[gcc r16-8485] c++/modules: Fix propagating noexcept for templates [PR124785]

[Nathaniel Shead]

https://gcc.gnu.org/g:ca20148d0b784ce52f83a33fc3abb2693a0d6a41

commit r16-8485-gca20148d0b784ce52f83a33fc3abb2693a0d6a41
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Mon Apr 6 00:34:33 2026 +1000

    c++/modules: Fix propagating noexcept for templates [PR124785]
    
    We triggered a checking assertion because when propagating deduced
    noexcept, we were updating the type of the existing TEMPLATE_DECL but
    not the type of its DECL_TEMPLATE_RESULT, violating assumptions made
    later on during modules streaming.
    
    But actually there was nothing to propagate here anyway, these
    declarations are identical, so this patch also fixes the condition for
    checking whether we need to propagate anything.  And so now I don't
    think there is ever a case we should have a noexcept-spec to propagate
    for a TEMPLATE_DECL, so add an assertion to validate this.
    
            PR c++/124785
    
    gcc/cp/ChangeLog:
    
            * module.cc (trees_in::is_matching_decl): Narrow condition for
            when noexcept propagation occurs; assert that we don't propagate
            noexcept-specs for TEMPLATE_DECLs.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/noexcept-5.h: New test.
            * g++.dg/modules/noexcept-5_a.C: New test.
            * g++.dg/modules/noexcept-5_b.C: New test.
            * g++.dg/modules/noexcept-5_c.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/module.cc                            |  4 +++-
 gcc/testsuite/g++.dg/modules/noexcept-5.h   |  4 ++++
 gcc/testsuite/g++.dg/modules/noexcept-5_a.C |  7 +++++++
 gcc/testsuite/g++.dg/modules/noexcept-5_b.C |  7 +++++++
 gcc/testsuite/g++.dg/modules/noexcept-5_c.C | 11 +++++++++++
 5 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 5828748a3e69..1514711f8474 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -12619,12 +12619,14 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
       tree d_spec = TYPE_RAISES_EXCEPTIONS (d_type);
       if (DECL_MAYBE_DELETED (e_inner) || DEFERRED_NOEXCEPT_SPEC_P (e_spec))
 	{
-	  if (!DEFERRED_NOEXCEPT_SPEC_P (d_spec)
+	  if (!(DECL_MAYBE_DELETED (d_inner)
+		|| DEFERRED_NOEXCEPT_SPEC_P (d_spec))
 	      || (UNEVALUATED_NOEXCEPT_SPEC_P (e_spec)
 		  && !UNEVALUATED_NOEXCEPT_SPEC_P (d_spec)))
 	    {
 	      dump (dumper::MERGE)
 		&& dump ("Propagating instantiated noexcept to %N", existing);
+	      gcc_checking_assert (existing == e_inner);
 	      TREE_TYPE (existing) = d_type;
 
 	      /* Propagate to existing clones.  */
diff --git a/gcc/testsuite/g++.dg/modules/noexcept-5.h b/gcc/testsuite/g++.dg/modules/noexcept-5.h
new file mode 100644
index 000000000000..78d91322a1ab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/noexcept-5.h
@@ -0,0 +1,4 @@
+// PR c++/124785
+template <typename T> struct Iterator {
+  constexpr friend auto operator<=>(const Iterator&, const Iterator&) = default;
+};
diff --git a/gcc/testsuite/g++.dg/modules/noexcept-5_a.C b/gcc/testsuite/g++.dg/modules/noexcept-5_a.C
new file mode 100644
index 000000000000..72085717e604
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/noexcept-5_a.C
@@ -0,0 +1,7 @@
+// PR c++/124785
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules" }
+// { dg-module-cmi M:A }
+
+export module M:A;
+#include "noexcept-5.h"
diff --git a/gcc/testsuite/g++.dg/modules/noexcept-5_b.C b/gcc/testsuite/g++.dg/modules/noexcept-5_b.C
new file mode 100644
index 000000000000..928980b6c1df
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/noexcept-5_b.C
@@ -0,0 +1,7 @@
+// PR c++/124785
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules" }
+// { dg-module-cmi M:B }
+
+export module M:B;
+#include "noexcept-5.h"
diff --git a/gcc/testsuite/g++.dg/modules/noexcept-5_c.C b/gcc/testsuite/g++.dg/modules/noexcept-5_c.C
new file mode 100644
index 000000000000..d9349147766c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/noexcept-5_c.C
@@ -0,0 +1,11 @@
+// PR c++/124785
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules -fdump-lang-module-alias" }
+// { dg-module-cmi M }
+
+export module M;
+export import :A;
+export import :B;
+
+// The noexcept-specifiers are equivalent, no need to merge.
+// { dg-final { scan-lang-dump-not {Propagating instantiated noexcept} module } }