Rollup merge of #88406 - spastorino:tait-nest-infer-test, r=oli-obk

Tait nest infer test

r? `@oli-obk`

Related to #86727

src/test/ui/type-alias-impl-trait/nested-tait-inference.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+use std::fmt::Debug;
+
+type FooX = impl Debug;
+
+trait Foo<A> { }
+
+impl Foo<()> for () { }
+
+fn foo() -> impl Foo<FooX> {
+    ()
+}
+
+fn main() { }

src/test/ui/type-alias-impl-trait/nested-tait-inference2.rs

@@ -0,0 +1,19 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+use std::fmt::Debug;
+
+type FooX = impl Debug;
+//~^ ERROR: could not find defining uses
+
+trait Foo<A> {}
+
+impl Foo<()> for () {}
+impl Foo<u32> for () {}
+
+fn foo() -> impl Foo<FooX> {
+    //~^ ERROR: the trait bound `(): Foo<impl Debug>` is not satisfied [E0277]
+    ()
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/nested-tait-inference2.stderr

@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `(): Foo<impl Debug>` is not satisfied
+  --> $DIR/nested-tait-inference2.rs:14:13
+   |
+LL | fn foo() -> impl Foo<FooX> {
+   |             ^^^^^^^^^^^^^^ the trait `Foo<impl Debug>` is not implemented for `()`
+   |
+   = help: the following implementations were found:
+             <() as Foo<()>>
+             <() as Foo<u32>>
+
+error: could not find defining uses
+  --> $DIR/nested-tait-inference2.rs:6:13
+   |
+LL | type FooX = impl Debug;
+   |             ^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.