Apply defaults if available to type substitutions

When we get defaults specified to Type parameters we must resolve the type
before insert the name into the name scope otherwise we can end up with
forward declared type parameters being resolved.

When we need to apply defaults during substitution we must check if they
themseleves need substitutions also with the mappings we have generated
so far.

Fixes #307

Author: philberty

gcc/rust/resolve/rust-ast-resolve-type.h

@@ -262,6 +262,12 @@ class ResolveGenericParam : public ResolverBase
   {
     ok = true;
 
+    // if it has a type lets resolve it
+    if (param.has_type ())
+      {
+	ResolveType::go (param.get_type ().get (), param.get_node_id ());
+      }
+
     // for now lets focus on handling the basics: like struct<T> { a:T, ....}
     resolver->get_type_scope ().insert (
       CanonicalPath (param.get_type_representation ()), param.get_node_id (),
@@ -271,12 +277,6 @@ class ResolveGenericParam : public ResolverBase
 		       "generic param redefined multiple times");
 	rust_error_at (locus, "was defined here");
       });
-
-    // if it has a type lets resolve it
-    if (param.has_type ())
-      {
-	ResolveType::go (param.get_type ().get (), param.get_node_id ());
-      }
   }
 
 private:

gcc/rust/typecheck/rust-tyty.cc

@@ -271,6 +271,20 @@ SubstitutionRef::get_mappings_from_generic_args (HIR::GenericArgs &args)
 	  rust_assert (param.param_has_default_ty ());
 
 	  BaseType *resolved = param.get_default_ty ();
+	  if (resolved->get_kind () == TypeKind::ERROR)
+	    return SubstitutionArgumentMappings::error ();
+
+	  // this resolved default might already contain default parameters
+	  if (resolved->contains_type_parameters ())
+	    {
+	      SubstitutionArgumentMappings intermediate (mappings,
+							 args.get_locus ());
+	      resolved = Resolver::SubstMapperInternal::Resolve (resolved,
+								 intermediate);
+	      if (resolved->get_kind () == TypeKind::ERROR)
+		return SubstitutionArgumentMappings::error ();
+	    }
+
 	  SubstitutionArg subst_arg (&param, resolved);
 	  mappings.push_back (std::move (subst_arg));
 	}

gcc/testsuite/rust.test/compile/generics25.rs

@@ -0,0 +1,9 @@
+struct Foo<A, B = (A, A)>(A, B);
+
+fn main() {
+    let a: Foo<bool>;
+    a = Foo::<bool>(true, (false, true));
+
+    let b: (bool, bool);
+    b = a.1;
+}

gcc/testsuite/rust.test/xfail_compile/generics9.rs

@@ -0,0 +1,11 @@
+// { dg-excess-errors "Noisy error and debug" }
+struct Foo<A, B = (A, B)>(A, B);
+// { dg-error "failed to resolve TypePath: B" "" { target { *-*-* } } .-1 }
+
+fn main() {
+    let a: Foo<bool>;
+    a = Foo::<bool>(true, (false, true));
+
+    let b: (bool, bool);
+    b = a.1;
+}