Make "failed region resolution while normalizing ParamEnv" a span_delayed_bug.

Fixes rust-lang#121414.

Author: nnethercote

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -172,9 +172,7 @@ fn do_normalize_predicates<'tcx>(
     // the normalized predicates.
     let errors = infcx.resolve_regions(&outlives_env);
     if !errors.is_empty() {
-        // @lcnr: Let's still ICE here for now. I want a test case
-        // for that.
-        tcx.dcx().span_bug(
+        tcx.dcx().span_delayed_bug(
             span,
             format!("failed region resolution while normalizing {elaborated_env:?}: {errors:?}"),
         );

tests/ui/traits/issue-121414.rs

@@ -0,0 +1,15 @@
+trait Bar {
+    type Type;
+}
+struct Foo<'a>(&'a ());
+impl<'a> Bar for Foo<'f> { //~ ERROR undeclared lifetime
+    type Type = u32;
+}
+
+fn test() //~ ERROR implementation of `Bar` is not general enough
+where
+    for<'a> <Foo<'a> as Bar>::Type: Sized,
+{
+}
+
+fn main() {}

tests/ui/traits/issue-121414.stderr

@@ -0,0 +1,20 @@
+error[E0261]: use of undeclared lifetime name `'f`
+  --> $DIR/issue-121414.rs:5:22
+   |
+LL | impl<'a> Bar for Foo<'f> {
+   |      -               ^^ undeclared lifetime
+   |      |
+   |      help: consider introducing lifetime `'f` here: `'f,`
+
+error: implementation of `Bar` is not general enough
+  --> $DIR/issue-121414.rs:9:4
+   |
+LL | fn test()
+   |    ^^^^ implementation of `Bar` is not general enough
+   |
+   = note: `Bar` would have to be implemented for the type `Foo<'0>`, for any lifetime `'0`...
+   = note: ...but `Bar` is actually implemented for the type `Foo<'1>`, for some specific lifetime `'1`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0261`.