Accept less invalid Rust in rustdoc

tests/rustdoc-ui/not-wf-ambiguous-normalization.rs

@@ -0,0 +1,25 @@
+// compile-flags: -Znormalize-docs
+
+#![feature(type_alias_impl_trait)]
+
+trait Allocator {
+    type Buffer;
+}
+
+struct DefaultAllocator;
+
+// This unconstrained impl parameter causes the normalization of
+// `<DefaultAllocator as Allocator>::Buffer` to be ambiguous,
+// which caused an ICE with `-Znormalize-docs`.
+impl<T> Allocator for DefaultAllocator {
+    //~^ ERROR: type annotations needed
+    type Buffer = ();
+}
+
+type A = impl Fn(<DefaultAllocator as Allocator>::Buffer);
+
+fn foo() -> A {
+    |_| ()
+}
+
+fn main() {}