[C++ Patch/RFC] PR 67980 ("left shift count is negative [-Wshift-count-negative] generated for unreachable code")

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

Hi,

in the language of our implementations details, submitter noticed that 
in terms of warnings we handle in a different way COND_EXPRs in 
tsubst_copy_and_build - we use fold_non_dependent_expr and integer_zerop 
to suppress undesired warnings by bumping c_inhibit_evaluation_warnings 
- and IF_STMTs in tsubst_expr, where we don't. My patch below, which 
passes testing, tries in a rather straightforward way to adopt the same 
mechanisms in the latter. There are quite a few details I'm not sure 
about: whether we should only use fold_non_dependent_expr for the 
purpose of suppressing the warnings -  thus never touching 'tmp' in the 
pt.c code handling IF_STMTs - which would be completely conservative in 
terms of code generation; whether there are subtle interactions with the 
new if constexpr, which I'm missing at the moment.

Thanks!

Paolo.

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