Re: [C++ Patch] PR 51225

[Jason Merrill]

So, the issue here is that fold_non_dependent_expr_sfinae checks 
potential_constant_expression, and doesn't fold anything which isn't one.

One approach would be to only guard cxx_constant_value with 
require_potential_constant_expression within a template.

Another approach would be to check potential_constant_expression at the 
same place that we check type/value_dependent_expression_p, i.e. in 
cp_finish_decl and convert_template_argument.

Jason