Add a failing case to tests/ui/macros/macro-interpolation.

This test currently tests the successful paths for the
`Interpolated`/`NtTy`/`Path` case in `parse_path_inner`, but it doesn't
test the failure path.

Author: nnethercote

tests/ui/macros/macro-interpolation.rs

@@ -1,5 +1,3 @@
-// run-pass
-
 macro_rules! overly_complicated {
     ($fnname:ident, $arg:ident, $ty:ty, $body:block, $val:expr, $pat:pat, $res:path) =>
     ({
@@ -21,12 +19,14 @@ macro_rules! qpath {
 
     (ty, <$type:ty as $trait:ty>::$name:ident) => {
         <$type as $trait>::$name
+        //~^ ERROR expected identifier, found `!`
     };
 }
 
 pub fn main() {
     let _: qpath!(path, <str as ToOwned>::Owned);
     let _: qpath!(ty, <str as ToOwned>::Owned);
+    let _: qpath!(ty, <str as !>::Owned);
 
     assert!(overly_complicated!(f, x, Option<usize>, { return Some(x); },
                                Some(8), Some(y), y) == 8)

tests/ui/macros/macro-interpolation.stderr

@@ -0,0 +1,16 @@
+error: expected identifier, found `!`
+  --> $DIR/macro-interpolation.rs:21:19
+   |
+LL |         <$type as $trait>::$name
+   |                   ^^^^^^ expected identifier
+...
+LL |     let _: qpath!(ty, <str as !>::Owned);
+   |            -----------------------------
+   |            |
+   |            this macro call doesn't expand to a type
+   |            in this macro invocation
+   |
+   = note: this error originates in the macro `qpath` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+