On 2/21/26 1:57 PM, Nathaniel Shead wrote:
Thanks for the patch! I guess this is probably small enough it doesn't
need copyright assignment (can you confirm, Jason?), but you should
consider contributing either via DCO or with copyright assignment; see
https://gcc.gnu.org/contribute.html#legal.
This does seem trivial enough, yes.
But it also seems that we can do better; if the GCM is importable, we
should import it so that we get any macros defined. And as Nathaniel
suggested in the BZ, if the header itself exists, we could #include
instead of import. The latter will be more complicated to implement,
but the former is just
import->check_importable (reader)
Does this adjusted patch look good to you?
BTW, I don't see the original message, only Nathaniel's reply.
From 9fb28bc401c4c83ac6946d8ea071baf4995cc38c Mon Sep 17 00:00:00 2001
From: Jason Merrill <[email protected]>
Date: Sat, 21 Feb 2026 17:23:30 +0900
Subject: [PATCH] c++/modules: fix -MG for header units [PR123622]
To: [email protected]
With -MG we should allow a nonexistent header unit, as we do with a
nonexistent #include. But still import it if available.
PR c++/123622
gcc/cp/ChangeLog:
* module.cc (preprocess_module): Check deps.missing_files.
gcc/testsuite/ChangeLog:
* g++.dg/modules/dep-6.C: New test.
Co-authored-by: <[email protected]>
---
gcc/cp/module.cc | 8 +++++++-
gcc/testsuite/g++.dg/modules/dep-6.C | 6 ++++++
2 files changed, 13 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/modules/dep-6.C
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index ccbf124876d..450bc70bc0e 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -23319,7 +23319,13 @@ preprocess_module (module_state *module, location_t from_loc,
if (!(import->is_module ()
&& (import->is_partition () || import->is_exported ()))
&& import->loadedness == ML_NONE
- && (import->is_header () || !flag_preprocess_only))
+ && (!flag_preprocess_only
+ || (import->is_header ()
+ /* Allow a missing/unimportable GCM with -MG.
+ FIXME We should also try falling back to #include
+ before giving up entirely. */
+ && (!cpp_get_options (reader)->deps.missing_files
+ || import->check_importable (reader))))
{
unsigned n = dump.push (import);
import->do_import (reader, true);
diff --git a/gcc/testsuite/g++.dg/modules/dep-6.C b/gcc/testsuite/g++.dg/modules/dep-6.C
new file mode 100644
index 00000000000..8a7960585a1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/dep-6.C
@@ -0,0 +1,6 @@
+// PR c++/123622
+// { dg-additional-options "-fmodules -M -MG -MF dep-6.d" }
+
+import "dep-6.h"; // Must not exist!
+
+// { dg-final { scan-file dep-6.d {\nCXX_IMPORTS \+= \./dep-6\.h\.c\+\+-module} } }
--
2.53.0
