[C++ Patch] PR 59571

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

noticed this issue, which looks simple to fix. The ICE happens in 
cxx_eval_constant_expression, because it cannot handle a CAST_EXPR (or 
any othe *_CAST, for that matter). In fact check_narrowing calls 
maybe_constant_value, and, because we are in a template, the latter 
faces the unfolded CAST_EXPR. Thus it seems easy to just use 
fold_non_dependent_expr_sfinae. Tested x86_64-linux.

Thanks,
Paolo.

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PS: looking forward, I'm wondering if some semantics/typeck functions 
shouldn't try harder before building a tree node and returning, eg, 
instead of just checking processing_template_decl, actually checking if 
type and expr are dependent? Does this kind of audit make sense for next 
Stage 1?

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