`<random>`: `piecewise_linear_distribution<T>::param_type` should not inherit from `discrete_distribution<size_t>::param_type`

[statementreply]

Describe the bug

Currently, piecewise_constant_distribution<T>::param_type and piecewise_linear_distribution<T>::param_type inherit from discrete_distribution<size_t>::param_type, although conceptually they are unrelated types. piecewise_linear_distribution<T>::param_type even breaks the invariants of its base class.

Invalid code could compile and run unexpectedly, and return nonsense.

Command-line test case

C:\Users\He\source\test>type random_param_inherit.cpp

#include <cstddef>
#include <random>

using namespace std;

int main() {
    piecewise_linear_distribution<> d1;
    discrete_distribution<size_t> d2(d1.param());
    return 0;
}

C:\Users\He\source\test>cl /EHsc /W4 /WX random_param_inherit.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.28.29812 版
版权所有(C) Microsoft Corporation。保留所有权利。

random_param_inherit.cpp
Microsoft (R) Incremental Linker Version 14.28.29812.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:random_param_inherit.exe
random_param_inherit.obj

Expected behavior

The test code should not compile.

STL version

https://github.com/microsoft/STL/commit/721e3ad8952fce3447916ecdbff3863043686fa1

[StephanTLavavej]

I've recently been extremely aggressive about refactoring <random> to change representations, including altering the inheritance hierarchies of engines, without affecting layout. I am certain that we can make this much-less-impactful change in the same way while preserving ABI.