fold_inplace initial value is not constexpr

[Life4gal]

I need to use a custom allocator to manage memory (especially GC), but I found that MSVC refused to compile my code.

error C2131: expression did not evaluate to a constant
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.33.31629\include\xmemory(1062):
note: failure was caused by call of undefined function or one not declared 'constexpr' 
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.33.31629\include\xmemory(1062):
note: see usage of 'StlAllocator<_Newfirst>::allocate' with
[
    _Newfirst=std::_Container_proxy
]

Here is a godbolt link(GCC/CLang works well)

[foonathan]

Instead of passing an initial value, pass a lambda that returns that value: https://godbolt.org/z/xoa1xdbGj

[Life4gal]

It solves the problem, but why?

In Example 2. Parse a list of numbers, but store them in reverse, it also passes a container (initializer_list), but why can it run? I also tried to change it to vector (with std::allocator), and it still works

[foonathan]

Without the lambda, it requires that the initial element is constexpr. The other types are, but the container with custom allocator isn't on MSVC.

[Life4gal]

Another puzzling point is that once I put them into lambda, it will not compile. :(

[foonathan]

You need to immediately invoke the lambda:

rule = []{ ... }; -> rule = []{ ... }();

[Life4gal]

Oh, I forgot it!