Remove type parameter defaults

derive/src/lib.rs

@@ -74,18 +74,14 @@ fn generate(input: TokenStream2) -> Result<TokenStream2> {
 }
 
 fn generate_type(input: TokenStream2) -> Result<TokenStream2> {
-    let mut ast: DeriveInput = syn::parse2(input.clone())?;
+    let ast: DeriveInput = syn::parse2(input.clone())?;
 
     let scale_info = crate_name_ident("scale-info")?;
     let parity_scale_codec = crate_name_ident("parity-scale-codec")?;
 
     let ident = &ast.ident;
 
-    ast.generics
-        .lifetimes_mut()
-        .for_each(|l| *l = parse_quote!('static));
-
-    let (_, ty_generics, _) = ast.generics.split_for_impl();
+    let (impl_generics, ty_generics, _) = ast.generics.split_for_impl();
     let where_clause = trait_bounds::make_where_clause(
         ident,
         &ast.generics,
@@ -101,15 +97,13 @@ fn generate_type(input: TokenStream2) -> Result<TokenStream2> {
         }
     });
 
-    let ast: DeriveInput = syn::parse2(input.clone())?;
     let build_type = match &ast.data {
         Data::Struct(ref s) => generate_composite_type(s, &scale_info),
         Data::Enum(ref e) => generate_variant_type(e, &scale_info),
         Data::Union(_) => return Err(Error::new_spanned(input, "Unions not supported")),
     };
-    let generic_types = ast.generics.type_params();
     let type_info_impl = quote! {
-        impl <#( #generic_types ),*> :: #scale_info ::TypeInfo for #ident #ty_generics #where_clause {
+        impl #impl_generics :: #scale_info ::TypeInfo for #ident #ty_generics #where_clause {
             type Identity = Self;
             fn type_info() -> :: #scale_info ::Type {
                 :: #scale_info ::Type::builder()

derive/src/trait_bounds.rs

@@ -41,6 +41,11 @@ pub fn make_where_clause<'a>(
             predicates: Punctuated::new(),
         }
     });
+    for lifetime in generics.lifetimes() {
+        where_clause
+            .predicates
+            .push(parse_quote!(#lifetime: 'static))
+    }
 
     let type_params = generics.type_params();
     let ty_params_ids = type_params

test_suite/tests/derive.rs

@@ -280,6 +280,31 @@ fn scale_compact_types_work_in_enums() {
     assert_type!(MutilatedMultiAddress<u8, u16>, ty);
 }
 
+#[test]
+fn type_parameters_with_default_bound_works() {
+    trait Formy {
+        type Tip;
+    }
+    #[derive(TypeInfo)]
+    pub enum MetaFormy {}
+    impl Formy for MetaFormy {
+        type Tip = u8;
+    }
+
+    #[allow(unused)]
+    #[derive(TypeInfo)]
+    struct Bat<TTT: Formy = MetaFormy> {
+        one: TTT,
+    }
+
+    let ty = Type::builder()
+        .path(Path::new("Bat", "derive"))
+        .type_params(tuple_meta_type!(MetaFormy))
+        .composite(Fields::named().field_of::<MetaFormy>("one", "TTT"));
+
+    assert_type!(Bat<MetaFormy>, ty);
+}
+
 #[test]
 fn whitespace_scrubbing_works() {
     #[allow(unused)]