[PATCH] c++/modules: deduced return type merging [PR114795]

[Patrick Palka <ppalka@redhat.com>]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

-- >8 --

When merging an imported function template specialization with an
existing one, if the existing one has an undeduced return type and the
imported one's is already deduced, we need to propagate the deduced type
since once we install the imported definition we won't get a chance to
deduce it by normal means.

This patch makes is_matching_decl propagate the deduced return type
alongside the existing propagate of the existing specification.  I
suppose could instead propagate it later when installing the imported
definition from read_definition, but it seems best to propagate it
sooner rather than later.

	PR c++/114795

gcc/cp/ChangeLog:

	* module.cc (trees_in::is_matching_decl): Propagate deduced
	function return type.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/auto-4_a.H: New test.
	* g++.dg/modules/auto-4_b.C: New test.
---
 gcc/cp/module.cc                        |  5 +++++
 gcc/testsuite/g++.dg/modules/auto-4_a.H | 14 ++++++++++++++
 gcc/testsuite/g++.dg/modules/auto-4_b.C | 15 +++++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/modules/auto-4_a.H
 create mode 100644 gcc/testsuite/g++.dg/modules/auto-4_b.C

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index d94d8ff4df9..e10e19ac9f7 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -11537,6 +11537,11 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
       else if (!DEFERRED_NOEXCEPT_SPEC_P (d_spec)
 	       && !comp_except_specs (d_spec, e_spec, ce_type))
 	goto mismatch;
+
+      /* Similarly if EXISTING has an undeduced return type, but DECL's
+	 is already deduced.  */
+      if (undeduced_auto_decl (existing) && !undeduced_auto_decl (decl))
+	TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);
     }
   else if (is_typedef)
     {
diff --git a/gcc/testsuite/g++.dg/modules/auto-4_a.H b/gcc/testsuite/g++.dg/modules/auto-4_a.H
new file mode 100644
index 00000000000..52b50533982
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-4_a.H
@@ -0,0 +1,14 @@
+// PR c++/114795
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+template<class T>
+struct A {
+  auto f() { return 0; }
+};
+
+template<class T>
+inline void g() {
+  A<int> a;
+  a.f();
+}
diff --git a/gcc/testsuite/g++.dg/modules/auto-4_b.C b/gcc/testsuite/g++.dg/modules/auto-4_b.C
new file mode 100644
index 00000000000..378684ef6d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-4_b.C
@@ -0,0 +1,15 @@
+// PR c++/114795
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+template<class T>
+struct A {
+  auto f() { return T(); }
+};
+
+A<int> a;
+
+import "auto-4_a.H";
+
+int main() {
+  g<int>(); // { dg-bogus "before deduction of 'auto'" "" { target *-*-* } 0 }
+}
-- 
2.45.0.rc0