bug!() on const inference variable in opaque type

Add some tests to verify that this case is never hit

src/librustc_typeck/check/mod.rs

@@ -907,7 +907,15 @@ fn fixup_opaque_types<'tcx, T>(tcx: TyCtxt<'tcx>, val: &T) -> T where T: TypeFol
                                 },
                                 GenericArgKind::Const(old_const) => {
                                     if let ConstValue::Infer(_) = old_const.val {
-                                        self.tcx.mk_param_from_def(param)
+                        // This should never happen - we currently do not support
+                        // 'const projections', e.g.:
+                        // `impl<T: SomeTrait> MyTrait for T where <T as SomeTrait>::MyConst == 25`
+                        // which should be the only way for us to end up with a const inference
+                        // variable after projection. If Rust ever gains support for this kind
+                        // of projection, this should *probably* be changed to
+                        // `self.tcx.mk_param_from_def(param)`
+                                        bug!("Found infer const: `{:?}` in opaque type: {:?}",
+                                             old_const, ty);
                                     } else {
                                         old_param.fold_with(self)
                                     }

src/test/ui/type-alias-impl-trait/assoc-type-const.rs

@@ -0,0 +1,36 @@
+// Tests that we properly detect defining usages when using
+// const generics in an associated opaque type
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+trait UnwrapItemsExt<const C: usize> {
+    type Iter;
+    fn unwrap_items(self) -> Self::Iter;
+}
+
+struct MyStruct<const C: usize> {}
+
+trait MyTrait<'a, const C: usize> {
+    type MyItem;
+    const MY_CONST: usize;
+}
+
+impl<'a, const C: usize> MyTrait<'a, {C}> for MyStruct<{C}> {
+    type MyItem = u8;
+    const MY_CONST: usize = C;
+}
+
+impl<'a, I, const C: usize> UnwrapItemsExt<{C}> for I
+where
+{
+    type Iter = impl MyTrait<'a, {C}>;
+
+    fn unwrap_items(self) -> Self::Iter {
+        MyStruct::<{C}> {}
+    }
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/assoc-type-const.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/assoc-type-const.rs:6:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+

src/test/ui/type-alias-impl-trait/assoc-type-lifetime.rs

@@ -0,0 +1,28 @@
+// Tests that we still detect defining usages when
+// lifetimes are used in an associated opaque type
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+trait UnwrapItemsExt {
+    type Iter;
+    fn unwrap_items(self) -> Self::Iter;
+}
+
+struct MyStruct {}
+
+trait MyTrait<'a> {}
+
+impl<'a> MyTrait<'a> for MyStruct {}
+
+impl<'a, I> UnwrapItemsExt for I
+where
+{
+    type Iter = impl MyTrait<'a>;
+
+    fn unwrap_items(self) -> Self::Iter {
+        MyStruct {}
+    }
+}
+
+fn main() {}