Fix no-longer-valid shortcuts in expression_returns_set().

expression_returns_set() used to short-circuit its recursion upon
seeing certain node types, such as DistinctExpr, that it knew the
executor did not support set-valued arguments for.  That was never
inherent, though, just a reflection of laziness in execQual.c.
With the new implementation of SRFs there is no reason to think
that any scalar-valued expression node could not have a set-valued
subexpression, except for AggRefs and WindowFuncs where we know there
is a parser check rejecting it.  And indeed, the shortcut causes
unexpected failures for cases such as a SRF underneath DistinctExpr,
because the planner stops looking for SRFs too soon.

Discussion: https://postgr.es/m/5259.1497044025@sss.pgh.pa.us

src/backend/nodes/nodeFuncs.c

@@ -694,39 +694,11 @@ expression_returns_set_walker(Node *node, void *context)
 		/* else fall through to check args */
 	}
 
-	/* Avoid recursion for some cases that can't return a set */
+	/* Avoid recursion for some cases that parser checks not to return a set */
 	if (IsA(node, Aggref))
 		return false;
 	if (IsA(node, WindowFunc))
 		return false;
-	if (IsA(node, DistinctExpr))
-		return false;
-	if (IsA(node, NullIfExpr))
-		return false;
-	if (IsA(node, ScalarArrayOpExpr))
-		return false;
-	if (IsA(node, BoolExpr))
-		return false;
-	if (IsA(node, SubLink))
-		return false;
-	if (IsA(node, SubPlan))
-		return false;
-	if (IsA(node, AlternativeSubPlan))
-		return false;
-	if (IsA(node, ArrayExpr))
-		return false;
-	if (IsA(node, RowExpr))
-		return false;
-	if (IsA(node, RowCompareExpr))
-		return false;
-	if (IsA(node, CoalesceExpr))
-		return false;
-	if (IsA(node, MinMaxExpr))
-		return false;
-	if (IsA(node, SQLValueFunction))
-		return false;
-	if (IsA(node, XmlExpr))
-		return false;
 
 	return expression_tree_walker(node, expression_returns_set_walker,
 								  context);

src/test/regress/expected/tsrf.out

@@ -443,6 +443,14 @@ SELECT int4mul(generate_series(1,2), 10);
       20
 (2 rows)
 
+SELECT generate_series(1,3) IS DISTINCT FROM 2;
+ ?column? 
+----------
+ t
+ f
+ t
+(3 rows)
+
 -- but SRFs in function RTEs must be at top level (annoying restriction)
 SELECT * FROM int4mul(generate_series(1,2), 10);
 ERROR:  set-returning functions must appear at top level of FROM

src/test/regress/sql/tsrf.sql

@@ -98,6 +98,7 @@ VALUES(1, generate_series(1,2));
 
 -- We allow tSRFs that are not at top level
 SELECT int4mul(generate_series(1,2), 10);
+SELECT generate_series(1,3) IS DISTINCT FROM 2;
 
 -- but SRFs in function RTEs must be at top level (annoying restriction)
 SELECT * FROM int4mul(generate_series(1,2), 10);