etl::vector::insert allows inserting an incompatible type

[relativityspace-jsmith]

I was refactoring some code and it started segfaulting. After some debugging, I realized a made a basic type error (inserting a vector of values into a value of pointers). However, I am very surprised that this was able to compile in the first place as it seems to violate basic type rules.

Here's a minimal example:

struct SomeStruct
{
    int foo;
};

etl::vector<SomeStruct, 4> myStructs{{1}, {2}, {3}, {4}};

etl::vector<SomeStruct *, 4> structPointers;
structPointers.insert(structPointers.end(), myStructs.begin(), myStructs.end());
printf("structPointers[0] = %p\n", structPointers[0]);

This compiles, and on my 64-bit desktop prints

structPointers[0] = 0000000200000001

If I change it to instead use std::vector, we get the expected compile error:

C:/msys64/ucrt64/include/c++/15.2.0/bits/stl_algobase.h:407:16: error: cannot convert 'SomeStruct' to 'SomeStruct*' in assignment
  407 |         *__out = *__in;
      |         ~~~~~~~^~~~~~~

It looks like the issue here is that the underlying insert implementation (etl::pvoidvector::insert) erases the type information of the source iterators and just treats them as void ** (rather than, in this case, SomeStruct *). This leads to it basically memcpy-ing the SomeStruct entries into the destination vector.

I think the best fix here would be to add some checks in the upper level class, etl::ivector<T*>, that the iterators passed to insert() (and probably other functions that accept iterators) are compatible with the type of the vector's elements.

[jwellbelove]

I am raising a PR to only enable the assign and insert member functions for class ivector<T*> & class ivector<const T*> when the value_type the iterator references is a pointer to T.