From 08fc5e8acd1bc41fbf5b8a4b5c96a42c16bab158 Mon Sep 17 00:00:00 2001
From: Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de>
Date: Wed, 10 Jan 2024 16:03:05 +0000
Subject: [PATCH 1/2] Define hidden types in confirmation

---
 .../src/traits/project.rs                     |  2 +-
 .../nested_inference_failure.rs               | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/type-alias-impl-trait/nested_inference_failure.rs

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index dd4e69efe379e..abbc2066eac16 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -2466,7 +2466,7 @@ fn confirm_param_env_candidate<'cx, 'tcx>(
     debug!(?cache_projection, ?obligation_projection);
 
     match infcx.at(cause, param_env).eq(
-        DefineOpaqueTypes::No,
+        DefineOpaqueTypes::Yes,
         cache_projection,
         obligation_projection,
     ) {
diff --git a/tests/ui/type-alias-impl-trait/nested_inference_failure.rs b/tests/ui/type-alias-impl-trait/nested_inference_failure.rs
new file mode 100644
index 0000000000000..16b78f18aa347
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/nested_inference_failure.rs
@@ -0,0 +1,26 @@
+// check-pass
+
+//! This test checks that we can successfully infer
+//! the hidden type of `FooImpl` to be `Foo<i32, {closure}>`
+//! and `ImplT` to be `i32`. This test used to fail, because
+//! we were unable to make the connection that the closure
+//! argument is the same as the first argument of `Foo`.
+
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+use std::marker::PhantomData;
+
+struct Foo<T: Debug, F: FnOnce(T)> {
+    f: F,
+    _phantom: PhantomData<T>,
+}
+
+type ImplT = impl Debug;
+type FooImpl = Foo<ImplT, impl FnOnce(ImplT)>;
+
+fn bar() -> FooImpl {
+    Foo::<i32, _> { f: |_| (), _phantom: PhantomData }
+}
+
+fn main() {}

From 0ae00444dc075c814dd061aa1cfdc5c461c1f608 Mon Sep 17 00:00:00 2001
From: Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de>
Date: Wed, 10 Jan 2024 16:05:24 +0000
Subject: [PATCH 2/2] Make sure the new solver agrees

---
 tests/ui/type-alias-impl-trait/nested_inference_failure.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tests/ui/type-alias-impl-trait/nested_inference_failure.rs b/tests/ui/type-alias-impl-trait/nested_inference_failure.rs
index 16b78f18aa347..d2091ca96ea01 100644
--- a/tests/ui/type-alias-impl-trait/nested_inference_failure.rs
+++ b/tests/ui/type-alias-impl-trait/nested_inference_failure.rs
@@ -1,4 +1,6 @@
 // check-pass
+// revisions: new old
+//[new] compile-flags: -Znext-solver
 
 //! This test checks that we can successfully infer
 //! the hidden type of `FooImpl` to be `Foo<i32, {closure}>`