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Modern C++ (i.e., C++11 and later) introduced "move only" types: types such as std::unique_ptr<T> that can never be duplicated. They must never be copied (by assignments and constructors), they must be "moved". The implementation of lalr1.cc used to copy symbols (including their semantic values). This commit ensures that values are only moved in modern C++, yet remain compatible with C++98/C++03. Suggested by Frank Heckenbach, who provided a full implementation on top of C++17's std::variant. See http://lists.gnu.org/archive/html/bug-bison/2018-03/msg00002.html, and https://lists.gnu.org/archive/html/bison-patches/2018-04/msg00002.html. Symbols (terminal/non terminal) are handled by several functions that used to take const-refs, which resulted eventually in a copy pushed on the stack. With modern C++ (C++11 and later) the callers must use std::move, and the callees must take their arguments as rvalue refs (foo&&). In order to avoid duplicating these functions to support both legacy C++ and modern C++, let's introduce macros (YY_MOVE, YY_RVREF, etc.) that rely on copy-semantics for C++98/03, and move-semantics for modern C++. That's easy for inner types, when the parser's functions pass arguments to each other. Functions facing the user (make_NUMBER, make_STRING, etc.) should support both rvalue-refs (for instance to support move-only types: make_INT (std::make_unique<int> (1))), and lvalue-refs (so that we can pass a variable: make_INT (my_int)). To avoid the multiplication of the signatures (there is also the location), let's take the argument by value. See: https://lists.gnu.org/archive/html/bison-patches/2018-09/msg00024.html. * data/c++.m4 (b4_cxx_portability): New. (basic_symbol): In C++11, replace copy-ctors with move-ctors. In C++11, replace copies with moves. * data/lalr1.cc (stack_symbol_type, yypush_): Likewise. Use YY_MOVE to avoid useless copies. * data/variant.hh (variant): Support move-semantics. (make_SYMBOL): In C++11, in order to support both read-only lvalues, and rvalues, take the argument as a copy. * data/stack.hh (yypush_): Use rvalue-refs in C++11. * tests/c++.at: Use move semantics. * tests/headers.at: Adjust to the new macros (YY_MOVE, etc.). * configure.ac (CXX98_CXXFLAGS, CXX11_CXXFLAGS, CXX14_CXXFLAGS) (CXX17_CXXFLAGS, ENABLE_CXX11): New. * tests/atlocal.in: Receive them. * examples/variant.yy: Don't define things in std. * examples/variant-11.test, examples/variant-11.yy: New. Check the support of move-only types. * examples/README, examples/local.mk: Adjust.
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