Split const trait method test and impl ops::Add

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs

@@ -19,7 +19,7 @@ impl Plus for u32 {
 }
 
 pub const fn add_i32(a: i32, b: i32) -> i32 {
-    a.plus(b)
+    a.plus(b) // ok
 }
 
 pub const fn add_u32(a: u32, b: u32) -> u32 {

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr

@@ -1,5 +1,5 @@
 error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/call-const-trait-method.rs:26:5
+  --> $DIR/call-const-trait-method-fail.rs:26:5
    |
 LL |     a.plus(b)
    |     ^^^^^^^^^

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-pass.rs

@@ -0,0 +1,41 @@
+// run-pass
+
+#![allow(incomplete_features)]
+#![feature(const_trait_impl)]
+#![feature(const_fn)]
+
+struct Int(i32);
+
+impl const std::ops::Add for Int {
+    type Output = Int;
+
+    fn add(self, rhs: Self) -> Self {
+        Int(self.0.plus(rhs.0))
+    }
+}
+
+impl const PartialEq for Int {
+    fn eq(&self, rhs: &Self) -> bool {
+        self.0 == rhs.0
+    }
+}
+
+pub trait Plus {
+    fn plus(self, rhs: Self) -> Self;
+}
+
+impl const Plus for i32 {
+    fn plus(self, rhs: Self) -> Self {
+        self + rhs
+    }
+}
+
+pub const fn add_i32(a: i32, b: i32) -> i32 {
+    a.plus(b)
+}
+
+const ADD_INT: Int = Int(1i32) + Int(2i32);
+
+fn main() {
+    assert!(ADD_INT == Int(3i32));
+}