Fix compilation of a test.

[bangerth]

This should fix #16794, broken by #16790.

The test as fixed compiles for me (it didn't without the changes) but produces different output on my machine. That said, the differences in output are produced in other parts of the test, not by the function here, so I'm pretty confident that the patch is correct.

[masterleinad]

What about lines 139 through 147? We should also replace begin_raw and end_raw there.

[bangerth]

I had done so until I realized that (i) these are for SymmetricTensor, for which the function is not deprecated or removed yet, and (ii) SymmetricTensor<4,dim> does not implement operator[].

I did verify that the test compiles with my changes. I'd like to leave everything else for a later patch.

[masterleinad]

I had done so until I realized that (i) these are for SymmetricTensor, for which the function is not deprecated or removed yet, and (ii) SymmetricTensor<4,dim> does not implement operator[].

It seems they are, see

dealii/include/deal.II/base/symmetric_tensor.h

Lines 797 to 844 in 16f23ad

	/**
	* Return a pointer to the first element of the underlying storage.
	*
	* @deprecated This function suggests that the elements of a SymmetricTensor
	* object are stored as a contiguous array, but this is not in fact true
	* and one should not pretend that this so. As a consequence, this function
	* is deprecated.
	*/
	DEAL_II_DEPRECATED
	Number *
	begin_raw();
	/**
	* Return a const pointer to the first element of the underlying storage.
	*
	* @deprecated This function suggests that the elements of a SymmetricTensor
	* object are stored as a contiguous array, but this is not in fact true
	* and one should not pretend that this so. As a consequence, this function
	* is deprecated.
	*/
	DEAL_II_DEPRECATED
	const Number *
	begin_raw() const;
	/**
	* Return a pointer to the element past the end of the underlying storage.
	*
	* @deprecated This function suggests that the elements of a SymmetricTensor
	* object are stored as a contiguous array, but this is not in fact true
	* and one should not pretend that this so. As a consequence, this function
	* is deprecated.
	*/
	DEAL_II_DEPRECATED
	Number *
	end_raw();
	/**
	* Return a const pointer to the element past the end of the underlying
	* storage.
	*
	* @deprecated This function suggests that the elements of a SymmetricTensor
	* object are stored as a contiguous array, but this is not in fact true
	* and one should not pretend that this so. As a consequence, this function
	* is deprecated.
	*/
	DEAL_II_DEPRECATED
	const Number *
	end_raw() const;

and https://github.com/dealii/dealii/blob/master/include/deal.II/base/symmetric_tensor.h#L1021-L1037.

[bangerth]

Oh. Still -- let's leave that for a non-emergency situation.