Keep the data of TK_TYPEPARAM pointing to the original definition.

iftype is implemented by shadowing the type parameter with a new definition
and a new constraint. Storing the pointer of the original definition
allows us to compare identity of type parameters easily.

This replaces the more complicated subtyping-based check to determine
whether a type parameter should be reified.

src/libponyc/pass/scope.c

@@ -193,7 +193,8 @@ static ast_t* make_iftype_typeparam(pass_opt_t* opt, ast_t* subtype,
       TREE(new_constraint)
       NONE));
 
-  ast_setdata(typeparam, typeparam);
+  // keep data pointing to the original def
+  ast_setdata(typeparam, ast_data(def));
 
   return typeparam;
 }

src/libponyc/type/reify.c

@@ -6,31 +6,25 @@
 #include "../ast/token.h"
 #include "ponyassert.h"
 
-static void reify_typeparamref(pass_opt_t* opt, ast_t** astp, ast_t* typeparam, ast_t* typearg)
+static void reify_typeparamref(ast_t** astp, ast_t* typeparam, ast_t* typearg)
 {
   ast_t* ast = *astp;
   pony_assert(ast_id(ast) == TK_TYPEPARAMREF);
   pony_assert(ast_id(typeparam) == TK_TYPEPARAM);
 
   ast_t* ref_def = (ast_t*)ast_data(ast);
-  ast_t* param_def = (ast_t*)ast_data(typeparam);
+
+  // We can't compare ref_def and typeparam, as they could be a copy or
+  // a iftype shadowing. However, their data points back to the original
+  // typeparam definition, which can be compared.
+  ref_def = (ast_t*)ast_data(ref_def);
+  typeparam = (ast_t*)ast_data(typeparam);
 
   pony_assert(ref_def != NULL);
-  pony_assert(param_def != NULL);
-  AST_GET_CHILDREN(ref_def, ref_name, ref_constraint);
-  AST_GET_CHILDREN(param_def, param_name, param_constraint);
+  pony_assert(typeparam != NULL);
 
-  if(ref_def != param_def)
-  {
-    if(ast_name(ref_name) == ast_name(param_name))
-    {
-      if((ast_id(param_constraint) != TK_TYPEPARAMREF) &&
-        !is_subtype(ref_constraint, param_constraint, NULL, opt))
-        return;
-    } else {
-      return;
-    }
-  }
+  if(ref_def != typeparam)
+    return;
 
   // Keep ephemerality.
   switch(ast_id(ast_childidx(ast, 2)))
@@ -117,7 +111,7 @@ static void reify_one(pass_opt_t* opt, ast_t** astp, ast_t* typeparam, ast_t* ty
   switch(ast_id(ast))
   {
     case TK_TYPEPARAMREF:
-      reify_typeparamref(opt, astp, typeparam, typearg);
+      reify_typeparamref(astp, typeparam, typearg);
       break;
 
     case TK_ARROW:

test/libponyc/iftype.cc

@@ -273,3 +273,24 @@ TEST_F(IftypeTest, UnconstrainedTypeparam)
 
   TEST_COMPILE(src);
 }
+
+TEST_F(IftypeTest, NestedCond)
+{
+  const char* src =
+    "trait T\n"
+    "class C is T\n"
+    "  fun tag c() => None\n"
+
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    foo[C](C)\n"
+
+    "  fun foo[A](x: A) =>\n"
+    "    iftype A <: T then\n"
+    "      iftype A <: C then\n"
+    "        x.c()\n"
+    "      end\n"
+    "    end";
+
+  TEST_COMPILE(src);
+}