Re: CREATE CAST allows creation of binary-coercible cast to range over domain

I wrote:
> I think it's an oversight that we allow binary-coercible casts
> involving range types at all.  There are no other nesting cases to
> worry about, because CreateCast already rejects binary-coercible casts
> applied to composites and arrays, explaining
>          * We know that composite, enum and array types are never binary-
>          * compatible with each other.  They all have OIDs embedded in them.
> But range types also embed their own OID, so I don't see why that
> concern doesn't apply to them.

In short, more or less as attached.  (I didn't bother with a
regression test, since none of the adjacent error checks are
covered either.)

I'm unsure whether to back-patch this.  If somebody were using
such a cast today, it might seem to mostly work, as long as the
two kinds of ranges had the same behavior.  So they might not
appreciate us breaking it in a minor release.  Maybe we should
put this into 17 but not further back.

            regards, tom lane

diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 6593fd7d81..149642ab1b 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -1689,13 +1689,18 @@ CreateCast(CreateCastStmt *stmt)
                      errmsg("source and target data types are not physically compatible")));

         /*
-         * We know that composite, enum and array types are never binary-
-         * compatible with each other.  They all have OIDs embedded in them.
+         * We know that composite, array, range and enum types are never
+         * binary-compatible with each other.  They all have OIDs embedded in
+         * them.
          *
          * Theoretically you could build a user-defined base type that is
-         * binary-compatible with a composite, enum, or array type.  But we
-         * disallow that too, as in practice such a cast is surely a mistake.
-         * You can always work around that by writing a cast function.
+         * binary-compatible with such a type.  But we disallow that too, as
+         * in practice such a cast is surely a mistake.  You can always work
+         * around that by writing a cast function.
+         *
+         * NOTE: if we didn't exclude every kind of container type for this
+         * reason, we'd likely need to recursively apply all of these same
+         * checks to the contained type(s).
          */
         if (sourcetyptype == TYPTYPE_COMPOSITE ||
             targettyptype == TYPTYPE_COMPOSITE)
@@ -1703,6 +1708,14 @@ CreateCast(CreateCastStmt *stmt)
                     (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
                      errmsg("composite data types are not binary-compatible")));

+        if (sourcetyptype == TYPTYPE_RANGE ||
+            targettyptype == TYPTYPE_RANGE ||
+            sourcetyptype == TYPTYPE_MULTIRANGE ||
+            targettyptype == TYPTYPE_MULTIRANGE)
+            ereport(ERROR,
+                    (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+                     errmsg("range data types are not binary-compatible")));
+
         if (sourcetyptype == TYPTYPE_ENUM ||
             targettyptype == TYPTYPE_ENUM)
             ereport(ERROR,