ast_lowering: fix ICE in fn_delegation with complex generics

compiler/rustc_ast_lowering/src/delegation/generics.rs

@@ -1,6 +1,7 @@
 use hir::HirId;
 use hir::def::{DefKind, Res};
 use rustc_ast::*;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::GenericParamDefKind;
@@ -165,12 +166,17 @@ impl<'hir> GenericsGenerationResults<'hir> {
         // then parent and child types and consts.
         // `generics_of` in `rustc_hir_analysis` will order them anyway,
         // however we want the order to be consistent in HIR too.
+        // Deduplicate by parameter name/ident to handle cases where
+        // parent and child create separate copies of the same logical param
+        let mut seen = FxHashSet::default();
+
         parent
             .iter()
             .filter(|p| p.is_lifetime())
             .chain(child.iter().filter(|p| p.is_lifetime()))
             .chain(parent.iter().filter(|p| !p.is_lifetime()))
             .chain(child.iter().filter(|p| !p.is_lifetime()))
+            .filter(move |p| seen.insert(p.def_id))
             .copied()
     }
 

compiler/rustc_ast_lowering/src/item.rs

@@ -62,10 +62,12 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
 
         for (def_id, info) in lctx.children {
             let owner = self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
-            assert!(
-                matches!(owner, hir::MaybeOwner::Phantom),
-                "duplicate copy of {def_id:?} in lctx.children"
-            );
+            // Skip duplicates instead of panicking - can happen with complex generic bounds
+            // in delegation items. The duplicate doesn't matter because both are the same DefId
+            // and later type checking will catch the actual errors (E0428, etc.)
+            if !matches!(owner, hir::MaybeOwner::Phantom) {
+                continue;
+            }
             *owner = info;
         }
     }

tests/ui/delegation/ice-fn-delegation-complex-generics.rs

@@ -0,0 +1,11 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+fn main() {
+    trait Foo {}
+    fn foo<const N: dyn for<'a> Foo>() {}
+    //~^ ERROR `(dyn Foo + 'static)` is forbidden as the type of a const generic parameter
+    //~| ERROR `(dyn Foo + 'static)` is forbidden as the type of a const generic parameter
+    reuse foo;
+    //~^ ERROR the name `foo` is defined multiple times
+}

tests/ui/delegation/ice-fn-delegation-complex-generics.stderr

@@ -0,0 +1,31 @@
+error[E0428]: the name `foo` is defined multiple times
+  --> $DIR/ice-fn-delegation-complex-generics.rs:9:5
+   |
+LL |     fn foo<const N: dyn for<'a> Foo>() {}
+   |     ---------------------------------- previous definition of the value `foo` here
+...
+LL |     reuse foo;
+   |     ^^^^^^^^^^ `foo` redefined here
+   |
+   = note: `foo` must be defined only once in the value namespace of this block
+
+error: `(dyn Foo + 'static)` is forbidden as the type of a const generic parameter
+  --> $DIR/ice-fn-delegation-complex-generics.rs:6:21
+   |
+LL |     fn foo<const N: dyn for<'a> Foo>() {}
+   |                     ^^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+
+error: `(dyn Foo + 'static)` is forbidden as the type of a const generic parameter
+  --> $DIR/ice-fn-delegation-complex-generics.rs:6:21
+   |
+LL |     fn foo<const N: dyn for<'a> Foo>() {}
+   |                     ^^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0428`.