Removing obsolete eigen.h warning suppression pragmas. (#3198)

* Removing all pragma from eigen.h

* Removing -Werror or equivalent from tests/CMakeLists.txt

* Restoring tests/CMakeLists.txt from master.

* Adding 4 PYBIND11_SILENCE_MSVC_C4127.

* Compatibility with -Wconversion, -Wdeprecated

* Introducing PYBIND11_COMPATIBILITY_WDEPRECATED_COPY

* Systematically using --verbose for compilations where possible (cmake 3.14 or newer).

Also changing all `cmake -t` to `--target`, `-v` to `--verbose`, `check` to `pytest`, for consistency (to make it easier to pin-point all commands of a certain type).

Also removing one `-j 2` for `pytest` in hopes of reducing flakes due to races in test_iostream and in prints from destructors.

* Commenting out pragmas as an experiment to reproduce previous observation.

* Removing all (newly added, but already commented-out) pragma code, adding HINT use -isystem (as cmake does).

* Restoring ci.yml from master. Those changes are better handled separately. BTW: in the last CI run there was still a test_iostream flake, even without the -j 2 for running the tests (verfied by inspecting the log).

Author: rwgk

include/pybind11/eigen.h

@@ -9,29 +9,13 @@
 
 #pragma once
 
-#include "numpy.h"
-
-#if defined(__INTEL_COMPILER)
-#  pragma warning(disable: 1682) // implicit conversion of a 64-bit integral type to a smaller integral type (potential portability problem)
-#elif defined(__GNUG__) || defined(__clang__)
-#  pragma GCC diagnostic push
-#  pragma GCC diagnostic ignored "-Wconversion"
-#  pragma GCC diagnostic ignored "-Wdeprecated-declarations"
-#  ifdef __clang__
-//   Eigen generates a bunch of implicit-copy-constructor-is-deprecated warnings with -Wdeprecated
-//   under Clang, so disable that warning here:
-#    pragma GCC diagnostic ignored "-Wdeprecated"
-#  endif
-#  if __GNUC__ >= 7
-#    pragma GCC diagnostic ignored "-Wint-in-bool-context"
-#  endif
-#endif
+/* HINT: To suppress warnings originating from the Eigen headers, use -isystem.
+   See also:
+       https://stackoverflow.com/questions/2579576/i-dir-vs-isystem-dir
+       https://stackoverflow.com/questions/1741816/isystem-for-ms-visual-studio-c-compiler
+*/
 
-#if defined(_MSC_VER)
-#  pragma warning(push)
-#  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
-#  pragma warning(disable: 4996) // warning C4996: std::unary_negate is deprecated in C++17
-#endif
+#include "numpy.h"
 
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
@@ -153,7 +137,8 @@ template <typename Type_> struct EigenProps {
                 np_cols = a.shape(1),
                 np_rstride = a.strides(0) / static_cast<ssize_t>(sizeof(Scalar)),
                 np_cstride = a.strides(1) / static_cast<ssize_t>(sizeof(Scalar));
-            if ((fixed_rows && np_rows != rows) || (fixed_cols && np_cols != cols))
+            if ((PYBIND11_SILENCE_MSVC_C4127(fixed_rows) && np_rows != rows) ||
+                (PYBIND11_SILENCE_MSVC_C4127(fixed_cols) && np_cols != cols))
                 return false;
 
             return {np_rows, np_cols, np_rstride, np_cstride};
@@ -165,7 +150,7 @@ template <typename Type_> struct EigenProps {
               stride = a.strides(0) / static_cast<ssize_t>(sizeof(Scalar));
 
         if (vector) { // Eigen type is a compile-time vector
-            if (fixed && size != n)
+            if (PYBIND11_SILENCE_MSVC_C4127(fixed) && size != n)
                 return false; // Vector size mismatch
             return {rows == 1 ? 1 : n, cols == 1 ? 1 : n, stride};
         }
@@ -179,7 +164,7 @@ template <typename Type_> struct EigenProps {
             if (cols != n) return false;
             return {1, n, stride};
         } // Otherwise it's either fully dynamic, or column dynamic; both become a column vector
-            if (fixed_rows && rows != n) return false;
+            if (PYBIND11_SILENCE_MSVC_C4127(fixed_rows) && rows != n) return false;
             return {n, 1, stride};
     }
 
@@ -596,9 +581,3 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
 
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
-
-#if defined(__GNUG__) || defined(__clang__)
-#  pragma GCC diagnostic pop
-#elif defined(_MSC_VER)
-#  pragma warning(pop)
-#endif