add regression test for issue rust-lang#78632

Author: yerke

src/test/ui/traits/issue-78632.rs

@@ -0,0 +1,59 @@
+// check-pass
+//
+// Regression test for issue #78632
+
+#![crate_type = "lib"]
+
+pub trait Corge<T> {
+    type Fred;
+}
+
+impl Corge<u8> for () {
+    type Fred = u32;
+}
+
+pub trait Waldo {
+    type Quax;
+}
+
+impl Waldo for u32 {
+    type Quax = u8;
+}
+
+pub trait Grault
+where
+    (): Corge<Self::Thud>,
+{
+    type Thud;
+    fn bar(_: <() as Corge<Self::Thud>>::Fred) {}
+}
+
+impl<T> Grault for T
+where
+    T: Waldo,
+    (): Corge<T::Quax>,
+    <() as Corge<T::Quax>>::Fred: Waldo,
+{
+    type Thud = u8;
+}
+
+pub trait Plugh<I> {
+    fn baz();
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct Qiz<T> {
+    foo: T,
+}
+
+impl<T> Plugh<<() as Corge<T::Thud>>::Fred> for Qiz<T>
+where
+    T: Grault,
+    (): Corge<T::Thud>,
+{
+    fn baz() {}
+}
+
+pub fn test() {
+    <Qiz<u32> as Plugh<u32>>::baz();
+}