Fix Unsubtyping on extern.convert_any (#8062)

We were not previously recording that the input to extern.convert_any
has to be a subtype of anyref. This meant that it was possible for
Unsubtyping to optimize too aggressively and break casts.

Author: tlively

src/ir/subtype-exprs.h

@@ -445,8 +445,19 @@ struct SubtypingDiscoverer : public OverriddenVisitor<SubType> {
     self()->noteSubtype(curr->replacement, type);
   }
   void visitRefAs(RefAs* curr) {
-    if (curr->op == RefAsNonNull) {
-      self()->noteCast(curr->value, curr);
+    switch (curr->op) {
+      case RefAsNonNull:
+        self()->noteCast(curr->value, curr);
+        return;
+      case AnyConvertExtern:
+        return;
+      case ExternConvertAny:
+        if (curr->type != Type::unreachable) {
+          auto any =
+            HeapTypes::any.getBasic(curr->type.getHeapType().getShared());
+          self()->noteSubtype(curr->value, Type(any, Nullable));
+        }
+        return;
     }
   }
   void visitStringNew(StringNew* curr) {}

test/lit/passes/unsubtyping.wast

@@ -1920,3 +1920,85 @@
     )
   )
 )
+
+;; extern.convert_any requires its input to remain a subtype of any.
+(module
+  ;; CHECK:      (rec
+  ;; CHECK-NEXT:  (type $A (sub (struct)))
+  (type $A (sub (struct)))
+  ;; CHECK:       (type $B (sub $A (struct)))
+  (type $B (sub $A (struct)))
+
+  ;; CHECK:       (type $2 (func (result externref)))
+
+  ;; CHECK:       (type $3 (func (param anyref)))
+
+  ;; CHECK:      (func $test (type $2) (result externref)
+  ;; CHECK-NEXT:  (extern.convert_any
+  ;; CHECK-NEXT:   (struct.new_default $B)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $test (result externref)
+    (extern.convert_any
+      (struct.new $B)
+    )
+  )
+
+  ;; CHECK:      (func $cast (type $3) (param $0 anyref)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.cast (ref null $A)
+  ;; CHECK-NEXT:    (local.get $0)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $cast (param anyref)
+    (drop
+      ;; Since $B flows into any, it must remain a subtype of $A to continue
+      ;; passing this cast.
+      (ref.cast (ref null $A)
+        (local.get 0)
+      )
+    )
+  )
+)
+
+;; Same as above with shared types.
+(module
+  ;; CHECK:      (rec
+  ;; CHECK-NEXT:  (type $A (sub (shared (struct))))
+  (type $A (sub (shared (struct))))
+  ;; CHECK:       (type $B (sub $A (shared (struct))))
+  (type $B (sub $A (shared (struct))))
+
+  ;; CHECK:       (type $2 (func (result (ref null (shared extern)))))
+
+  ;; CHECK:       (type $3 (func (param (ref null (shared any)))))
+
+  ;; CHECK:      (func $test (type $2) (result (ref null (shared extern)))
+  ;; CHECK-NEXT:  (extern.convert_any
+  ;; CHECK-NEXT:   (struct.new_default $B)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $test (result (ref null (shared extern)))
+    (extern.convert_any
+      (struct.new $B)
+    )
+  )
+
+  ;; CHECK:      (func $cast (type $3) (param $0 (ref null (shared any)))
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (ref.cast (ref null $A)
+  ;; CHECK-NEXT:    (local.get $0)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $cast (param (ref null (shared any)))
+    (drop
+      ;; Since $B flows into any, it must remain a subtype of $A to continue
+      ;; passing this cast.
+      (ref.cast (ref null $A)
+        (local.get 0)
+      )
+    )
+  )
+)