Limiting pragma for ignoring GCC 7 -Wnoexcept-type to the scope of pybind11.h. (#3161)

* Moving pragma for ignoring -Wnoexcept-type to the one location where it is needed.

* Trying a second location.

* The previous commit worked (GitHub Actions green), but see the added comment about the dicy nature of -Wnoexcept-type ("if and only if").

* Applying reviewer suggestion.

include/pybind11/pybind11.h

@@ -18,9 +18,6 @@
 #  pragma GCC diagnostic push
 #  pragma GCC diagnostic ignored "-Wunused-but-set-parameter"
 #  pragma GCC diagnostic ignored "-Wattributes"
-#  if __GNUC__ >= 7
-#    pragma GCC diagnostic ignored "-Wnoexcept-type"
-#  endif
 #endif
 
 #include "attr.h"
@@ -48,6 +45,18 @@
 #  include <cxxabi.h>
 #endif
 
+/* https://stackoverflow.com/questions/46798456/handling-gccs-noexcept-type-warning
+   This warning is about ABI compatibility, not code health.
+   It is only actually needed in a couple places, but apparently GCC 7 "generates this warning if
+   and only if the first template instantiation ... involves noexcept" [stackoverflow], therefore
+   it could get triggered from seemingly random places, depending on user code.
+   No other GCC version generates this warning.
+ */
+#if defined(__GNUC__) && __GNUC__ == 7
+#    pragma GCC diagnostic push
+#    pragma GCC diagnostic ignored "-Wnoexcept-type"
+#endif
+
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 PYBIND11_NAMESPACE_BEGIN(detail)
@@ -2376,6 +2385,10 @@ inline function get_overload(const T *this_ptr, const char *name) {
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 
+#if defined(__GNUC__) && __GNUC__ == 7
+#    pragma GCC diagnostic pop // -Wnoexcept-type
+#endif
+
 #if defined(_MSC_VER) && !defined(__INTEL_COMPILER)
 #  pragma warning(pop)
 #elif defined(__GNUG__) && !defined(__clang__) && !defined(__INTEL_COMPILER)