Why has std::accumulate not been made constexpr in C++20?

P1645R1 was actually adopted in the Belfast meeting for inclusion in C++20 in response to NB comment US 320.

As a result, all of the following algorithms will be constexpr in C++20 (except for the overloads of them that take an ExecutionPolicy):

• accumulate
• reduce
• inner_product
• transform_reduce
• partial_sum
• exclusive_scan
• inclusive_scan
• transform_exclusive_scan
• transform_inclusive_scan
• adjacent_difference
• iota

TL;DR;

There is a proposal in process. We won't know the result until the process is done but it won't make C++20.

Longer answer

There was indeed a proposal: constexpr for algorithms, quoting the relevant part:

This proposal is to add constexpr to the following function templates in , excepting the function templates that accept an ExecutionPolicy.

• accumulate

...

We can see from the cplusplus/papers issue 432 that the paper was moved to Language Evolution Working Group:

R0, needs to be looked at / forwarded by LEWG. Removing the LWG tag.

and the milestone was moved to 2019-11:

modified the milestones: 2019-07, 2019-11

which would be the upcomoing Belfast meeting, so it will not make C++20.