C++ PATCH for c++/81073, constexpr and static var in statement-expression

[Jason Merrill <jason at redhat dot com>]

The testcase successfully compiles, but then fails to link because
we've optimized away the declaration of the variable.  We catch this
in potential_constant_expression_1, but this path wasn't calling it.

Fixed on trunk by always calling that function, not just in templates.
With that change, I needed to adjust pce1 to not require that a
variable be initialized yet, so that we can check it within the
initializer.  To avoid that causing some missed errors,
decl_maybe_constant_var_p now considers the initializer if it is
already known.

Fixed on 7 branch more simply, by calling p_c_e from
cxx_eval_constant_expression.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.

Attachment: 81073.diff
Description: Text document

Attachment: 81073-7.diff
Description: Text document