[PATCH] c++/modules: Prevent overwriting arguments when merging duplicates [PR112588]

[Nathaniel Shead <nathanieloshead@gmail.com>]

Ping for https://gcc.gnu.org/pipermail/gcc-patches/2023-November/637768.html.
(I've changed the summary message a little from that email but the patch
is otherwise unchanged.)

On Wed, Nov 22, 2023 at 10:33:15PM +1100, Nathaniel Shead wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu. I don't have write
> access.
> 
> -- >8 --
> 
> When merging duplicate instantiations of function templates, currently
> read_function_def overwrites the arguments with that of the existing
> duplicate. This is problematic, however, since this means that the
> PARM_DECLs in the body of the function definition no longer match with
> the PARM_DECLs in the argument list, which causes issues when it comes
> to generating RTL.
> 
> There doesn't seem to be any reason to do this replacement, so this
> patch removes that logic.
> 
> 	PR c++/112588
> 
> gcc/cp/ChangeLog:
> 
> 	* module.cc (trees_in::read_function_def): Don't overwrite
> 	arguments.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/merge-16.h: New test.
> 	* g++.dg/modules/merge-16_a.C: New test.
> 	* g++.dg/modules/merge-16_b.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  gcc/cp/module.cc                          |  2 --
>  gcc/testsuite/g++.dg/modules/merge-16.h   | 10 ++++++++++
>  gcc/testsuite/g++.dg/modules/merge-16_a.C |  7 +++++++
>  gcc/testsuite/g++.dg/modules/merge-16_b.C |  5 +++++
>  4 files changed, 22 insertions(+), 2 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-16.h
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-16_a.C
>  create mode 100644 gcc/testsuite/g++.dg/modules/merge-16_b.C
> 
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index 4f5b6e2747a..2520ab659cc 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -11665,8 +11665,6 @@ trees_in::read_function_def (tree decl, tree maybe_template)
>        DECL_RESULT (decl) = result;
>        DECL_INITIAL (decl) = initial;
>        DECL_SAVED_TREE (decl) = saved;
> -      if (maybe_dup)
> -	DECL_ARGUMENTS (decl) = DECL_ARGUMENTS (maybe_dup);
>  
>        if (context)
>  	SET_DECL_FRIEND_CONTEXT (decl, context);
> diff --git a/gcc/testsuite/g++.dg/modules/merge-16.h b/gcc/testsuite/g++.dg/modules/merge-16.h
> new file mode 100644
> index 00000000000..fdb38551103
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-16.h
> @@ -0,0 +1,10 @@
> +// PR c++/112588
> +
> +void f(int*);
> +
> +template <typename T>
> +struct S {
> +  void g(int n) { f(&n); }
> +};
> +
> +template struct S<void>;
> diff --git a/gcc/testsuite/g++.dg/modules/merge-16_a.C b/gcc/testsuite/g++.dg/modules/merge-16_a.C
> new file mode 100644
> index 00000000000..c243224c875
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-16_a.C
> @@ -0,0 +1,7 @@
> +// PR c++/112588
> +// { dg-additional-options "-fmodules-ts" }
> +// { dg-module-cmi merge16 }
> +
> +module;
> +#include "merge-16.h"
> +export module merge16;
> diff --git a/gcc/testsuite/g++.dg/modules/merge-16_b.C b/gcc/testsuite/g++.dg/modules/merge-16_b.C
> new file mode 100644
> index 00000000000..8c7b1f0511f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/merge-16_b.C
> @@ -0,0 +1,5 @@
> +// PR c++/112588
> +// { dg-additional-options "-fmodules-ts" }
> +
> +#include "merge-16.h"
> +import merge16;
> -- 
> 2.42.0
>