From a1379613b3471995fe00293ad42ca22d64545322 Mon Sep 17 00:00:00 2001
From: cpplearner <cpplearner@outlook.com>
Date: Sat, 17 Oct 2026 20:03:26 +0800
Subject: [PATCH] Fix SFINAE for function template with deduced return type and
 pending definition [GH #453]

---
 src/exprutil.c                                   |   8 +++++---
 src/pending_changes/gh-453.txt                   |   7 +++++++
 .../modules/gh-453-template-return-auto/A.ifc    | Bin 0 -> 1855 bytes
 .../modules/gh-453-template-return-auto/A.ixx    |   7 +++++++
 .../gh-453-template-return-auto/test.mft.cpp     |  13 +++++++++++++
 5 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 src/pending_changes/gh-453.txt
 create mode 100644 tests/tests/regressions/modules/gh-453-template-return-auto/A.ifc
 create mode 100644 tests/tests/regressions/modules/gh-453-template-return-auto/A.ixx
 create mode 100644 tests/tests/regressions/modules/gh-453-template-return-auto/test.mft.cpp

diff --git a/src/exprutil.c b/src/exprutil.c
index c9bb061c90..7a46ecca17 100644
--- a/src/exprutil.c
+++ b/src/exprutil.c
@@ -33,6 +33,7 @@ exprutil.c -- Expression scanning utility routines.
 #include "lower_il.h"
 #endif /* DO_IL_LOWERING */
 #include "statements.h"
+#include "modules.h"
 #if USE_X86_FUNCTION_MULTIVERSIONING
 #include "sys_predef.h"
 #endif /* USE_X86_FUNCTION_MULTIVERSIONING */
@@ -18149,11 +18150,12 @@ reference entry, or is NULL if none is needed.
       if (tip != NULL) {
         a_symbol_ptr  tsym;
         tsym = prototype_template_if_template_symbol(tip->template_sym);
-        if (tsym->defined) {
+        if (tsym->defined ||
+            has_pending_template_definition_from_module(
+                                                    routine->assoc_template)) {
           could_be_instantiated = TRUE;
         }  /* if */
-      } else if (routine_sym->variant.routine.ptr
-                            ->friend_defined_in_instantiation) {
+      } else if (routine->friend_defined_in_instantiation) {
         could_be_instantiated = TRUE;
       }  /* if */
       if (!could_be_instantiated) {
diff --git a/src/pending_changes/gh-453.txt b/src/pending_changes/gh-453.txt
new file mode 100644
index 0000000000..4c26018108
--- /dev/null
+++ b/src/pending_changes/gh-453.txt
@@ -0,0 +1,7 @@
+[GH #453]
+SFINAE for function template with pending definition and deduced return type
+
+With modules, it is possible that a function template definition exists in a
+module but the definition is not yet loaded.  Previously, substitution failed
+if such a function template is called in a SFINAE context and the function
+template has a deduced return type.  This is now fixed.
diff --git a/tests/tests/regressions/modules/gh-453-template-return-auto/A.ifc b/tests/tests/regressions/modules/gh-453-template-return-auto/A.ifc
new file mode 100644
index 0000000000000000000000000000000000000000..a4e1db37bf2f3cc27508524b2a653e046db99cbd
GIT binary patch
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literal 0
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diff --git a/tests/tests/regressions/modules/gh-453-template-return-auto/A.ixx b/tests/tests/regressions/modules/gh-453-template-return-auto/A.ixx
new file mode 100644
index 0000000000..c45ea008db
--- /dev/null
+++ b/tests/tests/regressions/modules/gh-453-template-return-auto/A.ixx
@@ -0,0 +1,7 @@
+export module A;
+
+export template<class T>
+auto fn(T&)
+{
+    return 42;
+}
diff --git a/tests/tests/regressions/modules/gh-453-template-return-auto/test.mft.cpp b/tests/tests/regressions/modules/gh-453-template-return-auto/test.mft.cpp
new file mode 100644
index 0000000000..201eefd8ce
--- /dev/null
+++ b/tests/tests/regressions/modules/gh-453-template-return-auto/test.mft.cpp
@@ -0,0 +1,13 @@
+//type:fp
+//options_all:--ms_c++20 -tused --modules --microsoft_version 1952 --ms_mod_file_map A.ifc --target win64
+
+import A;
+
+template<class T>
+concept C = requires(T& t) {
+    ::fn(t);
+};
+
+static_assert(C<int>);
+
+int main() {}