pgsql: Don't let JSON constructor coercions block SQL function inlining

Don't let JSON constructor coercions block SQL function inlining

contain_context_dependent_node_walker() reports a CaseTestExpr as
context-dependent unless it sits under a simple CaseExpr or the
elemexpr of an ArrayCoerceExpr. A JsonConstructorExpr whose RETURNING
type requires a coercion also carries a CaseTestExpr placeholder in
that coercion, so any SQL function called with such a constructor as
an argument was refused inlining.

Teach the walker that a CaseTestExpr is expected within the coercion
of a JsonConstructorExpr, the same way it already handles the elemexpr
of an ArrayCoerceExpr. This is safe now that eval_const_expressions
no longer lets an enclosing simple CASE clobber that placeholder.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs48A=VCFbteTkuCoknO1_0-Cu0aMBT0M07dm7vj1QyixDg@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9d0adee8768852fc0e5713ec27cf52d46d97279d

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 23 ++++++++++++++++++++++-
src/test/regress/expected/sqljson.out | 18 ++++++++++++++++++
src/test/regress/sql/sqljson.sql | 8 ++++++++
3 files changed, 48 insertions(+), 1 deletion(-)