Add test for forward declared const param defaults

compiler/rustc_resolve/src/diagnostics.rs

@@ -450,7 +450,7 @@ impl<'a> Resolver<'a> {
                 err.span_label(shadowed_binding_span, msg);
                 err
             }
-            ResolutionError::ForwardDeclaredTyParam => {
+            ResolutionError::ForwardDeclaredGenericParam => {
                 let mut err = struct_span_err!(
                     self.session,
                     span,

compiler/rustc_resolve/src/lib.rs

@@ -242,7 +242,7 @@ enum ResolutionError<'a> {
         shadowed_binding_span: Span,
     },
     /// Error E0128: generic parameters with a default cannot use forward-declared identifiers.
-    ForwardDeclaredTyParam, // FIXME(const_generics_defaults)
+    ForwardDeclaredGenericParam,
     /// ERROR E0770: the type of const parameters must not depend on other generic parameters.
     ParamInTyOfConstParam(Symbol),
     /// generic parameters must not be used inside const evaluations.
@@ -2617,7 +2617,7 @@ impl<'a> Resolver<'a> {
                 let res_error = if rib_ident.name == kw::SelfUpper {
                     ResolutionError::SelfInTyParamDefault
                 } else {
-                    ResolutionError::ForwardDeclaredTyParam
+                    ResolutionError::ForwardDeclaredGenericParam
                 };
                 self.report_error(span, res_error);
             }

src/test/ui/const-generics/defaults/forward-declared.rs

@@ -0,0 +1,15 @@
+#![feature(const_generics_defaults)]
+
+struct Foo<const N: usize = M, const M: usize = 10>;
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
+
+enum Bar<const N: usize = M, const M: usize = 10> {}
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
+
+struct Foo2<const N: usize = N>;
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
+
+enum Bar2<const N: usize = N> {}
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
+
+fn main() {}

src/test/ui/const-generics/defaults/forward-declared.stderr

@@ -0,0 +1,27 @@
+error[E0128]: generic parameters with a default cannot use forward declared identifiers
+  --> $DIR/forward-declared.rs:3:29
+   |
+LL | struct Foo<const N: usize = M, const M: usize = 10>;
+   |                             ^ defaulted generic parameters cannot be forward declared
+
+error[E0128]: generic parameters with a default cannot use forward declared identifiers
+  --> $DIR/forward-declared.rs:6:27
+   |
+LL | enum Bar<const N: usize = M, const M: usize = 10> {}
+   |                           ^ defaulted generic parameters cannot be forward declared
+
+error[E0128]: generic parameters with a default cannot use forward declared identifiers
+  --> $DIR/forward-declared.rs:9:30
+   |
+LL | struct Foo2<const N: usize = N>;
+   |                              ^ defaulted generic parameters cannot be forward declared
+
+error[E0128]: generic parameters with a default cannot use forward declared identifiers
+  --> $DIR/forward-declared.rs:12:28
+   |
+LL | enum Bar2<const N: usize = N> {}
+   |                            ^ defaulted generic parameters cannot be forward declared
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0128`.