only use generic info when ty var belong it in orphan check

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -554,12 +554,12 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for TyVarReplacer<'cx, 'tcx> {
             return ty.super_fold_with(self);
         };
         let origin = self.infcx.type_var_origin(vid);
-        if let Some(def_id) = origin.param_def_id {
+        if let Some(def_id) = origin.param_def_id
+            && let Some(index) = self.generics.param_def_id_to_index.get(&def_id)
+        {
             // The generics of an `impl` don't have a parent, we can index directly.
-            let index = self.generics.param_def_id_to_index[&def_id];
-            let name = self.generics.own_params[index as usize].name;
-
-            Ty::new_param(self.infcx.tcx, index, name)
+            let name = self.generics.own_params[*index as usize].name;
+            Ty::new_param(self.infcx.tcx, *index, name)
         } else {
             ty
         }

tests/ui/coherence/impl-for-assoc-with-ty-var.rs

@@ -0,0 +1,21 @@
+//@ aux-build:coherence_lib.rs
+
+// issue#132826
+
+extern crate coherence_lib;
+
+use coherence_lib::{Pair, Remote, Remote1};
+
+trait MyTrait {
+    type Item;
+}
+
+impl<M> MyTrait for Pair<M, M> {
+    type Item = Pair<M, M>;
+}
+
+impl<K> Remote for <Pair<K, K> as MyTrait>::Item {}
+//~^ ERROR: the type parameter `K` is not constrained by the impl trait, self type, or predicates
+//~| ERROR: only traits defined in the current crate can be implemented for arbitrary types
+
+fn main() {}

tests/ui/coherence/impl-for-assoc-with-ty-var.stderr

@@ -0,0 +1,22 @@
+error[E0207]: the type parameter `K` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/impl-for-assoc-with-ty-var.rs:17:6
+   |
+LL | impl<K> Remote for <Pair<K, K> as MyTrait>::Item {}
+   |      ^ unconstrained type parameter
+
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/impl-for-assoc-with-ty-var.rs:17:1
+   |
+LL | impl<K> Remote for <Pair<K, K> as MyTrait>::Item {}
+   | ^^^^^^^^^^^^^^^^^^^-----------------------------
+   |                    |
+   |                    `Pair` is not defined in the current crate
+   |
+   = note: impl doesn't have any local type before any uncovered type parameters
+   = note: for more information see https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules
+   = note: define and implement a trait or new type instead
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0117, E0207.
+For more information about an error, try `rustc --explain E0117`.