[clang] [Clang] prevent an assertion failure caused by C++ constant expression checks in C23 floating conversions (PR #174113)

[Shafik Yaghmour via cfe-commits]

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@@ -401,3 +401,11 @@ bool issue155507(v2int16_t a, v2int16_t b) {
 
 constexpr bool b2 = (bool)nullptr;
 _Static_assert(!b2);
+
+double ghissue173847(double a) {
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shafik wrote:

We should have also tested `long double` which also crashes in this case. 

nit, I thought the naming convention was `gh173847`?

https://github.com/llvm/llvm-project/pull/174113