[C++ Patch] PR 83921 ("[7/8 Regression] GCC rejects constexpr initialization of empty aggregate")

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

Hi,

I'm finishing testing on x86_64-linux the below - which anyway seems 
very unlikely to cause regressions because we aren't really stress 
testing the relevant checks in potential_constant_expression_1 much, if 
at all (surely stmtexpr19.C tests static).

Anyway, the issue is the following. In 239268 aka "Implement C++17 
constexpr lambda" Jason added some checks to 
potential_constant_expression_1 covering static, thread_local and 
uninitialized var declaration in constexpr function context. Then 
extended to constexpr context more generally in 249382 aka "constexpr 
and static var in statement-expression", with ext/stmtexpr19.C covering 
the static case. Now, it looks like the check for uninitialized vars in 
constexpr functions context is more correctly carried out by 
check_for_uninitialized_const_var instead, because the simple check in 
potential_constant_expression_1 as-is causes the regression pointed out 
by this bug. Thus the fix below which just restricts the check in 
potential_constant_expression_1, and the testcases, one for this bug 
proper, plus one, very similar to stmtexpr19.C, double checking that we 
are still diagnosing in the statement-expression context. I also 
verified under the debugger how for constexpr-83921.C we are actually 
running check_for_uninitialized_const_var on 'f' - which obviously passes.

Thanks, Paolo.

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