Don't suggest assoc ty bound on non-angle-bracketed problematic assoc ty binding

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -447,17 +447,30 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     fn maybe_suggest_assoc_ty_bound(&self, self_ty: &hir::Ty<'_>, diag: &mut Diag<'_>) {
         let mut parents = self.tcx().hir_parent_iter(self_ty.hir_id);
 
-        if let Some((_, hir::Node::AssocItemConstraint(constraint))) = parents.next()
+        if let Some((c_hir_id, hir::Node::AssocItemConstraint(constraint))) = parents.next()
             && let Some(obj_ty) = constraint.ty()
+            && let Some((_, hir::Node::TraitRef(trait_ref))) = parents.next()
         {
-            if let Some((_, hir::Node::TraitRef(..))) = parents.next()
-                && let Some((_, hir::Node::Ty(ty))) = parents.next()
+            if let Some((_, hir::Node::Ty(ty))) = parents.next()
                 && let hir::TyKind::TraitObject(..) = ty.kind
             {
                 // Assoc ty bounds aren't permitted inside trait object types.
                 return;
             }
 
+            if trait_ref
+                .path
+                .segments
+                .iter()
+                .find_map(|seg| {
+                    seg.args.filter(|args| args.constraints.iter().any(|c| c.hir_id == c_hir_id))
+                })
+                .is_none_or(|args| args.parenthesized != hir::GenericArgsParentheses::No)
+            {
+                // Only consider angle-bracketed args (where we have a `=` to replace with `:`).
+                return;
+            }
+
             let lo = if constraint.gen_args.span_ext.is_dummy() {
                 constraint.ident.span
             } else {

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.rs

@@ -1,4 +1,4 @@
-// Regression test for issue #105056.
+// issue: <https://github.com/rust-lang/rust/issues/105056>
 //@ edition: 2021
 
 fn f(_: impl Trait<T = Copy>) {}
@@ -23,4 +23,11 @@ type Obj = dyn Trait<T = Clone>;
 
 trait Trait { type T; }
 
+// Don't suggest assoc ty bounds when we have parenthesized args (the underlying assoc type
+// binding `Output` isn't introduced by `=` but by `->`, suggesting `:` wouldn't be valid).
+// issue: <https://github.com/rust-lang/rust/issues/140543>
+fn i(_: impl Fn() -> std::fmt::Debug) {}
+//~^ ERROR expected a type, found a trait
+//~| HELP you can add the `dyn` keyword if you want a trait object
+
 fn main() {}

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.stderr

@@ -57,6 +57,17 @@ help: you can add the `dyn` keyword if you want a trait object
 LL | type Obj = dyn Trait<T = dyn Clone>;
    |                          +++
 
-error: aborting due to 4 previous errors
+error[E0782]: expected a type, found a trait
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:29:22
+   |
+LL | fn i(_: impl Fn() -> std::fmt::Debug) {}
+   |                      ^^^^^^^^^^^^^^^
+   |
+help: you can add the `dyn` keyword if you want a trait object
+   |
+LL | fn i(_: impl Fn() -> dyn std::fmt::Debug) {}
+   |                      +++
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0782`.