Deny parenthetical notation for negative bounds

compiler/rustc_ast_passes/messages.ftl

@@ -188,6 +188,9 @@ ast_passes_module_nonascii = trying to load file for module `{$name}` with non-a
 ast_passes_negative_bound_not_supported =
     negative bounds are not supported
 
+ast_passes_negative_bound_with_parenthetical_notation =
+    parenthetical notation may not be used for negative bounds
+
 ast_passes_nested_impl_trait = nested `impl Trait` is not allowed
     .outer = outer `impl Trait`
     .inner = nested `impl Trait` here

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1257,13 +1257,24 @@ fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
         if let GenericBound::Trait(trait_ref, modifiers) = bound
             && let BoundPolarity::Negative(_) = modifiers.polarity
             && let Some(segment) = trait_ref.trait_ref.path.segments.last()
-            && let Some(ast::GenericArgs::AngleBracketed(args)) = segment.args.as_deref()
         {
-            for arg in &args.args {
-                if let ast::AngleBracketedArg::Constraint(constraint) = arg {
-                    self.dcx()
-                        .emit_err(errors::ConstraintOnNegativeBound { span: constraint.span });
+            match segment.args.as_deref() {
+                Some(ast::GenericArgs::AngleBracketed(args)) => {
+                    for arg in &args.args {
+                        if let ast::AngleBracketedArg::Constraint(constraint) = arg {
+                            self.dcx().emit_err(errors::ConstraintOnNegativeBound {
+                                span: constraint.span,
+                            });
+                        }
+                    }
                 }
+                // The lowered form of parenthesized generic args contains a type binding.
+                Some(ast::GenericArgs::Parenthesized(args)) => {
+                    self.dcx().emit_err(errors::NegativeBoundWithParentheticalNotation {
+                        span: args.span,
+                    });
+                }
+                None => {}
             }
         }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -745,6 +745,13 @@ pub struct ConstraintOnNegativeBound {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_negative_bound_with_parenthetical_notation)]
+pub struct NegativeBoundWithParentheticalNotation {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_invalid_unnamed_field_ty)]
 pub struct InvalidUnnamedFieldTy {

tests/ui/traits/negative-bounds/associated-constraints.rs

@@ -16,4 +16,7 @@ fn test3<T: !Trait<Assoc: Send>>() {}
 fn test4<T>() where T: !Trait<Assoc: Send> {}
 //~^ ERROR associated type constraints not allowed on negative bounds
 
+fn test5<T>() where T: !Fn() -> i32 {}
+//~^ ERROR parenthetical notation may not be used for negative bounds
+
 fn main() {}

tests/ui/traits/negative-bounds/associated-constraints.stderr

@@ -22,4 +22,11 @@ error: associated type constraints not allowed on negative bounds
 LL | fn test4<T>() where T: !Trait<Assoc: Send> {}
    |                               ^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: parenthetical notation may not be used for negative bounds
+  --> $DIR/associated-constraints.rs:19:25
+   |
+LL | fn test5<T>() where T: !Fn() -> i32 {}
+   |                         ^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
+