Re: Possible to declare move constructor of basic_symbol as noexcept?

[Akim Demaille]

Adrian,

See below for my stab at this issue.  Unfortunately it seems to hit
a problem in GCC10, as the CI features several failures as follows:

> In file included from ../examples/c++/calc++/driver.hh:24,
>                  from ../examples/c++/calc++/driver.cc:20:
> ./examples/c++/calc++/parser.hh: In instantiation of 
> 'yy::parser::basic_symbol<Base>::basic_symbol(typename Base::kind_type, 
> int&&, yy::parser::location_type&&) [with Base = yy::parser::by_kind; 
> typename Base::kind_type = yy::parser::token::token_kind_type; 
> yy::parser::location_type = yy::location]':
> ./examples/c++/calc++/parser.hh:742:68:   required from here
> ./examples/c++/calc++/parser.hh:583:7: internal compiler error: Segmentation 
> fault
>   583 |       basic_symbol (typename Base::kind_type t, int&& v, 
> location_type&& l)
>       |       ^~~~~~~~~~~~
> Please submit a full bug report,
> with preprocessed source if appropriate.
> See <file:///usr/share/doc/gcc-10/README.Bugs> for instructions.

I'm not super happy to have to play tricks with compiler macros
to be able to know if I can trust the compiler :(  If you see
something wrong in my approach, if you see a nice way to address
this portability issue, please state it.

The logs of the CI for this commit are here:

https://travis-ci.org/github/akimd/bison/builds/755133452

Cheers!

commit a7d1aa221efad101d82817345361647fd02a215c
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Mon Jan 18 08:03:44 2021 +0100

    c++: use YY_NOEXCEPT on the constructor when possible
    
    Suggested by Adrian <withoutpointk@gmail.com>.
    https://lists.gnu.org/r/help-bison/2021-01/msg00008.html
    
    * data/skeletons/variant.hh (b4_is_nothrow_move_constructible): New.
    * data/skeletons/c++.m4: Use it to qualify the basic_symbol ctor.

diff --git a/data/skeletons/c++.m4 b/data/skeletons/c++.m4
index cfd4e6ed7..3496450e2 100644
--- a/data/skeletons/c++.m4
+++ b/data/skeletons/c++.m4
@@ -309,7 +309,9 @@ m4_define([b4_symbol_type_define],
 
 #if 201103L <= YY_CPLUSPLUS
       /// Move constructor.
-      basic_symbol (basic_symbol&& that)
+      basic_symbol (basic_symbol&& that)]b4_variant_if([
+        YY_NOEXCEPT (]b4_type_foreach([b4_is_nothrow_move_constructible], [[
+                     && ]])[)])[
         : Base (std::move (that))
         , value (]b4_variant_if([], [std::move (that.value)]))b4_locations_if([
         , location (std::move (that.location))])[
diff --git a/data/skeletons/variant.hh b/data/skeletons/variant.hh
index ac65c622b..644c5a788 100644
--- a/data/skeletons/variant.hh
+++ b/data/skeletons/variant.hh
@@ -83,7 +83,11 @@ m4_define([b4_variant_includes],
 # include <cassert>
 # define ]b4_assert[ assert
 #endif
-]])])
+]])
+#if 201103L <= YY_CPLUSPLUS
+# include <type_traits>
+#endif
+])
 
 
 
@@ -497,7 +501,8 @@ m4_define([b4_basic_symbol_constructor_define],
       basic_symbol (]b4_join(
           [typename Base::kind_type t],
           b4_symbol_if([$1], [has_type], [b4_symbol([$1], [type])&& v]),
-          b4_locations_if([location_type&& l]))[)
+          b4_locations_if([location_type&& l]))[)]b4_symbol_if([$1], 
[has_type], [
+        YY_NOEXCEPT (b4_is_nothrow_move_constructible([$1]))])[
         : Base (t)]b4_symbol_if([$1], [has_type], [
         , value (std::move (v))])[]b4_locations_if([
         , location (std::move (l))])[
@@ -515,6 +520,15 @@ m4_define([b4_basic_symbol_constructor_define],
 ]])
 
 
+# b4_is_nothrow_move_constructible(SYMBOL-NUM)
+# --------------------------------------------
+# Participates in the predicate to enable basic_symbol's ctor to be noexpect.
+m4_define([b4_is_nothrow_move_constructible],
+[b4_symbol_if([$1], [has_type],
+              [[std::is_nothrow_move_constructible< ]b4_symbol([$1], [type])[ 
>::value]],
+              [true])])
+
+
 # b4_token_constructor_define
 # ---------------------------
 # Define the overloaded versions of make_symbol for all the value types.