[v2,PR91979] Updated the fix:

Changlogs
gcc
------
2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>

        PR c++/91979 - mangling nullptr expression
        * mangle.c (write_template_arg_literal): Handle nullptr
        mangling.
        * testsuite/g++.dg/cpp0x/nullptr27.C: Modify Test.
        * testsuite/g++.dg/cpp0x/pr91979.C: New Test.

libiberty
-----------
2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>

        * cp-demangle.c (d_expr_primary): Handle
        nullptr demangling.
        * testsuite/demangle-expected: Added test.

gcc/c-family
----------
2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>

        * c-opts.c (c_common_post_options): Updated
        latest_abi_version.
---
 gcc/c-family/c-opts.c                  |  2 +-
 gcc/cp/mangle.c                        |  4 +++-
 gcc/testsuite/g++.dg/cpp0x/nullptr27.C |  2 +-
 gcc/testsuite/g++.dg/cpp0x/pr91979.C   | 15 +++++++++++++++
 libiberty/cp-demangle.c                | 11 +++++++++++
 libiberty/testsuite/demangle-expected  |  4 ++++
 6 files changed, 35 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr91979.C

On 11/1/19 11:47 PM, Kamlesh Kumar wrote:
> Changlogs
> gcc
> ------
> 2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>
> 
>          PR c++/91979 - mangling nullptr expression
>          * mangle.c (write_template_arg_literal): Handle nullptr
>          mangling.
>          * testsuite/g++.dg/cpp0x/nullptr27.C: Modify Test.
>          * testsuite/g++.dg/cpp0x/pr91979.C: New Test.
> 
> libiberty
> -----------
> 2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>
> 
>          * cp-demangle.c (d_expr_primary): Handle
>          nullptr demangling.
>          * testsuite/demangle-expected: Added test.
> 
> gcc/c-family
> ----------
> 2019-11-02  Kamlesh Kumar  <kamleshbhalui@gmail.com>
> 
>          * c-opts.c (c_common_post_options): Updated
>          latest_abi_version.
> ---
>   gcc/c-family/c-opts.c                  |  2 +-
>   gcc/cp/mangle.c                        |  4 +++-
>   gcc/testsuite/g++.dg/cpp0x/nullptr27.C |  2 +-
>   gcc/testsuite/g++.dg/cpp0x/pr91979.C   | 15 +++++++++++++++
>   libiberty/cp-demangle.c                | 11 +++++++++++
>   libiberty/testsuite/demangle-expected  |  4 ++++
>   6 files changed, 35 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr91979.C
> 
> diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
> index 0fffe60b140..d4c77be5cd5 100644
> --- a/gcc/c-family/c-opts.c
> +++ b/gcc/c-family/c-opts.c
> @@ -937,7 +937,7 @@ c_common_post_options (const char **pfilename)
>   
>     /* Change flag_abi_version to be the actual current ABI level, for the
>        benefit of c_cpp_builtins, and to make comparison simpler.  */
> -  const int latest_abi_version = 13;
> +  const int latest_abi_version = 14;

Please also add a comment about ABI v14 above fabi-version= in 
gcc/common.opt.

>     /* Generate compatibility aliases for ABI v11 (7.1) by default.  */
>     const int abi_compat_default = 11;
>   
> diff --git a/gcc/cp/mangle.c b/gcc/cp/mangle.c
> index a9333b84349..234a975781e 100644
> --- a/gcc/cp/mangle.c
> +++ b/gcc/cp/mangle.c
> @@ -3400,7 +3400,9 @@ write_template_arg_literal (const tree value)
>         case INTEGER_CST:
>   	gcc_assert (!same_type_p (TREE_TYPE (value), boolean_type_node)
>   		    || integer_zerop (value) || integer_onep (value));
> -	write_integer_cst (value);
> +	if (abi_version_at_least(14)
> +	    && !NULLPTR_TYPE_P (TREE_TYPE (value)))

This logic is wrong: it will never emit the value for ABI version lower 
than 14.  And you need a space before (.

> +	  write_integer_cst (value);
>   	break;
>   
>         case REAL_CST:
> diff --git a/gcc/testsuite/g++.dg/cpp0x/nullptr27.C b/gcc/testsuite/g++.dg/cpp0x/nullptr27.C
> index 2510dc80634..edd11606266 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/nullptr27.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/nullptr27.C
> @@ -1,7 +1,7 @@
>   // PR c++/52706
>   // { dg-do compile { target c++11 } }
>   // { dg-options "-fabi-version=0" }
> -// { dg-final { scan-assembler "_Z1fIDnLDn0EEiT_" } }
> +// { dg-final { scan-assembler "_Z1fIDnLDnEEiT_" } }
>   
>   template<class T, decltype(nullptr) = nullptr>
>   int f(T);

Please also add a test for the old mangled name with -fabi-version=13.

> diff --git a/gcc/testsuite/g++.dg/cpp0x/pr91979.C b/gcc/testsuite/g++.dg/cpp0x/pr91979.C
> new file mode 100644
> index 00000000000..7fcd56b27f0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/pr91979.C
> @@ -0,0 +1,15 @@
> +// PR c++/91989
> +// { dg-do compile { target c++11 } }
> +// { dg-final { scan-assembler "_Z3fooILPv0EEvPN9enable_ifIXeqT_LDnEEvE4typeE" } }
> +
> +template <bool, typename T = void>
> +struct enable_if {};
> +
> +template <typename T>
> +struct enable_if<true, T> { typedef T type; };
> +
> +template <void *P>
> +void foo(typename enable_if<P == nullptr>::type* = 0) {}
> +
> +template void foo<(void *)0>(void *);
> +
> diff --git a/libiberty/cp-demangle.c b/libiberty/cp-demangle.c
> index 5b674d7d93c..3fb1c56409e 100644
> --- a/libiberty/cp-demangle.c
> +++ b/libiberty/cp-demangle.c
> @@ -3577,6 +3577,17 @@ d_expr_primary (struct d_info *di)
>   	  && type->u.s_builtin.type->print != D_PRINT_DEFAULT)
>   	di->expansion -= type->u.s_builtin.type->len;
>   
> +      if (type->type == DEMANGLE_COMPONENT_BUILTIN_TYPE
> +	  && strncmp (type->u.s_builtin.type->name,
> +		      cplus_demangle_builtin_types[33].name, 17) == 0)

Let's use strcmp rather than hardcode 17 here.

> +	{
> +	  if (d_peek_char (di) == 'E')
> +	    {
> +	      d_advance (di, 1);
> +	      return type;
> +	    }
> +	}
> +
>         /* Rather than try to interpret the literal value, we just
>   	 collect it as a string.  Note that it's possible to have a
>   	 floating point literal here.  The ABI specifies that the
> diff --git a/libiberty/testsuite/demangle-expected b/libiberty/testsuite/demangle-expected
> index 61681484d0e..f68a8a71aaf 100644
> --- a/libiberty/testsuite/demangle-expected
> +++ b/libiberty/testsuite/demangle-expected
> @@ -1446,3 +1446,7 @@ Foo<int>()::X::fn
>   _ZZZ3FooIiEfvENKUlT_E_clIcEEDaS0_EN1X2fnEv
>   Foo<int>()::{lambda(auto:1)#1}::operator()<char>(char) const::X::fn()
>   Foo<int>()::{lambda(auto:1)#1}::operator()<char>(char) const::X::fn
> +#PR91979 demangling nullptr expression
> +
> +_Z3fooILPv0EEvPN9enable_ifIXeqT_LDnEEvE4typeE
> +void foo<(void*)0>(enable_if<((void*)0)==((decltype(nullptr))), void>::type*)
>