Fix failure to validate the result of select_common_type().

Although select_common_type() has a failure-return convention, an
apparent successful return just provides a type OID that *might* work
as a common supertype; we've not validated that the required casts
actually exist.  In the mainstream use-cases that doesn't matter,
because we'll proceed to invoke coerce_to_common_type() on each input,
which will fail appropriately if the proposed common type doesn't
actually work.  However, a few callers didn't read the (nonexistent)
fine print, and thought that if they got back a nonzero OID then the
coercions were sure to work.

This affects in particular the recently-added "anycompatible"
polymorphic types; we might think that a function/operator using
such types matches cases it really doesn't.  A likely end result
of that is unexpected "ambiguous operator" errors, as for example
in bug #17387 from James Inform.  Another, much older, case is that
the parser might try to transform an "x IN (list)" construct to
a ScalarArrayOpExpr even when the list elements don't actually have
a common supertype.

It doesn't seem desirable to add more checking to select_common_type
itself, as that'd just slow down the mainstream use-cases.  Instead,
write a separate function verify_common_type that performs the
missing checks, and add a call to that where necessary.  Likewise add
verify_common_type_from_oids to go with select_common_type_from_oids.

Back-patch to v13 where the "anycompatible" types came in.  (The
symptom complained of in bug #17387 doesn't appear till v14, but
that's just because we didn't get around to converting || to use
anycompatible till then.)  In principle the "x IN (list)" fix could
go back all the way, but I'm not currently convinced that it makes
much difference in real-world cases, so I won't bother for now.

Discussion: https://postgr.es/m/17387-5dfe54b988444963@postgresql.org

src/backend/parser/parse_coerce.c

@@ -1298,6 +1298,10 @@ parser_coercion_errposition(ParseState *pstate,
  * rather than throwing an error on failure.
  * 'which_expr': if not NULL, receives a pointer to the particular input
  * expression from which the result type was taken.
+ *
+ * Caution: "failure" just means that there were inputs of different type
+ * categories.  It is not guaranteed that all the inputs are coercible to the
+ * selected type; caller must check that (see verify_common_type).
  */
 Oid
 select_common_type(ParseState *pstate, List *exprs, const char *context,
@@ -1426,6 +1430,10 @@ select_common_type(ParseState *pstate, List *exprs, const char *context,
  * earlier entries in the array have some preference over later ones.
  * On failure, return InvalidOid if noerror is true, else throw an error.
  *
+ * Caution: "failure" just means that there were inputs of different type
+ * categories.  It is not guaranteed that all the inputs are coercible to the
+ * selected type; caller must check that (see verify_common_type_from_oids).
+ *
  * Note: neither caller will pass any UNKNOWNOID entries, so the tests
  * for that in this function are dead code.  However, they don't cost much,
  * and it seems better to keep this logic as close to select_common_type()
@@ -1548,6 +1556,48 @@ coerce_to_common_type(ParseState *pstate, Node *node,
 	return node;
 }
 
+/*
+ * verify_common_type()
+ *		Verify that all input types can be coerced to a proposed common type.
+ *		Return true if so, false if not all coercions are possible.
+ *
+ * Most callers of select_common_type() don't need to do this explicitly
+ * because the checks will happen while trying to convert input expressions
+ * to the right type, e.g. in coerce_to_common_type().  However, if a separate
+ * check step is needed to validate the applicability of the common type, call
+ * this.
+ */
+bool
+verify_common_type(Oid common_type, List *exprs)
+{
+	ListCell   *lc;
+
+	foreach(lc, exprs)
+	{
+		Node	   *nexpr = (Node *) lfirst(lc);
+		Oid			ntype = exprType(nexpr);
+
+		if (!can_coerce_type(1, &ntype, &common_type, COERCION_IMPLICIT))
+			return false;
+	}
+	return true;
+}
+
+/*
+ * verify_common_type_from_oids()
+ *		As above, but work from an array of type OIDs.
+ */
+static bool
+verify_common_type_from_oids(Oid common_type, int nargs, const Oid *typeids)
+{
+	for (int i = 0; i < nargs; i++)
+	{
+		if (!can_coerce_type(1, &typeids[i], &common_type, COERCION_IMPLICIT))
+			return false;
+	}
+	return true;
+}
+
 /*
  * select_common_typmod()
  *		Determine the common typmod of a list of input expressions.
@@ -1917,7 +1967,13 @@ check_generic_type_consistency(const Oid *actual_arg_types,
 										 true);
 
 		if (!OidIsValid(anycompatible_typeid))
-			return false;		/* there's no common supertype */
+			return false;		/* there's definitely no common supertype */
+
+		/* We have to verify that the selected type actually works */
+		if (!verify_common_type_from_oids(anycompatible_typeid,
+										  n_anycompatible_args,
+										  anycompatible_actual_types))
+			return false;
 
 		if (have_anycompatible_nonarray)
 		{
@@ -2494,6 +2550,14 @@ enforce_generic_type_consistency(const Oid *actual_arg_types,
 											 anycompatible_actual_types,
 											 false);
 
+			/* We have to verify that the selected type actually works */
+			if (!verify_common_type_from_oids(anycompatible_typeid,
+											  n_anycompatible_args,
+											  anycompatible_actual_types))
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("arguments of anycompatible family cannot be cast to a common type")));
+
 			if (have_anycompatible_array)
 			{
 				anycompatible_array_typeid = get_array_type(anycompatible_typeid);

src/backend/parser/parse_expr.c

@@ -1126,6 +1126,11 @@ transformAExprIn(ParseState *pstate, A_Expr *a)
 		allexprs = list_concat(list_make1(lexpr), rnonvars);
 		scalar_type = select_common_type(pstate, allexprs, NULL, NULL);
 
+		/* We have to verify that the selected type actually works */
+		if (OidIsValid(scalar_type) &&
+			!verify_common_type(scalar_type, allexprs))
+			scalar_type = InvalidOid;
+
 		/*
 		 * Do we have an array type to use?  Aside from the case where there
 		 * isn't one, we don't risk using ScalarArrayOpExpr when the common

src/include/parser/parse_coerce.h

@@ -70,6 +70,7 @@ extern Oid	select_common_type(ParseState *pstate, List *exprs,
 extern Node *coerce_to_common_type(ParseState *pstate, Node *node,
 								   Oid targetTypeId,
 								   const char *context);
+extern bool verify_common_type(Oid common_type, List *exprs);
 
 extern int32 select_common_typmod(ParseState *pstate, List *exprs, Oid common_type);
 

src/test/regress/expected/arrays.out

@@ -747,6 +747,18 @@ SELECT ARRAY[1.1] || ARRAY[2,3,4];
  {1.1,2,3,4}
 (1 row)
 
+SELECT array_agg(x) || array_agg(x) FROM (VALUES (ROW(1,2)), (ROW(3,4))) v(x);
+             ?column?              
+-----------------------------------
+ {"(1,2)","(3,4)","(1,2)","(3,4)"}
+(1 row)
+
+SELECT ROW(1,2) || array_agg(x) FROM (VALUES (ROW(3,4)), (ROW(5,6))) v(x);
+         ?column?          
+---------------------------
+ {"(1,2)","(3,4)","(5,6)"}
+(1 row)
+
 SELECT * FROM array_op_test WHERE i @> '{32}' ORDER BY seqno;
  seqno |                i                |                                                                 t                                                                  
 -------+---------------------------------+------------------------------------------------------------------------------------------------------------------------------------

src/test/regress/expected/expressions.out

@@ -232,6 +232,36 @@ explain (verbose, costs off) select * from bpchar_view
 
 rollback;
 --
+-- Ordinarily, IN/NOT IN can be converted to a ScalarArrayOpExpr
+-- with a suitably-chosen array type.
+--
+explain (verbose, costs off)
+select random() IN (1, 4, 8.0);
+                         QUERY PLAN                         
+------------------------------------------------------------
+ Result
+   Output: (random() = ANY ('{1,4,8}'::double precision[]))
+(2 rows)
+
+explain (verbose, costs off)
+select random()::int IN (1, 4, 8.0);
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Result
+   Output: (((random())::integer)::numeric = ANY ('{1,4,8.0}'::numeric[]))
+(2 rows)
+
+-- However, if there's not a common supertype for the IN elements,
+-- we should instead try to produce "x = v1 OR x = v2 OR ...".
+-- In most cases that'll fail for lack of all the requisite = operators,
+-- but it can succeed sometimes.  So this should complain about lack of
+-- an = operator, not about cast failure.
+select '(0,0)'::point in ('(0,0,0,0)'::box, point(0,0));
+ERROR:  operator does not exist: point = box
+LINE 1: select '(0,0)'::point in ('(0,0,0,0)'::box, point(0,0));
+                              ^
+HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.
+--
 -- Tests for ScalarArrayOpExpr with a hashfn
 --
 -- create a stable function so that the tests below are not

src/test/regress/sql/arrays.sql

@@ -317,6 +317,8 @@ SELECT ARRAY[[1,2],[3,4]] || ARRAY[5,6] AS "{{1,2},{3,4},{5,6}}";
 SELECT ARRAY[0,0] || ARRAY[1,1] || ARRAY[2,2] AS "{0,0,1,1,2,2}";
 SELECT 0 || ARRAY[1,2] || 3 AS "{0,1,2,3}";
 SELECT ARRAY[1.1] || ARRAY[2,3,4];
+SELECT array_agg(x) || array_agg(x) FROM (VALUES (ROW(1,2)), (ROW(3,4))) v(x);
+SELECT ROW(1,2) || array_agg(x) FROM (VALUES (ROW(3,4)), (ROW(5,6))) v(x);
 
 SELECT * FROM array_op_test WHERE i @> '{32}' ORDER BY seqno;
 SELECT * FROM array_op_test WHERE i && '{32}' ORDER BY seqno;

src/test/regress/sql/expressions.sql

@@ -103,6 +103,22 @@ explain (verbose, costs off) select * from bpchar_view
 rollback;
 
 
+--
+-- Ordinarily, IN/NOT IN can be converted to a ScalarArrayOpExpr
+-- with a suitably-chosen array type.
+--
+explain (verbose, costs off)
+select random() IN (1, 4, 8.0);
+explain (verbose, costs off)
+select random()::int IN (1, 4, 8.0);
+-- However, if there's not a common supertype for the IN elements,
+-- we should instead try to produce "x = v1 OR x = v2 OR ...".
+-- In most cases that'll fail for lack of all the requisite = operators,
+-- but it can succeed sometimes.  So this should complain about lack of
+-- an = operator, not about cast failure.
+select '(0,0)'::point in ('(0,0,0,0)'::box, point(0,0));
+
+
 --
 -- Tests for ScalarArrayOpExpr with a hashfn
 --