[wasm2wat] Write select type immediate (#1451)

The main fix is in `wat-writer.cc`, where the type immediate was never
being printed. But I've also included a change to how `select` type
immediates are represented in wabt.

Previously, a bare `select` instruction would be stored with the type
`Type::Any`. This is not a real wasm type, and is primarily used for
type validation. The spec instead considers this form of `select` to
have an empty type immediate, which is closer to the `Type::Void` type.

This commit now uses `Type::Void` (or an empty `TypeVector`) to
represent the bare `select` instruction.

Fixes #1444.

1 files changed, 7 insertions, 2 deletions

diff --git a/src/validator.cc b/src/validator.cc
index 8884059e..e23d5be1 100644
--- a/src/validator.cc
+++ b/src/validator.cc
@@ -464,8 +464,13 @@ Result Validator::OnReturnCallIndirectExpr(ReturnCallIndirectExpr* expr) {
 }
 
 Result Validator::OnSelectExpr(SelectExpr* expr) {
-  assert(expr->result_type.size());
-  result_ |= validator_.OnSelect(expr->loc, expr->result_type[0]);
+  Type result_type;
+  if (expr->result_type.empty()) {
+    result_type = Type::Void;
+  } else {
+    result_type = expr->result_type[0];
+  }
+  result_ |= validator_.OnSelect(expr->loc, result_type);
   // TODO: Existing behavior fails when select fails.
 #if 0
   return Result::Ok;