Hi,

When implementing some of the recent stdlib class template argument deduction changes, we noticed that p0620 substantially changes the rules for deduction guides to the point where (eg) our standard library's testsuite fails on a compiler that defines __cpp_deduction_guides to the latest version described by SD-6.

I suggest we bump the version from 201611 to 201703 for implementations that support P0620. (This leaves Clang 6.0.0 as defining the macro to the older version but providing the newer functionality, but that seems preferable to making the "full" rule set undetectable.)

Thoughts?