Provide an error cursor for "can't subscript" error messages.

Commit c7aba7c1 didn't add this, but after more fooling with the
feature I feel that it'd be useful.  To make this possible, refactor
getSubscriptingRoutines() so that the caller is responsible for
throwing any error.  (In clauses.c, I just chose to make the
most conservative assumption rather than throwing an error.  We don't
expect failures there anyway really, so the code space for an error
message would be a poor investment.)

src/backend/executor/execExpr.c

@@ -2536,6 +2536,14 @@ ExecInitSubscriptingRef(ExprEvalStep *scratch, SubscriptingRef *sbsref,
 
 	/* Look up the subscripting support methods */
 	sbsroutines = getSubscriptingRoutines(sbsref->refcontainertype, NULL);
+	if (!sbsroutines)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("cannot subscript type %s because it does not support subscripting",
+						format_type_be(sbsref->refcontainertype)),
+				 state->parent ?
+				 executor_errposition(state->parent->state,
+									  exprLocation((Node *) sbsref)) : 0));
 
 	/* Allocate sbsrefstate, with enough space for per-subscript arrays too */
 	sbsrefstate = palloc0(MAXALIGN(sizeof(SubscriptingRefState)) +

src/backend/optimizer/util/clauses.c

@@ -848,7 +848,7 @@ contain_nonstrict_functions_walker(Node *node, void *context)
 			return true;
 		/* Otherwise we must look up the subscripting support methods */
 		sbsroutines = getSubscriptingRoutines(sbsref->refcontainertype, NULL);
-		if (!sbsroutines->fetch_strict)
+		if (!(sbsroutines && sbsroutines->fetch_strict))
 			return true;
 		/* else fall through to check args */
 	}
@@ -1144,7 +1144,8 @@ contain_leaked_vars_walker(Node *node, void *context)
 				/* Consult the subscripting support method info */
 				sbsroutines = getSubscriptingRoutines(sbsref->refcontainertype,
 													  NULL);
-				if (!(sbsref->refassgnexpr != NULL ?
+				if (!sbsroutines ||
+					!(sbsref->refassgnexpr != NULL ?
 					  sbsroutines->store_leakproof :
 					  sbsroutines->fetch_leakproof))
 				{

src/backend/parser/parse_node.c

@@ -271,6 +271,12 @@ transformContainerSubscripts(ParseState *pstate,
 	 * functions and typelem.
 	 */
 	sbsroutines = getSubscriptingRoutines(containerType, &elementType);
+	if (!sbsroutines)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("cannot subscript type %s because it does not support subscripting",
+						format_type_be(containerType)),
+				 parser_errposition(pstate, exprLocation(containerBase))));
 
 	/*
 	 * Detect whether any of the indirection items are slice specifiers.

src/backend/utils/cache/lsyscache.c

@@ -3031,7 +3031,7 @@ get_typsubscript(Oid typid, Oid *typelemp)
  * getSubscriptingRoutines
  *
  *		Given the type OID, fetch the type's subscripting methods struct.
- *		Fail if type is not subscriptable.
+ *		Return NULL if type is not subscriptable.
  *
  * If typelemp isn't NULL, we also store the type's typelem value there.
  * This saves some callers an extra catalog lookup.
@@ -3042,10 +3042,7 @@ getSubscriptingRoutines(Oid typid, Oid *typelemp)
 	RegProcedure typsubscript = get_typsubscript(typid, typelemp);
 
 	if (!OidIsValid(typsubscript))
-		ereport(ERROR,
-				(errcode(ERRCODE_DATATYPE_MISMATCH),
-				 errmsg("cannot subscript type %s because it does not support subscripting",
-						format_type_be(typid))));
+		return NULL;
 
 	return (const struct SubscriptRoutines *)
 		DatumGetPointer(OidFunctionCall0(typsubscript));

src/test/regress/expected/arrays.out

@@ -238,6 +238,8 @@ ERROR:  array subscript in assignment must not be null
 -- Un-subscriptable type
 SELECT (now())[1];
 ERROR:  cannot subscript type timestamp with time zone because it does not support subscripting
+LINE 1: SELECT (now())[1];
+                ^
 -- test slices with empty lower and/or upper index
 CREATE TEMP TABLE arrtest_s (
   a       int2[],