sql: fix internal error when planning void IS NULL or void is NOT NULL

This commit adds support for comparisons between columns of type void
and NULL using col IS NULL or col IS NOT NULL, by adding an explicit
overload.

Fixes #93572

Release note (bug fix): Fixed an internal error that could occur
when comparing a column of type void to NULL using col IS NULL
or col IS NOT NULL.

docs/generated/sql/operators.md

@@ -457,9 +457,11 @@
 <tr><td>tsvector <code>IS NOT DISTINCT FROM</code> tsvector</td><td><a href="bool.html">bool</a></td></tr>
 <tr><td>tuple <code>IS NOT DISTINCT FROM</code> tuple</td><td><a href="bool.html">bool</a></td></tr>
 <tr><td>unknown <code>IS NOT DISTINCT FROM</code> unknown</td><td><a href="bool.html">bool</a></td></tr>
+<tr><td>unknown <code>IS NOT DISTINCT FROM</code> void</td><td><a href="bool.html">bool</a></td></tr>
 <tr><td><a href="uuid.html">uuid</a> <code>IS NOT DISTINCT FROM</code> <a href="uuid.html">uuid</a></td><td><a href="bool.html">bool</a></td></tr>
 <tr><td><a href="uuid.html">uuid[]</a> <code>IS NOT DISTINCT FROM</code> <a href="uuid.html">uuid[]</a></td><td><a href="bool.html">bool</a></td></tr>
 <tr><td>varbit <code>IS NOT DISTINCT FROM</code> varbit</td><td><a href="bool.html">bool</a></td></tr>
+<tr><td>void <code>IS NOT DISTINCT FROM</code> unknown</td><td><a href="bool.html">bool</a></td></tr>
 </tbody></table>
 <table><thead>
 <tr><td><code>LIKE</code></td><td>Return</td></tr>

pkg/sql/logictest/testdata/logic_test/void

@@ -41,15 +41,20 @@ SELECT crdb_internal.void_func()
 ----
 ·
 
-# Regression test for #83754. Postgres does not error out, but is inconsistent
+# Regression test for #83754. Note that Postgres is inconsistent
 # in evaluation. For example, `SELECT ''::VOID IS DISTINCT FROM NULL::UNKNOWN;`
 # errors out, but `SELECT ''::VOID IS DISTINCT FROM NULL;` does not.
 # This is due to normalization into an IS NOT NULL op when one operand is NULL.
 # The NULL with type cast is not recognized as NULL.
-# We will stay consistent and have both cases error out to avoid different
-# behavior between normalized and unnormalized expressions.
-statement error pq: unsupported comparison operator: <void> IS DISTINCT FROM <unknown>
+query B
 SELECT ''::VOID IS DISTINCT FROM NULL
+----
+true
+
+query B
+SELECT ''::VOID IS DISTINCT FROM NULL::UNKNOWN
+----
+true
 
 statement ok
 SET vectorize=on
@@ -147,3 +152,24 @@ NULL
 
 statement ok
 RESET vectorize
+
+# Regression test for #93572. This should not fail with an internal error.
+query B
+WITH tab(x) AS (VALUES ('':::VOID)) SELECT x IS NULL FROM tab
+----
+false
+
+query B
+WITH tab(x) AS (VALUES (NULL:::VOID)) SELECT x IS NULL FROM tab
+----
+true
+
+query B
+WITH tab(x) AS (VALUES ('':::VOID)) SELECT x IS NOT NULL FROM tab
+----
+true
+
+query B
+WITH tab(x) AS (VALUES (NULL:::VOID)) SELECT x IS NOT NULL FROM tab
+----
+false

pkg/sql/sem/tree/eval.go

@@ -1728,6 +1728,15 @@ var CmpOps = cmpOpFixups(map[treecmp.ComparisonOperatorSymbol]*CmpOpOverloads{
 		makeIsFn(types.Time, types.TimeTZ, volatility.Stable),
 		makeIsFn(types.TimeTZ, types.Time, volatility.Stable),
 
+		// Void is unique in that it is not equivalent with itself, so implicit
+		// equivalence with Unknown in function ArgTypes.MatchAt due to the check
+		// `(typ.Family() == types.UnknownFamily || a[i].Typ.Equivalent(typ))` does
+		// not occur. Therefore, to allow the comparison
+		// `''::VOID IS DISTINCT FROM NULL`, an explicit equivalence with Unknown is
+		// added:
+		makeIsFn(types.Void, types.Unknown, volatility.Stable),
+		makeIsFn(types.Unknown, types.Void, volatility.Stable),
+
 		// Tuple comparison.
 		{
 			LeftType:          types.AnyTuple,