SubtypingDiscoverer: Differentiate non-flow subtyping constraints (#6344)

When we do a local.set of a value into a local then we have both a subtyping constraint - for
the value to be valid to put in that local - and also a flow of a value, which can then reach
more places. Such flow then interacts with casts in Unsubtyping, since it needs to know
what can flow where in order to know how casts force us to keep subtyping relations.

That regressed in the not-actually-NFC #6323 in which I added the innocuous lines
to add subtyping constraints in ref.eq. It seems fine to require that the arms of a
RefEq must be of type eqref, but Unsubtyping then assuming those arms flowed into
a location of type eqref... which means casts might force us to not optimize some
things.

To fix this, differentiate the rare case of non-flowing subtyping constraints, which is
basically only RefEq. There are perhaps a few more cases (like i31 operations) but they
do not matter in practice for Unsubtyping anyhow; I suggest we land this first to undo
the regression and then at our leisure investigate the other instructions.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/passes/Unsubtyping.cpp b/src/passes/Unsubtyping.cpp
index 67a3c4e85..897511c35 100644
--- a/src/passes/Unsubtyping.cpp
+++ b/src/passes/Unsubtyping.cpp
@@ -193,6 +193,22 @@ struct Unsubtyping
     noteSubtype(sub->type, super->type);
   }
 
+  void noteNonFlowSubtype(Expression* sub, Type super) {
+    // This expression's type must be a subtype of |super|, but the value does
+    // not flow anywhere - this is a static constraint. As the value does not
+    // flow, it cannot reach anywhere else, which means we need this in order to
+    // validate but it does not interact with casts. Given that, if super is a
+    // basic type then we can simply ignore this: we only remove subtyping
+    // between user types, so subtyping wrt basic types is unchanged, and so
+    // this constraint will never be a problem.
+    if (super.isRef() && super.getHeapType().isBasic()) {
+      return;
+    }
+
+    // Otherwise, we must take this into account.
+    noteSubtype(sub, super);
+  }
+
   void noteCast(HeapType src, HeapType dest) {
     if (src == dest || dest.isBottom()) {
       return;