From 37f5cc22391e693523e8320a1d5c214610790911 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Mon, 2 Sep 2019 18:03:54 -0700
Subject: [PATCH] Do not complain about unconstrained params when Self is Ty
 Error

---
 .../constrained_generic_params.rs              | 18 ++++++++----------
 src/librustc_typeck/impl_wf_check.rs           |  4 ++++
 src/test/ui/issues/issue-36836.rs              |  5 +++++
 src/test/ui/issues/issue-36836.stderr          |  9 +++++++++
 4 files changed, 26 insertions(+), 10 deletions(-)
 create mode 100644 src/test/ui/issues/issue-36836.rs
 create mode 100644 src/test/ui/issues/issue-36836.stderr

diff --git a/src/librustc_typeck/constrained_generic_params.rs b/src/librustc_typeck/constrained_generic_params.rs
index 79a04b9423a8..c95f81506cd5 100644
--- a/src/librustc_typeck/constrained_generic_params.rs
+++ b/src/librustc_typeck/constrained_generic_params.rs
@@ -20,10 +20,10 @@ impl From<ty::ParamConst> for Parameter {
 }
 
 /// Returns the set of parameters constrained by the impl header.
-pub fn parameters_for_impl<'tcx>(impl_self_ty: Ty<'tcx>,
-                                 impl_trait_ref: Option<ty::TraitRef<'tcx>>)
-                                 -> FxHashSet<Parameter>
-{
+pub fn parameters_for_impl<'tcx>(
+    impl_self_ty: Ty<'tcx>,
+    impl_trait_ref: Option<ty::TraitRef<'tcx>>,
+) -> FxHashSet<Parameter> {
     let vec = match impl_trait_ref {
         Some(tr) => parameters_for(&tr, false),
         None => parameters_for(&impl_self_ty, false),
@@ -36,12 +36,10 @@ pub fn parameters_for_impl<'tcx>(impl_self_ty: Ty<'tcx>,
 /// uniquely determined by `t` (see RFC 447). If it is true, return the list
 /// of parameters whose values are needed in order to constrain `ty` - these
 /// differ, with the latter being a superset, in the presence of projections.
-pub fn parameters_for<'tcx, T>(t: &T,
-                               include_nonconstraining: bool)
-                               -> Vec<Parameter>
-    where T: TypeFoldable<'tcx>
-{
-
+pub fn parameters_for<'tcx, T: TypeFoldable<'tcx>>(
+    t: &T,
+    include_nonconstraining: bool,
+) -> Vec<Parameter> {
     let mut collector = ParameterCollector {
         parameters: vec![],
         include_nonconstraining,
diff --git a/src/librustc_typeck/impl_wf_check.rs b/src/librustc_typeck/impl_wf_check.rs
index fcfd9adef54d..8e69fbd9a79e 100644
--- a/src/librustc_typeck/impl_wf_check.rs
+++ b/src/librustc_typeck/impl_wf_check.rs
@@ -99,6 +99,10 @@ fn enforce_impl_params_are_constrained(
 ) {
     // Every lifetime used in an associated type must be constrained.
     let impl_self_ty = tcx.type_of(impl_def_id);
+    if impl_self_ty.sty == ty::Error {
+        // Don't complain about unconstrained type params when self ty doesn't exist. (#36836)
+        return;
+    }
     let impl_generics = tcx.generics_of(impl_def_id);
     let impl_predicates = tcx.predicates_of(impl_def_id);
     let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
diff --git a/src/test/ui/issues/issue-36836.rs b/src/test/ui/issues/issue-36836.rs
new file mode 100644
index 000000000000..56d5a7cca456
--- /dev/null
+++ b/src/test/ui/issues/issue-36836.rs
@@ -0,0 +1,5 @@
+trait Foo {}
+
+impl<T> Foo for Bar<T> {} //~ ERROR cannot find type `Bar` in this scope
+
+fn main() {}
diff --git a/src/test/ui/issues/issue-36836.stderr b/src/test/ui/issues/issue-36836.stderr
new file mode 100644
index 000000000000..bfda9b0bbdd7
--- /dev/null
+++ b/src/test/ui/issues/issue-36836.stderr
@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `Bar` in this scope
+  --> $DIR/issue-36836.rs:3:17
+   |
+LL | impl<T> Foo for Bar<T> {}
+   |                 ^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.