Fix error message on type mismatch in generator

Instead of "closure is expected to take 0 arguments"
we now get the expected type mismatch error.

Author: jonas-schievink

src/librustc_typeck/check/closure.rs

@@ -265,8 +265,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 _ => return None,
             }
         } else {
-            // Generators cannot have explicit arguments.
-            vec![]
+            // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
+            // else they must have exactly 1 argument. For now though, just give up in this case.
+            return None;
         };
 
         let ret_param_ty = projection.skip_binder().ty;

src/test/ui/generator/type-mismatch-error.rs

@@ -0,0 +1,22 @@
+//! Test that we get the expected type mismatch error instead of "closure is expected to take 0
+//! arguments" (which got introduced after implementing resume arguments).
+
+#![feature(generators, generator_trait)]
+
+use std::ops::Generator;
+
+fn f<G: Generator>(_: G, _: G::Return) {}
+
+fn main() {
+    f(
+        |a: u8| {
+            if false {
+                yield ();
+            } else {
+                a
+                //~^ error: `if` and `else` have incompatible types
+            }
+        },
+        0u8,
+    );
+}

src/test/ui/generator/type-mismatch-error.stderr

@@ -0,0 +1,19 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/type-mismatch-error.rs:16:17
+   |
+LL | /             if false {
+LL | |                 yield ();
+   | |                 ---------
+   | |                 |       |
+   | |                 |       help: consider removing this semicolon
+   | |                 expected because of this
+LL | |             } else {
+LL | |                 a
+   | |                 ^ expected `()`, found `u8`
+LL | |
+LL | |             }
+   | |_____________- `if` and `else` have incompatible types
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.