TAITs do not constrain generic params

compiler/rustc_hir_analysis/src/constrained_generic_params.rs

@@ -59,7 +59,7 @@ struct ParameterCollector {
 impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ParameterCollector {
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         match *t.kind() {
-            ty::Alias(ty::Projection | ty::Inherent, ..) if !self.include_nonconstraining => {
+            ty::Alias(..) if !self.include_nonconstraining => {
                 // projections are not injective
                 return ControlFlow::Continue(());
             }

tests/ui/type-alias-impl-trait/coherence.rs

@@ -11,7 +11,7 @@ fn use_alias<T>(val: T) -> AliasOfForeignType<T> {
     foreign_crate::ForeignType(val)
 }
 
-impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
+impl foreign_crate::ForeignTrait for AliasOfForeignType<()> {}
 //~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
 
 fn main() {}

tests/ui/type-alias-impl-trait/coherence.stderr

@@ -1,10 +1,10 @@
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
   --> $DIR/coherence.rs:14:1
    |
-LL | impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------------
-   | |                                       |
-   | |                                       `AliasOfForeignType<T>` is not defined in the current crate
+LL | impl foreign_crate::ForeignTrait for AliasOfForeignType<()> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^----------------------
+   | |                                    |
+   | |                                    `AliasOfForeignType<()>` is not defined in the current crate
    | impl doesn't use only types from inside the current crate
    |
    = note: define and implement a trait or new type instead

tests/ui/type-alias-impl-trait/coherence_generalization.rs

@@ -2,14 +2,15 @@
 
 // FIXME(type_alias_impl_trait): What does this test? This needs a comment
 // explaining what we're worried about here.
+
 #![feature(type_alias_impl_trait)]
 trait Trait {}
 type Opaque<T> = impl Sized;
 fn foo<T>() -> Opaque<T> {
     ()
 }
 
-impl<T, V> Trait for (T, V, V, u32) {}
-impl<U, V> Trait for (Opaque<U>, V, i32, V) {}
+impl<T, U, V> Trait for (T, U, V, V, u32) {}
+impl<U, V> Trait for (Opaque<U>, U, V, i32, V) {}
 
 fn main() {}

tests/ui/type-alias-impl-trait/unconstrained-impl-param.rs

@@ -0,0 +1,25 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Display;
+
+type Opaque<X> = impl Sized + 'static;
+fn define<X>() -> Opaque<X> {}
+
+trait Trait {
+    type Assoc: Display;
+}
+impl<'a> Trait for Opaque<&'a str> {
+    //~^ ERROR the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+    type Assoc = &'a str;
+}
+
+// ======= Exploit =======
+
+fn extend<T: Trait + 'static>(s: T::Assoc) -> Box<dyn Display> {
+    Box::new(s)
+}
+
+fn main() {
+    let val = extend::<Opaque<&'_ str>>(&String::from("blah blah blah"));
+    println!("{}", val);
+}

tests/ui/type-alias-impl-trait/unconstrained-impl-param.stderr

@@ -0,0 +1,9 @@
+error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unconstrained-impl-param.rs:11:6
+   |
+LL | impl<'a> Trait for Opaque<&'a str> {
+   |      ^^ unconstrained lifetime parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0207`.