add test for ICE Where clause Binder(..) was applicable to `Obligat…

…ion(..)` but now is not

Fixes #84727

tests/ui/traits/trait-selection-ice-84727.rs

@@ -0,0 +1,38 @@
+// ICE Where clause `Binder(..)` was applicable to `Obligation(..)` but now is not
+// issue: rust-lang/rust#84727
+
+struct Cell<Fg, Bg = Fg> {
+    foreground: Color<Fg>,
+    //~^ ERROR cannot find type `Color` in this scope
+    background: Color<Bg>,
+    //~^ ERROR cannot find type `Color` in this scope
+}
+
+trait Over<Bottom, Output> {
+    fn over(self) -> Output;
+}
+
+impl<TopFg, TopBg, BottomFg, BottomBg, NewFg, NewBg>
+    Over<Cell<BottomFg, BottomBg>, Cell<NewFg, NewBg>> for Cell<TopFg, TopBg>
+where
+    Self: Over<Color<BottomBg>, Cell<NewFg>>,
+    //~^ ERROR cannot find type `Color` in this scope
+{
+    fn over(self) -> Cell<NewFg> {
+    //~^ ERROR mismatched types
+        self.over();
+    }
+}
+
+impl<'b, TopFg, TopBg, BottomFg, BottomBg> Over<&Cell<BottomFg, BottomBg>, ()>
+    for Cell<TopFg, TopBg>
+where
+    Cell<TopFg, TopBg>: Over<Cell<BottomFg>, Cell<BottomFg>>,
+{
+    fn over(self) -> Cell<NewBg> {
+    //~^ ERROR cannot find type `NewBg` in this scope
+        self.over();
+    }
+}
+
+pub fn main() {}

tests/ui/traits/trait-selection-ice-84727.stderr

@@ -0,0 +1,47 @@
+error[E0412]: cannot find type `Color` in this scope
+  --> $DIR/trait-selection-ice-84727.rs:5:17
+   |
+LL |     foreground: Color<Fg>,
+   |                 ^^^^^ not found in this scope
+
+error[E0412]: cannot find type `Color` in this scope
+  --> $DIR/trait-selection-ice-84727.rs:7:17
+   |
+LL |     background: Color<Bg>,
+   |                 ^^^^^ not found in this scope
+
+error[E0412]: cannot find type `Color` in this scope
+  --> $DIR/trait-selection-ice-84727.rs:18:16
+   |
+LL |     Self: Over<Color<BottomBg>, Cell<NewFg>>,
+   |                ^^^^^ not found in this scope
+
+error[E0412]: cannot find type `NewBg` in this scope
+  --> $DIR/trait-selection-ice-84727.rs:32:27
+   |
+LL |     fn over(self) -> Cell<NewBg> {
+   |                           ^^^^^ not found in this scope
+   |
+help: you might be missing a type parameter
+   |
+LL | impl<'b, TopFg, TopBg, BottomFg, BottomBg, NewBg> Over<&Cell<BottomFg, BottomBg>, ()>
+   |                                          +++++++
+
+error[E0308]: mismatched types
+  --> $DIR/trait-selection-ice-84727.rs:21:22
+   |
+LL |     fn over(self) -> Cell<NewFg> {
+   |        ----          ^^^^^^^^^^^ expected `Cell<NewFg>`, found `()`
+   |        |
+   |        implicitly returns `()` as its body has no tail or `return` expression
+LL |
+LL |         self.over();
+   |                    - help: remove this semicolon to return this value
+   |
+   = note: expected struct `Cell<NewFg>`
+           found unit type `()`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0308, E0412.
+For more information about an error, try `rustc --explain E0308`.