diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index a441a635c1e..e00c5789fe9 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -647,6 +647,22 @@ pub struct WhereBoundPredicate<'hir> {
     pub bounds: GenericBounds<'hir>,
 }
 
+impl WhereBoundPredicate<'hir> {
+    /// Returns `true` if `param_def_id` matches the `bounded_ty` of this predicate.
+    pub fn is_param_bound(&self, param_def_id: DefId) -> bool {
+        let path = match self.bounded_ty.kind {
+            TyKind::Path(QPath::Resolved(None, path)) => path,
+            _ => return false,
+        };
+        match path.res {
+            Res::Def(DefKind::TyParam, def_id) | Res::SelfTy(Some(def_id), None) => {
+                def_id == param_def_id
+            }
+            _ => false,
+        }
+    }
+}
+
 /// A lifetime predicate (e.g., `'a: 'b + 'c`).
 #[derive(Debug, HashStable_Generic)]
 pub struct WhereRegionPredicate<'hir> {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 79194718d1e..f8df0e25959 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -2009,6 +2009,19 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
             Some(param) => param,
             _ => return,
         };
+        let param_def_id = self.tcx.hir().local_def_id(param.hir_id).to_def_id();
+        let preds = generics.where_clause.predicates.iter();
+        let explicitly_sized = preds
+            .filter_map(|pred| match pred {
+                hir::WherePredicate::BoundPredicate(bp) => Some(bp),
+                _ => None,
+            })
+            .filter(|bp| bp.is_param_bound(param_def_id))
+            .flat_map(|bp| bp.bounds)
+            .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
+        if explicitly_sized {
+            return;
+        }
         debug!("maybe_suggest_unsized_generics: param={:?}", param);
         match node {
             hir::Node::Item(
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 2274db76c05..2f427305782 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -28,7 +28,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
-use rustc_hir::def::{CtorKind, DefKind, Res};
+use rustc_hir::def::{CtorKind, DefKind};
 use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::weak_lang_items;
@@ -668,6 +668,7 @@ impl ItemCtxt<'tcx> {
             })
             .flat_map(|b| predicates_from_bound(self, ty, b));
 
+        let param_def_id = self.tcx.hir().local_def_id(param_id).to_def_id();
         let from_where_clauses = ast_generics
             .where_clause
             .predicates
@@ -677,7 +678,7 @@ impl ItemCtxt<'tcx> {
                 _ => None,
             })
             .flat_map(|bp| {
-                let bt = if is_param(self.tcx, bp.bounded_ty, param_id) {
+                let bt = if bp.is_param_bound(param_def_id) {
                     Some(ty)
                 } else if !only_self_bounds.0 {
                     Some(self.to_ty(bp.bounded_ty))
@@ -714,23 +715,6 @@ impl ItemCtxt<'tcx> {
     }
 }
 
-/// Tests whether this is the AST for a reference to the type
-/// parameter with ID `param_id`. We use this so as to avoid running
-/// `ast_ty_to_ty`, because we want to avoid triggering an all-out
-/// conversion of the type to avoid inducing unnecessary cycles.
-fn is_param(tcx: TyCtxt<'_>, ast_ty: &hir::Ty<'_>, param_id: hir::HirId) -> bool {
-    if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = ast_ty.kind {
-        match path.res {
-            Res::SelfTy(Some(def_id), None) | Res::Def(DefKind::TyParam, def_id) => {
-                def_id == tcx.hir().local_def_id(param_id).to_def_id()
-            }
-            _ => false,
-        }
-    } else {
-        false
-    }
-}
-
 fn convert_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
     let it = tcx.hir().item(item_id);
     debug!("convert: item {} with id {}", it.ident, it.hir_id());
diff --git a/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.rs b/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.rs
new file mode 100644
index 00000000000..5cfaf4be96a
--- /dev/null
+++ b/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.rs
@@ -0,0 +1,8 @@
+// Regression test for #85945: Don't suggest `?Sized` bound if an explicit
+// `Sized` bound is already in a `where` clause.
+fn foo<T>(_: &T) where T: Sized {}
+fn bar() { foo(""); }
+//~^ERROR the size for values of type
+
+pub fn main() {
+}
diff --git a/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.stderr b/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.stderr
new file mode 100644
index 00000000000..92be9f764cc
--- /dev/null
+++ b/src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.stderr
@@ -0,0 +1,18 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/issue-85945-check-where-clause-before-suggesting-unsized.rs:4:16
+   |
+LL | fn bar() { foo(""); }
+   |            --- ^^ doesn't have a size known at compile-time
+   |            |
+   |            required by a bound introduced by this call
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by a bound in `foo`
+  --> $DIR/issue-85945-check-where-clause-before-suggesting-unsized.rs:3:8
+   |
+LL | fn foo<T>(_: &T) where T: Sized {}
+   |        ^ required by this bound in `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.