Auto merge of #93783 - oli-obk:lazy_tait_regression_fix, r=jackh726

Fix regression from lazy opaque types

The breakage was found in https://github.com/rust-lang/rust/pull/92007#issuecomment-1032203011 and has not hit nightly yet.

5 files changed, 94 insertions, 2 deletions

diff --git a/src/test/ui/impl-trait/hidden-type-is-opaque-2.rs b/src/test/ui/impl-trait/hidden-type-is-opaque-2.rs
new file mode 100644
index 00000000000..1b65685a6c0
--- /dev/null
+++ b/src/test/ui/impl-trait/hidden-type-is-opaque-2.rs
@@ -0,0 +1,34 @@
+// This doesn't work, because we don't flow information from opaque types
+// into function arguments via the function's generic parameters
+// FIXME(oli-obk): make `expected_inputs_for_expected_output` support this
+
+fn reify_as() -> Thunk<impl FnOnce(Continuation) -> Continuation> {
+    Thunk::new(|mut cont| { //~ ERROR type annotations needed
+        cont.reify_as();
+        cont
+    })
+}
+
+#[must_use]
+struct Thunk<F>(F);
+
+impl<F> Thunk<F> {
+    fn new(f: F) -> Self
+    where
+        F: ContFn,
+    {
+        Thunk(f)
+    }
+}
+
+trait ContFn {}
+
+impl<F: FnOnce(Continuation) -> Continuation> ContFn for F {}
+
+struct Continuation;
+
+impl Continuation {
+    fn reify_as(&mut self) {}
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/hidden-type-is-opaque-2.stderr b/src/test/ui/impl-trait/hidden-type-is-opaque-2.stderr
new file mode 100644
index 00000000000..e538aaeb4c5
--- /dev/null
+++ b/src/test/ui/impl-trait/hidden-type-is-opaque-2.stderr
@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/hidden-type-is-opaque-2.rs:6:17
+   |
+LL |     Thunk::new(|mut cont| {
+   |                 ^^^^^^^^ consider giving this closure parameter a type
+   |
+   = note: type must be known at this point
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/src/test/ui/impl-trait/hidden-type-is-opaque.rs b/src/test/ui/impl-trait/hidden-type-is-opaque.rs
new file mode 100644
index 00000000000..b302ae36ef4
--- /dev/null
+++ b/src/test/ui/impl-trait/hidden-type-is-opaque.rs
@@ -0,0 +1,32 @@
+// check-pass
+
+fn reify_as() -> Thunk<impl ContFn> {
+    Thunk::new(|mut cont| {
+        cont.reify_as();
+        cont
+    })
+}
+
+#[must_use]
+struct Thunk<F>(F);
+
+impl<F> Thunk<F> {
+    fn new(f: F) -> Self
+    where
+        F: FnOnce(Continuation) -> Continuation,
+    {
+        Thunk(f)
+    }
+}
+
+trait ContFn {}
+
+impl<F: FnOnce(Continuation) -> Continuation> ContFn for F {}
+
+struct Continuation;
+
+impl Continuation {
+    fn reify_as(&mut self) {}
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/issues/issue-70877.rs b/src/test/ui/impl-trait/issues/issue-70877.rs
index 8169cfafac7..1a86fa00ed1 100644
--- a/src/test/ui/impl-trait/issues/issue-70877.rs
+++ b/src/test/ui/impl-trait/issues/issue-70877.rs
@@ -13,7 +13,7 @@ impl Iterator for Bar {
     type Item = FooItem;
 
     fn next(&mut self) -> Option<Self::Item> {
-        Some(Box::new(quux))
+        Some(Box::new(quux)) //~ ERROR mismatched types
     }
 }
 
diff --git a/src/test/ui/impl-trait/issues/issue-70877.stderr b/src/test/ui/impl-trait/issues/issue-70877.stderr
index 8813bff3c35..7cbd58bdabf 100644
--- a/src/test/ui/impl-trait/issues/issue-70877.stderr
+++ b/src/test/ui/impl-trait/issues/issue-70877.stderr
@@ -1,3 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-70877.rs:16:9
+   |
+LL | type FooRet = impl std::fmt::Debug;
+   |               -------------------- the expected opaque type
+...
+LL |     fn next(&mut self) -> Option<Self::Item> {
+   |                           ------------------ expected `Option<Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> FooRet + 'static)>>` because of return type
+LL |         Some(Box::new(quux))
+   |         ^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn Fn`, found fn item
+   |
+   = note: expected enum `Option<Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> FooRet + 'static)>>`
+              found enum `Option<Box<for<'r> fn(&'r (dyn ToString + 'r)) -> FooRet {quux}>>`
+
 error: opaque type's hidden type cannot be another opaque type from the same scope
   --> $DIR/issue-70877.rs:31:12
    |
@@ -15,5 +29,6 @@ note: opaque type being used as hidden type
 LL | type FooRet = impl std::fmt::Debug;
    |               ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.