Do not ICE on constant evaluation failure in GVN.

compiler/rustc_middle/src/mir/consts.rs

@@ -520,11 +520,13 @@ impl<'tcx> Const<'tcx> {
                 // types are fine though.
                 ty::ConstKind::Value(_) => c.ty().is_primitive(),
                 ty::ConstKind::Unevaluated(..) | ty::ConstKind::Expr(..) => false,
+                // This can happen if evaluation of a constant failed. The result does not matter
+                // much since compilation is doomed.
+                ty::ConstKind::Error(..) => false,
                 // Should not appear in runtime MIR.
                 ty::ConstKind::Infer(..)
                 | ty::ConstKind::Bound(..)
-                | ty::ConstKind::Placeholder(..)
-                | ty::ConstKind::Error(..) => bug!(),
+                | ty::ConstKind::Placeholder(..) => bug!(),
             },
             Const::Unevaluated(..) => false,
             // If the same slice appears twice in the MIR, we cannot guarantee that we will

tests/ui/consts/const-eval/issue-50814-2.mir-opt.stderr

@@ -0,0 +1,21 @@
+error[E0080]: evaluation of `<A<()> as Foo<()>>::BAR` failed
+  --> $DIR/issue-50814-2.rs:16:24
+   |
+LL |     const BAR: usize = [5, 6, 7][T::BOO];
+   |                        ^^^^^^^^^^^^^^^^^ index out of bounds: the length is 3 but the index is 42
+
+note: erroneous constant encountered
+  --> $DIR/issue-50814-2.rs:20:6
+   |
+LL |     &<A<T> as Foo<T>>::BAR
+   |      ^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/issue-50814-2.rs:20:5
+   |
+LL |     &<A<T> as Foo<T>>::BAR
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/const-eval/issue-50814-2.stderr → tests/ui/consts/const-eval/issue-50814-2.normal.stderr

@@ -1,17 +1,17 @@
 error[E0080]: evaluation of `<A<()> as Foo<()>>::BAR` failed
-  --> $DIR/issue-50814-2.rs:14:24
+  --> $DIR/issue-50814-2.rs:16:24
    |
 LL |     const BAR: usize = [5, 6, 7][T::BOO];
    |                        ^^^^^^^^^^^^^^^^^ index out of bounds: the length is 3 but the index is 42
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:18:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^
 
 note: the above error was encountered while instantiating `fn foo::<()>`
-  --> $DIR/issue-50814-2.rs:30:22
+  --> $DIR/issue-50814-2.rs:32:22
    |
 LL |     println!("{:x}", foo::<()>() as *const usize as usize);
    |                      ^^^^^^^^^^^

tests/ui/consts/const-eval/issue-50814-2.rs

@@ -1,4 +1,6 @@
 // build-fail
+// revisions: normal mir-opt
+// [mir-opt]compile-flags: -Zmir-opt-level=4
 
 trait C {
     const BOO: usize;