Don't report a hard error if there are inference failures until

after we check casts, because sometimes casts can influence inference,
unfortunately. We do re-run `select_all_trait_obligations` during
regionck anyhow.

src/librustc_typeck/check/mod.rs

@@ -445,9 +445,8 @@ fn check_bare_fn<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
 
             fcx.select_all_obligations_and_apply_defaults();
             upvar::closure_analyze_fn(&fcx, fn_id, decl, body);
-            fcx.select_all_obligations_or_error();
+            fcx.select_obligations_where_possible();
             fcx.check_casts();
-
             fcx.select_all_obligations_or_error(); // Casts can introduce new obligations.
 
             regionck::regionck_fn(&fcx, fn_id, fn_span, decl, body);