c++: tsubst_function_decl and excess arg levels [PR100102]

Here, when instantiating the dependent alias template
duration::__is_harmonic with args={{T,U},{int}}, we find ourselves
substituting the function decl _S_gcd.  Since we have more arg levels
than _S_gcd has parm levels, an old special case in tsubst_function_decl
causes us to unwantedly reduce args to its innermost level, yielding
args={int}, which leads to a nonsensical substitution into the decl's
context and an eventual crash.

The comment for this special case refers to three examples for which we
ought to see more arg levels than parm levels here, but none of the
examples actually demonstrate this.  In the first example, when
defining S<int>::f(U) parms_depth is 2 and args_depth is 1, and
later when instantiating say S<int>::f<char> both depths are 2.  In the
second example, when substituting the template friend declaration
parms_depth is 2 and args_depth is 1, and later when instantiating f
both depths are 1.  Finally, the third example is invalid since we can't
specialize a member template of an unspecialized class template like
that.

Given that this reduction code seems no longer relevant for its
documented purpose and that it causes problems as in the PR, this patch
just removes it.  Note that as far as bootstrap/regtest is concerned,
this code is dead; the below two tests would be the first to trigger the
removed code.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and perhaps backports?  Also tested on various other libraries,
e.g. range-v3 and cmcstl2.

	PR c++/100102

gcc/cp/ChangeLog:

	* pt.c (tsubst_function_decl): Remove old code for reducing
	args when it has excess levels.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-72.C: New test.
	* g++.dg/cpp0x/alias-decl-72a.C: New test.
---
 gcc/cp/pt.c                                 | 39 ---------------------
 gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C  |  9 +++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C |  9 +++++
 3 files changed, 18 insertions(+), 39 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C

On 6/3/21 11:46 PM, Patrick Palka wrote:
> Here, when instantiating the dependent alias template
> duration::__is_harmonic with args={{T,U},{int}}, we find ourselves
> substituting the function decl _S_gcd.  Since we have more arg levels
> than _S_gcd has parm levels, an old special case in tsubst_function_decl
> causes us to unwantedly reduce args to its innermost level, yielding
> args={int}, which leads to a nonsensical substitution into the decl's
> context and an eventual crash.
> 
> The comment for this special case refers to three examples for which we
> ought to see more arg levels than parm levels here, but none of the
> examples actually demonstrate this.  In the first example, when
> defining S<int>::f(U) parms_depth is 2 and args_depth is 1, and
> later when instantiating say S<int>::f<char> both depths are 2.  In the
> second example, when substituting the template friend declaration
> parms_depth is 2 and args_depth is 1, and later when instantiating f
> both depths are 1.  Finally, the third example is invalid since we can't
> specialize a member template of an unspecialized class template like
> that.
> 
> Given that this reduction code seems no longer relevant for its
> documented purpose and that it causes problems as in the PR, this patch
> just removes it.  Note that as far as bootstrap/regtest is concerned,
> this code is dead; the below two tests would be the first to trigger the
> removed code.

Interesting!

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk and perhaps backports?  Also tested on various other libraries,
> e.g. range-v3 and cmcstl2.

OK I think for 10/11/12; 9 doesn't have the <chrono> change that 
revealed this issue.

> 	PR c++/100102
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (tsubst_function_decl): Remove old code for reducing
> 	args when it has excess levels.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/alias-decl-72.C: New test.
> 	* g++.dg/cpp0x/alias-decl-72a.C: New test.
> ---
>   gcc/cp/pt.c                                 | 39 ---------------------
>   gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C  |  9 +++++
>   gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C |  9 +++++
>   3 files changed, 18 insertions(+), 39 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 3cac073ed50..a6acdf864d1 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -13909,45 +13909,6 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
>   	  if (tree spec = retrieve_specialization (gen_tmpl, argvec, hash))
>   	    return spec;
>   	}
> -
> -      /* We can see more levels of arguments than parameters if
> -	 there was a specialization of a member template, like
> -	 this:
> -
> -	 template <class T> struct S { template <class U> void f(); }
> -	 template <> template <class U> void S<int>::f(U);
> -
> -	 Here, we'll be substituting into the specialization,
> -	 because that's where we can find the code we actually
> -	 want to generate, but we'll have enough arguments for
> -	 the most general template.
> -
> -	 We also deal with the peculiar case:
> -
> -	 template <class T> struct S {
> -	   template <class U> friend void f();
> -	 };
> -	 template <class U> void f() {}
> -	 template S<int>;
> -	 template void f<double>();
> -
> -	 Here, the ARGS for the instantiation of will be {int,
> -	 double}.  But, we only need as many ARGS as there are
> -	 levels of template parameters in CODE_PATTERN.  We are
> -	 careful not to get fooled into reducing the ARGS in
> -	 situations like:
> -
> -	 template <class T> struct S { template <class U> void f(U); }
> -	 template <class T> template <> void S<T>::f(int) {}
> -
> -	 which we can spot because the pattern will be a
> -	 specialization in this case.  */
> -      int args_depth = TMPL_ARGS_DEPTH (args);
> -      int parms_depth =
> -	TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (DECL_TI_TEMPLATE (t)));
> -
> -      if (args_depth > parms_depth && !DECL_TEMPLATE_SPECIALIZATION (t))
> -	args = get_innermost_template_args (args, parms_depth);
>       }
>     else
>       {
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C
> new file mode 100644
> index 00000000000..8009756dcba
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-72.C
> @@ -0,0 +1,9 @@
> +// PR c++/100102
> +// { dg-do compile { target c++11 } }
> +
> +template<int()> struct ratio;
> +template<class T, class U> struct duration {
> +  static constexpr int _S_gcd();
> +  template<class> using __is_harmonic = ratio<_S_gcd>;
> +  using type = __is_harmonic<int>;
> +};
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C
> new file mode 100644
> index 00000000000..a4443e18f9d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-72a.C
> @@ -0,0 +1,9 @@
> +// PR c++/100102
> +// { dg-do compile { target c++11 } }
> +
> +template<int> struct ratio;
> +template<class T> struct duration {
> +  static constexpr int _S_gcd();
> +  template<class> using __is_harmonic = ratio<(duration::_S_gcd)()>;
> +  using type = __is_harmonic<int>;
> +};
>