add regression tests

tests/ui/traits/new-solver/projection/param-env-trait-candidate-1.rs

@@ -0,0 +1,14 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// See https://github.com/rust-lang/trait-system-refactor-initiative/issues/1
+// a minimization of a pattern in core.
+fn next<T: Iterator<Item = U>, U>(t: &mut T) -> Option<U> {
+    t.next()
+}
+
+fn foo<T: Iterator>(t: &mut T) {
+    let _: Option<T::Item> = next(t);
+}
+
+fn main() {}

tests/ui/traits/new-solver/projection/param-env-trait-candidate-2.rs

@@ -0,0 +1,29 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// See https://github.com/rust-lang/trait-system-refactor-initiative/issues/1,
+// a minimization of a pattern in core.
+
+trait Iterator {
+    type Item;
+}
+
+struct Flatten<I>(I);
+
+impl<I, U> Iterator for Flatten<I>
+where
+    I: Iterator<Item = U>,
+{
+    type Item = U;
+}
+
+fn needs_iterator<I: Iterator>() {}
+
+fn environment<J>()
+where
+    J: Iterator,
+{
+    needs_iterator::<Flatten<J>>();
+}
+
+fn main() {}