Fix issue #78 and have a better error message.

core/_preload.odin

@@ -417,7 +417,7 @@ __get_map_header :: proc(m: ^map[$K]$V) -> __MapHeader #cc_contextless {
 		value: V;
 	}
 
-	_, is_string := type_info_base(type_info(K)).(^TypeInfo.String);
+	_, is_string := type_info_base(type_info(K)).variant.(TypeInfo.String);
 	header.is_key_string = is_string;
 	header.entry_size    = size_of(Entry);
 	header.entry_align   = align_of(Entry);

src/check_expr.cpp

@@ -6980,7 +6980,7 @@ ExprKind check_expr_base_internal(Checker *c, Operand *o, AstNode *node, Type *t
 			if (!ok) {
 				gbString expr_str = expr_to_string(o->expr);
 				gbString dst_type_str = type_to_string(t);
-				error(o->expr, "Cannot type assert `%s` to `%s`", expr_str, dst_type_str);
+				error(o->expr, "Cannot type assert `%s` to `%s` as it is not a variant of that union", expr_str, dst_type_str);
 				gb_string_free(dst_type_str);
 				gb_string_free(expr_str);
 				o->mode = Addressing_Invalid;
@@ -7000,7 +7000,9 @@ ExprKind check_expr_base_internal(Checker *c, Operand *o, AstNode *node, Type *t
 			add_type_info_type(c, o->type);
 			add_type_info_type(c, t);
 		} else {
-			error(o->expr, "Type assertions can only operate on unions and `any`");
+			gbString str = type_to_string(o->type);
+			error(o->expr, "Type assertions can only operate on unions and `any`, got %s", str);
+			gb_string_free(str);
 			o->mode = Addressing_Invalid;
 			o->expr = node;
 			return kind;