fix #104513, Delay a span bug if we see generic params with err durin…

…g writeback

compiler/rustc_hir_typeck/src/writeback.rs

@@ -325,6 +325,20 @@ impl<'cx, 'tcx> Visitor<'tcx> for WritebackCx<'cx, 'tcx> {
         }
     }
 
+    fn visit_generic_arg(&mut self, arg: &'tcx hir::GenericArg<'tcx>) {
+        match arg {
+            hir::GenericArg::Type(ty) if let hir::TyKind::Err = ty.kind => {
+                // An generic argument with an error type will be reported
+                // We don't want to ICE here
+                self.tcx().sess.delay_span_bug(ty.span, format!("unexpected generic argument: {arg:?}"));
+            }
+            hir::GenericArg::Type(t) => self.visit_ty(t),
+            _ => {
+                // Nothing to write back here
+            }
+        }
+    }
+
     fn visit_block(&mut self, b: &'tcx hir::Block<'tcx>) {
         self.visit_node_id(b.span, b.hir_id);
         intravisit::walk_block(self, b);

src/test/ui/typeck/issue-104513-ice.rs

@@ -0,0 +1,6 @@
+struct S;
+fn f() {
+    let _: S<impl Oops> = S; //~ ERROR cannot find trait `Oops` in this scope
+    //~^ ERROR `impl Trait` only allowed in function and inherent method return types
+}
+fn main() {}

src/test/ui/typeck/issue-104513-ice.stderr

@@ -0,0 +1,18 @@
+error[E0405]: cannot find trait `Oops` in this scope
+  --> $DIR/issue-104513-ice.rs:3:19
+   |
+LL | fn f() {
+   |     - help: you might be missing a type parameter: `<Oops>`
+LL |     let _: S<impl Oops> = S;
+   |                   ^^^^ not found in this scope
+
+error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+  --> $DIR/issue-104513-ice.rs:3:14
+   |
+LL |     let _: S<impl Oops> = S;
+   |              ^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0405, E0562.
+For more information about an error, try `rustc --explain E0405`.