test: Add test for issues 45510 and 18952.

Those tests are directly taken from the comments on the bug reports.

Author: cjgillot

src/test/run-pass/issue-18952.rs

@@ -0,0 +1,48 @@
+// This issue tests fn_traits overloading on arity.
+
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+
+struct Foo;
+
+impl Fn<(isize, isize)> for Foo {
+    extern "rust-call" fn call(&self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+
+impl FnMut<(isize, isize)> for Foo {
+    extern "rust-call" fn call_mut(&mut self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+
+impl FnOnce<(isize, isize)> for Foo {
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: (isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+
+impl Fn<(isize, isize, isize)> for Foo {
+    extern "rust-call" fn call(&self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+
+impl FnMut<(isize, isize, isize)> for Foo {
+    extern "rust-call" fn call_mut(&mut self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+impl FnOnce<(isize, isize, isize)> for Foo {
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: (isize, isize, isize)) -> Self::Output {
+        println!("{:?}", args);
+    }
+}
+
+fn main() {
+    let foo = Foo;
+    foo(1, 1);
+}

src/test/run-pass/issue-45510.rs

@@ -0,0 +1,27 @@
+// Test overloaded resolution of fn_traits.
+
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+
+struct Ishmael;
+struct Maybe;
+struct CallMe;
+
+impl FnOnce<(Ishmael,)> for CallMe {
+    type Output = ();
+    extern "rust-call" fn call_once(self, _args: (Ishmael,)) -> () {
+        println!("Split your lungs with blood and thunder!");
+    }
+}
+
+impl FnOnce<(Maybe,)> for CallMe {
+    type Output = ();
+    extern "rust-call" fn call_once(self, _args: (Maybe,)) -> () {
+        println!("So we just met, and this is crazy");
+    }
+}
+
+fn main() {
+    CallMe(Ishmael);
+    CallMe(Maybe);
+}