Do not ICE on assoc type with bad placeholder

src/librustc_typeck/collect.rs

@@ -730,7 +730,13 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::HirId) {
             placeholder_type_error(tcx, None, &[], visitor.0, false);
         }
 
-        hir::TraitItemKind::Type(_, None) => {}
+        hir::TraitItemKind::Type(_, None) => {
+            // #74612: Visit and try to find bad placeholders
+            // even if there is no concrete type.
+            let mut visitor = PlaceholderHirTyCollector::default();
+            visitor.visit_trait_item(trait_item);
+            placeholder_type_error(tcx, None, &[], visitor.0, false);
+        }
     };
 
     tcx.ensure().predicates_of(def_id);

src/test/ui/typeck/typeck_type_placeholder_item.rs

@@ -194,6 +194,8 @@ trait Qux {
     const D: _ = 42;
     //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
     // type E: _; // FIXME: make the parser propagate the existence of `B`
+    type F: std::ops::Fn(_);
+    //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
 }
 impl Qux for Struct {
     type A = _;

src/test/ui/typeck/typeck_type_placeholder_item.stderr

@@ -29,7 +29,7 @@ LL | struct BadStruct2<_, T>(_, T);
    |                   ^ expected identifier, found reserved identifier
 
 error: associated constant in `impl` without body
-  --> $DIR/typeck_type_placeholder_item.rs:203:5
+  --> $DIR/typeck_type_placeholder_item.rs:205:5
    |
 LL |     const C: _;
    |     ^^^^^^^^^^-
@@ -545,6 +545,12 @@ LL |     const D: _ = 42;
    |              not allowed in type signatures
    |              help: replace `_` with the correct type: `i32`
 
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/typeck_type_placeholder_item.rs:197:26
+   |
+LL |     type F: std::ops::Fn(_);
+   |                          ^ not allowed in type signatures
+
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item.rs:40:24
    |
@@ -582,33 +588,33 @@ LL |         fn clone(&self) -> _ { FnTest9 }
    |                            help: replace with the correct return type: `main::FnTest9`
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:199:14
+  --> $DIR/typeck_type_placeholder_item.rs:201:14
    |
 LL |     type A = _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:201:14
+  --> $DIR/typeck_type_placeholder_item.rs:203:14
    |
 LL |     type B = _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:203:14
+  --> $DIR/typeck_type_placeholder_item.rs:205:14
    |
 LL |     const C: _;
    |              ^ not allowed in type signatures
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/typeck_type_placeholder_item.rs:206:14
+  --> $DIR/typeck_type_placeholder_item.rs:208:14
    |
 LL |     const D: _ = 42;
    |              ^
    |              |
    |              not allowed in type signatures
    |              help: replace `_` with the correct type: `i32`
 
-error: aborting due to 66 previous errors
+error: aborting due to 67 previous errors
 
 Some errors have detailed explanations: E0121, E0282, E0403.
 For more information about an error, try `rustc --explain E0121`.