Add regression test for rust-lang#140545

Author: moxian

tests/ui/impl-trait/issues/fixedpoint-issue-140545.rs

@@ -0,0 +1,12 @@
+// Regression test for ICE from issue #140545
+// The error message is confusing and wrong, but that's a different problem (#139350)
+//@ edition:2018
+
+trait Foo {}
+fn a(x: impl Foo) -> impl Foo {
+    if true { x } else { a(a(x)) }
+    //~^ ERROR: expected generic type parameter, found `impl Foo` [E0792]
+    //~| ERROR: type parameter `impl Foo` is part of concrete type but not used in parameter list for the `impl Trait` type alias
+}
+
+fn main(){}

tests/ui/impl-trait/issues/fixedpoint-issue-140545.stderr

@@ -0,0 +1,17 @@
+error[E0792]: expected generic type parameter, found `impl Foo`
+  --> $DIR/fixedpoint-issue-140545.rs:7:26
+   |
+LL | fn a(x: impl Foo) -> impl Foo {
+   |         -------- this generic parameter must be used with a generic type parameter
+LL |     if true { x } else { a(a(x)) }
+   |                          ^^^^^^^
+
+error: type parameter `impl Foo` is part of concrete type but not used in parameter list for the `impl Trait` type alias
+  --> $DIR/fixedpoint-issue-140545.rs:7:26
+   |
+LL |     if true { x } else { a(a(x)) }
+   |                          ^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0792`.