Add test for current behaviour.

This commit adds a test for the current behaviour of signature deduction
of generators when there is a type mismatch between the return type of
the function body and the signature.

src/test/ui/generator/type-mismatch-signature-deduction.rs

@@ -0,0 +1,17 @@
+#![feature(generators, generator_trait)]
+
+use std::ops::Generator;
+
+fn foo() -> impl Generator<Return = i32> { //~ ERROR type mismatch
+    || {
+        if false {
+            return Ok(6);
+        }
+
+        yield ();
+
+        5 //~ ERROR mismatched types [E0308]
+    }
+}
+
+fn main() {}

src/test/ui/generator/type-mismatch-signature-deduction.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-signature-deduction.rs:13:9
+   |
+LL |         5
+   |         ^ expected enum `std::result::Result`, found integer
+   |
+   = note: expected type `std::result::Result<{integer}, _>`
+              found type `{integer}`
+
+error[E0271]: type mismatch resolving `<[generator@$DIR/type-mismatch-signature-deduction.rs:6:5: 14:6 _] as std::ops::Generator>::Return == i32`
+  --> $DIR/type-mismatch-signature-deduction.rs:5:13
+   |
+LL | fn foo() -> impl Generator<Return = i32> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found i32
+   |
+   = note: expected type `std::result::Result<{integer}, _>`
+              found type `i32`
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0271, E0308.
+For more information about an error, try `rustc --explain E0271`.