Fix compiler crash related to type parameter references

This fixes a bug in rescoping. Rescoping happens after a method was
copied from a trait/interface to another type. It starts with a fresh
scope, visits all of the method's AST nodes and adds the visited symbols
to the method's symbol table. This includes type parameters. If a type
parameter reference is visited, the referenced type parameter is looked
up and added as data to the node.

As rescoping starts with a fresh scope, adding the type parameter to the
reference fails if the type parameter itself is not defined yet. This is
fixed by eagerly adding type parameters to the scope before visiting
type parameter references.

Fixes #3285

.release-notes/fix-param-type-reference-crash.md

@@ -0,0 +1,5 @@
+## Fix compiler crash related to type parameter references
+
+Previously, if a method signature in a trait or interface referenced a type
+parameter before the type parameter itself was defined, the compiler would
+crash. This is now fixed.

src/libponyc/pass/traits.c

@@ -417,7 +417,6 @@ static ast_result_t rescope(ast_t** astp, pass_opt_t* options)
     case TK_EMBED:
     case TK_PARAM:
     case TK_MATCH_CAPTURE:
-    case TK_TYPEPARAM:
     {
       pony_assert(ast_child(ast) != NULL);
       ast_set(ast, ast_name(ast_child(ast)), ast, SYM_DEFINED, true);
@@ -432,6 +431,19 @@ static ast_result_t rescope(ast_t** astp, pass_opt_t* options)
       break;
     }
 
+    case TK_TYPEPARAMS:
+    {
+      ast_t* typeparam = ast_child(ast);
+      while(typeparam != NULL)
+      {
+        pony_assert(ast_child(typeparam) != NULL);
+        ast_set(ast, ast_name(ast_child(typeparam)), typeparam, SYM_DEFINED, true);
+
+        typeparam = ast_sibling(typeparam);
+      }
+      break;
+    }
+
     case TK_TYPEPARAMREF:
     {
       pony_assert(ast_child(ast) != NULL);

test/libponyc/traits.cc

@@ -722,3 +722,15 @@ TEST_F(TraitsTest, TypeargInFunction2)
   // Tests typearg reification in methods inherited from parameterised traits.
   DO(test_compile(src, "expr"));
 }
+
+
+TEST_F(TraitsTest, TypeParamRefBeforeTypeParam)
+{
+  const char* src =
+    "trait T\n"
+    "  fun f[X: Y, Y: U32]() => None\n"
+
+    "class C is T";
+
+  TEST_COMPILE(src);
+}