gdb/python/python-internal.h: avoid uninitialized constexpr

The following recent change introduced a regression when building using
clang++:

    commit 764af878259768bb70c65bdf3f3285c2d6409bbd
    Date:   Wed Jun 12 18:58:49 2024 +0200

        [gdb/python] Add typesafe wrapper around PyObject_CallMethod

The error message is:

    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
      CXX    python/py-block.o
    1 error generated.
    make[2]: *** [Makefile:1959: python/py-arch.o] Error 1
    make[2]: *** Waiting for unfinished jobs....
    In file included from ../../gdb/python/py-auto-load.c:25:
    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
    1 error generated.
    make[2]: *** [Makefile:1959: python/py-auto-load.o] Error 1
    In file included from ../../gdb/python/py-block.c:23:
    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
    1 error generated.

This patch fixes this by changing gdbpy_method_format to be a templated
struct, and only have its specializations define the static constexpr
member "format".  This way, we avoid having an uninitialized constexpr
expression, regardless of it being instantiated or not.

Reviewed-By: Tom de Vries <tdevries@suse.de>
Change-Id: I5bec241144f13500ef78daea30f00d01e373692d

1 files changed, 18 insertions, 6 deletions

diff --git a/gdb/python/python-internal.h b/gdb/python/python-internal.h
index f4c35ba..fec0010 100644
--- a/gdb/python/python-internal.h
+++ b/gdb/python/python-internal.h
@@ -148,19 +148,31 @@ typedef long Py_hash_t;
 /* A template variable holding the format character (as for
    Py_BuildValue) for a given type.  */
 template<typename T>
-constexpr char gdbpy_method_format;
+struct gdbpy_method_format {};
 
 template<>
-constexpr char gdbpy_method_format<gdb_py_longest> = GDB_PY_LL_ARG[0];
+struct gdbpy_method_format<gdb_py_longest>
+{
+  static constexpr char format = GDB_PY_LL_ARG[0];
+};
 
 template<>
-constexpr char gdbpy_method_format<gdb_py_ulongest> = GDB_PY_LLU_ARG[0];
+struct gdbpy_method_format<gdb_py_ulongest>
+{
+  static constexpr char format = GDB_PY_LLU_ARG[0];
+};
 
 template<>
-constexpr char gdbpy_method_format<int> = 'i';
+struct gdbpy_method_format<int>
+{
+  static constexpr char format = 'i';
+};
 
 template<>
-constexpr char gdbpy_method_format<unsigned> = 'I';
+struct gdbpy_method_format<unsigned>
+{
+  static constexpr char format = 'I';
+};
 
 /* A helper function to compute the PyObject_CallMethod /
    Py_BuildValue format given the argument types.  */
@@ -169,7 +181,7 @@ template<typename... Args>
 constexpr std::array<char, sizeof... (Args) + 1>
 gdbpy_make_fmt ()
 {
-  return { gdbpy_method_format<Args>..., '\0' };
+  return { gdbpy_method_format<Args>::format..., '\0' };
 }
 
 /* Typesafe wrapper around PyObject_CallMethod.