compiler/rustc_typeck/src/check/check.rs      | 32 +++++--------------
 2 files changed, 13 insertions(+), 25 deletions(-)

diff --git a/compiler/rustc_infer/src/infer/type_variable.rs b/compiler/rustc_infer/src/infer/type_variable.rs
index f15268f6895..071cf6e1ea4 100644
--- a/compiler/rustc_infer/src/infer/type_variable.rs
+++ b/compiler/rustc_infer/src/infer/type_variable.rs
@@ -129,7 +129,11 @@ pub enum TypeVariableOriginKind {
     SubstitutionPlaceholder,
     AutoDeref,
     AdjustmentType,
-    DivergingFn,
+
+    /// In type check, when we are type checking a function that
+    /// returns `-> dyn Foo`, we substitute a type variable for the
+    /// return type for diagnostic purposes.
+    DynReturnFn,
     LatticeVariable,
 }
 
diff --git a/compiler/rustc_typeck/src/check/check.rs b/compiler/rustc_typeck/src/check/check.rs
index 1fd1253e527..3b9047318d4 100644
--- a/compiler/rustc_typeck/src/check/check.rs
+++ b/compiler/rustc_typeck/src/check/check.rs
@@ -241,32 +241,16 @@ pub(super) fn check_fn<'a, 'tcx>(
     // we saw and assigning it to the expected return type. This isn't
     // really expected to fail, since the coercions would have failed
     // earlier when trying to find a LUB.
-    //
-    // However, the behavior around `!` is sort of complex. In the
-    // event that the `actual_return_ty` comes back as `!`, that
-    // indicates that the fn either does not return or "returns" only
-    // values of type `!`. In this case, if there is an expected
-    // return type that is *not* `!`, that should be ok. But if the
-    // return type is being inferred, we want to "fallback" to `!`:
-    //
-    //     let x = move || panic!();
-    //
-    // To allow for that, I am creating a type variable with diverging
-    // fallback. This was deemed ever so slightly better than unifying
-    // the return value with `!` because it allows for the caller to
-    // make more assumptions about the return type (e.g., they could do
-    //
-    //     let y: Option<u32> = Some(x());
-    //
-    // which would then cause this return type to become `u32`, not
-    // `!`).
     let coercion = fcx.ret_coercion.take().unwrap().into_inner();
     let mut actual_return_ty = coercion.complete(&fcx);
-    if actual_return_ty.is_never() {
-        actual_return_ty = fcx.next_diverging_ty_var(TypeVariableOrigin {
-            kind: TypeVariableOriginKind::DivergingFn,
-            span,
-        });
+    debug!("actual_return_ty = {:?}", actual_return_ty);
+    if let ty::Dynamic(..) = declared_ret_ty.kind() {
+        // We have special-cased the case where the function is declared
+        // `-> dyn Foo` and we don't actually relate it to the
+        // `fcx.ret_coercion`, so just substitute a type variable.
+        actual_return_ty =
+            fcx.next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::DynReturnFn, span });
+        debug!("actual_return_ty replaced with {:?}", actual_return_ty);
     }
     fcx.demand_suptype(span, revealed_ret_ty, actual_return_ty);