Emit a proper error if bitflags enum is generic

This probably hasn't been hit by anyone but I noticed it in the AST,
so...

enumflags_derive/src/lib.rs

@@ -258,6 +258,11 @@ fn gen_enumflags(ast: &mut DeriveInput, default: Vec<Ident>) -> Result<TokenStre
         }
     };
 
+    if ast.generics.lt_token.is_some() || ast.generics.where_clause.is_some() {
+        return Err(syn::Error::new_spanned(&ast.generics,
+            "bitflags cannot be generic"));
+    }
+
     let repr = extract_repr(&ast.attrs)?
         .ok_or_else(|| syn::Error::new_spanned(ident,
                         "repr attribute missing. Add #[repr(u64)] or a similar attribute to specify the size of the bitfield."))?;

test_suite/ui/with_generics.rs

@@ -0,0 +1,7 @@
+#[enumflags2::bitflags]
+#[repr(u8)]
+enum Foo<A> {
+    Bar,
+}
+
+fn main() {}

test_suite/ui/with_generics.stderr

@@ -0,0 +1,14 @@
+error: bitflags cannot be generic
+ --> ui/with_generics.rs:3:9
+  |
+3 | enum Foo<A> {
+  |         ^^^
+
+error[E0392]: type parameter `A` is never used
+ --> ui/with_generics.rs:3:10
+  |
+3 | enum Foo<A> {
+  |          ^ unused type parameter
+  |
+  = help: consider removing `A`, referring to it in a field, or using a marker such as `PhantomData`
+  = help: if you intended `A` to be a const parameter, use `const A: /* Type */` instead