This patch fixes PR c++/88095: class nontype template parameter UDL string
literals doesn't accepts deduction placeholder

It also addresses a latent issue; literal operator templates with template
parameter packs of literal class type were previously accepted.  The patch
corrects this and adds a test (udlit-class-nttp-neg.C).

This fix is needed for one of the char8_t remediation approaches documented
in P1423, and may be helpful for existing code bases impacted by the char8_t
changes adopted via P0482 for C++20.

gcc/cp/ChangeLog:

2019-08-02  Tom Honermann  <tom@honermann.net>

        * parser.c (cp_parser_template_declaration_after_parameters): Enable
        class template argument deduction for non-type template parameters
        in literal operator templates.

gcc/testsuite/ChangeLog:

2019-08-02  Tom Honermann  <tom@honermann.net>

        PR c++/88095
        * g++.dg/cpp2a/udlit-class-nttp-ctad.C: New test.
        * g++.dg/cpp2a/udlit-class-nttp-ctad-neg.C: New test.
        * g++.dg/cpp2a/udlit-class-nttp-ctad-neg2.C: New test.
        * g++.dg/cpp2a/udlit-class-nttp.C: New test.
        * g++.dg/cpp2a/udlit-class-nttp-neg.C: New test.
        * g++.dg/cpp2a/udlit-class-nttp-neg2.C: New test.

From-SVN: r274123