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

[Nathaniel Shead <nathanieloshead@gmail.com>]

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