Feature request for better safety while storing the generator

This issue is related to the https://github.com/boostorg/spirit/issues/436

The problem is that it is very simple to misuse the Spirit (or Proto) by storing a result that holds references to temporaries. @Kojoley noted that there may be a way to static assert such cases or store the input by value if the value is an rvalue.

Test case and some discussion available at https://github.com/boostorg/spirit/issues/436

The issue names no repository files; start with the test case and discussion linked from Boost Spirit issue #436. Determine the agreed behavior for stored results that reference temporaries, including whether invalid cases should be rejected or rvalues stored by value. Done requires an agreed design and corresponding safety coverage.

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