Fix SFINAE calling a function template with deduced return type and pending definition

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) {

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.

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;
+}

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() {}