fix: add a test and comment

Author: henryiii

.github/workflows/ci.yml

@@ -69,7 +69,8 @@ jobs:
         python-version: ${{ matrix.python }}
 
     - name: Setup Boost (Linux)
-      if: runner.os == 'Linux'
+      # Can't use boost + define _
+      if: runner.os == 'Linux' && matrix.python != '3.6'
       run: sudo apt-get install libboost-dev
 
     - name: Setup Boost (macOS)

include/pybind11/detail/descr.h

@@ -58,6 +58,7 @@ constexpr descr<N - 1> const_str(char const(&text)[N]) { return descr<N - 1>(tex
 constexpr descr<0> const_str(char const(&)[1]) { return {}; }
 
 // The "_" might be defined as a macro - don't define it if so.
+// Repeating the const_str code to avoid introducing a #define.
 #ifndef _
 template <size_t N>
 constexpr descr<N - 1> _(char const(&text)[N]) { return descr<N - 1>(text); }

tests/test_custom_type_casters.cpp

@@ -18,7 +18,12 @@ class ArgAlwaysConverts { };
 namespace pybind11 { namespace detail {
 template <> struct type_caster<ArgInspector1> {
 public:
+    // Classic
+#ifndef _
+    PYBIND11_TYPE_CASTER(ArgInspector1, _("ArgInspector1"));
+#else
     PYBIND11_TYPE_CASTER(ArgInspector1, const_str("ArgInspector1"));
+#endif
 
     bool load(handle src, bool convert) {
         value.arg = "loading ArgInspector1 argument " +