Make sure we deny unimplemented RTN on qpath segments

compiler/rustc_ast_lowering/messages.ftl

@@ -43,6 +43,8 @@ ast_lowering_bad_return_type_notation_output =
     return type not allowed with return type notation
     .suggestion = remove the return type
 
+ast_lowering_bad_return_type_notation_position = return type notation not allowed in this position yet
+
 ast_lowering_base_expression_double_dot =
     base expression required after `..`
     .suggestion = add a base expression here

compiler/rustc_ast_lowering/src/errors.rs

@@ -399,6 +399,11 @@ pub enum BadReturnTypeNotation {
         #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
         span: Span,
     },
+    #[diag(ast_lowering_bad_return_type_notation_position)]
+    Position {
+        #[primary_span]
+        span: Span,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_lowering/src/path.rs

@@ -1,7 +1,8 @@
 use crate::ImplTraitPosition;
 
 use super::errors::{
-    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, GenericTypeWithParentheses, UseAngleBrackets,
+    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, BadReturnTypeNotation,
+    GenericTypeWithParentheses, UseAngleBrackets,
 };
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
@@ -276,8 +277,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         )
                     }
                 },
-                GenericArgs::ParenthesizedElided(_span) => {
-                    todo!()
+                GenericArgs::ParenthesizedElided(span) => {
+                    self.dcx().emit_err(BadReturnTypeNotation::Position { span: *span });
+                    (
+                        GenericArgsCtor {
+                            args: Default::default(),
+                            constraints: &[],
+                            parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
+                            span: *span,
+                        },
+                        false,
+                    )
                 }
             }
         } else {

tests/ui/associated-type-bounds/return-type-notation/bare-path.rs

@@ -0,0 +1,26 @@
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Tr {
+    const CONST: usize;
+
+    fn method() -> impl Sized;
+}
+
+fn foo<T: Tr>()
+where
+    T::method(..): Send,
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected type, found function
+    <T as Tr>::method(..): Send,
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected associated type, found associated function `Tr::method`
+{
+    let _ = T::CONST::(..);
+    //~^ ERROR return type notation not allowed in this position yet
+    let _: T::method(..);
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected type, found function
+}
+
+fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bare-path.stderr

@@ -0,0 +1,66 @@
+error[E0575]: expected associated type, found associated function `Tr::method`
+  --> $DIR/bare-path.rs:15:5
+   |
+LL |     <T as Tr>::method(..): Send,
+   |     ^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/bare-path.rs:1:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:19:23
+   |
+LL |     let _ = T::CONST::(..);
+   |                       ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:21:21
+   |
+LL |     let _: T::method(..);
+   |                     ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:12:14
+   |
+LL |     T::method(..): Send,
+   |              ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:15:22
+   |
+LL |     <T as Tr>::method(..): Send,
+   |                      ^^^^
+
+error: expected type, found function
+  --> $DIR/bare-path.rs:12:8
+   |
+LL |     T::method(..): Send,
+   |        ^^^^^^ unexpected function
+   |
+note: the associated function is defined here
+  --> $DIR/bare-path.rs:7:5
+   |
+LL |     fn method() -> impl Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expected type, found function
+  --> $DIR/bare-path.rs:21:15
+   |
+LL |     let _: T::method(..);
+   |               ^^^^^^ unexpected function
+   |
+note: the associated function is defined here
+  --> $DIR/bare-path.rs:7:5
+   |
+LL |     fn method() -> impl Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0575`.