Inference

Author: BoxyUwU

src/test/ui/const-generics/defaults/doesnt_infer.rs

@@ -0,0 +1,15 @@
+#![feature(const_generics_defaults)]
+
+// test that defaulted const params are not used to help type inference
+
+struct Foo<const N: u32 = 2>;
+
+impl<const N: u32> Foo<N> {
+    fn foo() -> Self { loop {} }
+}
+
+fn main() {
+    let foo = Foo::<1>::foo();
+    let foo = Foo::foo();
+    //~^ error: type annotations needed for `Foo<{_: u32}>`
+}

src/test/ui/const-generics/defaults/doesnt_infer.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed for `Foo<{_: u32}>`
+  --> $DIR/doesnt_infer.rs:13:15
+   |
+LL |     let foo = Foo::foo();
+   |         ---   ^^^^^^^^ cannot infer the value of const parameter `N`
+   |         |
+   |         consider giving `foo` the explicit type `Foo<{_: u32}>`, where the type parameter `N` is specified
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.