diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
index 92288c89827..9faa172a497 100644
--- a/compiler/rustc_middle/src/ty/flags.rs
+++ b/compiler/rustc_middle/src/ty/flags.rs
@@ -141,7 +141,9 @@ impl FlagComputation {
             &ty::Infer(infer) => {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
-                    ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {}
+                    ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
+                        self.add_flags(TypeFlags::HAS_TY_FRESH)
+                    }
 
                     ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => {
                         self.add_flags(TypeFlags::HAS_TY_INFER)
@@ -278,7 +280,7 @@ impl FlagComputation {
             ty::ConstKind::Infer(infer) => {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
-                    InferConst::Fresh(_) => {}
+                    InferConst::Fresh(_) => self.add_flags(TypeFlags::HAS_CT_FRESH),
                     InferConst::Var(_) => self.add_flags(TypeFlags::HAS_CT_INFER),
                 }
             }
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index 8fcdf813b41..0dbcd483c45 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -59,6 +59,15 @@ bitflags! {
                                           | TypeFlags::HAS_CT_INFER.bits
                                           | TypeFlags::HAS_TY_PLACEHOLDER.bits
                                           | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          // We consider 'freshened' types and constants
+                                          // to depend on a particular fn.
+                                          // The freshening process throws away information,
+                                          // which can make things unsuitable for use in a global
+                                          // cache. Note that there is no 'fresh lifetime' flag -
+                                          // freshening replaces all lifetimes with `ReErased`,
+                                          // which is different from how types/const are freshened.
+                                          | TypeFlags::HAS_TY_FRESH.bits
+                                          | TypeFlags::HAS_CT_FRESH.bits
                                           | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits;
 
         /// Does this have `Projection`?
@@ -90,6 +99,12 @@ bitflags! {
         /// Does this value have parameters/placeholders/inference variables which could be
         /// replaced later, in a way that would change the results of `impl` specialization?
         const STILL_FURTHER_SPECIALIZABLE = 1 << 17;
+
+        /// Does this value have `InferTy::FreshTy/FreshIntTy/FreshFloatTy`?
+        const HAS_TY_FRESH                = 1 << 18;
+
+        /// Does this value have `InferConst::Fresh`?
+        const HAS_CT_FRESH                = 1 << 19;
     }
 }
 
diff --git a/src/test/ui/traits/issue-84399-bad-fresh-caching.rs b/src/test/ui/traits/issue-84399-bad-fresh-caching.rs
new file mode 100644
index 00000000000..1494001564f
--- /dev/null
+++ b/src/test/ui/traits/issue-84399-bad-fresh-caching.rs
@@ -0,0 +1,55 @@
+// compile-flags: --crate-type lib
+// check-pass
+//
+// Regression test for issue #84399
+// Tests that we keep the full `ParamEnv` when
+// caching predicates with freshened types in the global cache
+
+use std::marker::PhantomData;
+pub trait Allocator<R> {
+    type Buffer;
+}
+pub struct DefaultAllocator;
+impl <R> Allocator<R> for DefaultAllocator {
+    type Buffer = ();
+}
+pub type Owned<R> = <DefaultAllocator as Allocator<R>>::Buffer;
+pub type MatrixMN<R> = Matrix<R, Owned<R>>;
+pub type Matrix4<N> = Matrix<N, ()>;
+pub struct Matrix<R, S> {
+    pub data: S,
+    _phantoms: PhantomData<R>,
+}
+pub fn set_object_transform(matrix: &Matrix4<()>) {
+    matrix.js_buffer_view();
+}
+pub trait Storable {
+    type Cell;
+    fn slice_to_items(_buffer: &()) -> &[Self::Cell] {
+        unimplemented!()
+    }
+}
+pub type Cell<T> = <T as Storable>::Cell;
+impl<R> Storable for MatrixMN<R>
+where
+    DefaultAllocator: Allocator<R>,
+{
+    type Cell = ();
+}
+pub trait JsBufferView {
+    fn js_buffer_view(&self) -> usize {
+        unimplemented!()
+    }
+}
+impl<R> JsBufferView for [MatrixMN<R>]
+where
+    DefaultAllocator: Allocator<R>,
+    MatrixMN<R>: Storable,
+    [Cell<MatrixMN<R>>]: JsBufferView,
+{
+    fn js_buffer_view(&self) -> usize {
+        <MatrixMN<R> as Storable>::slice_to_items(&()).js_buffer_view()
+    }
+}
+impl JsBufferView for [()] {}
+impl<R> JsBufferView for MatrixMN<R> where DefaultAllocator: Allocator<R> {}