Fix #91489

compiler/rustc_middle/src/ty/mod.rs

@@ -1319,6 +1319,11 @@ impl<'tcx> ParamEnv<'tcx> {
         self
     }
 
+    pub fn without_const(mut self) -> Self {
+        self.packed.set_tag(ParamTag { constness: hir::Constness::NotConst, ..self.packed.tag() });
+        self
+    }
+
     /// Returns a new parameter environment with the same clauses, but
     /// which "reveals" the true results of projections in all cases
     /// (even for associated types that are specializable). This is

compiler/rustc_typeck/src/check/closure.rs

@@ -80,7 +80,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let generator_types = check_fn(
             self,
-            self.param_env,
+            self.param_env.without_const(),
             liberated_sig,
             decl,
             expr.hir_id,

src/test/ui/rfc-2632-const-trait-impl/non-const-op-in-closure-in-const.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(const_trait_impl)]
+#![feature(const_fn_trait_bound)]
+
+trait Convert<T> {
+    fn to(self) -> T;
+}
+
+impl<A, B> const Convert<B> for A where B: ~const From<A> {
+    fn to(self) -> B {
+        B::from(self)
+    }
+}
+
+const FOO: fn() -> String = || "foo".to();
+
+fn main() {}