c++/modules: Propagate FNDECL_USED_AUTO when propagating deduced return types [PR118049]

In the linked testcase, we're erroring because the declared return types
of the functions do not appear to match.  This is because when merging
the deduced return types for 'foo' in 'auto-5_b.C', we overwrote the
return type for the declaration with the deduced return type from
'auto-5_a.C' but neglected to track that we were originally declared
with 'auto'.

As a drive-by improvement to QOI, also add checks for if the deduced
return types do not match; this is currently useful because we do not
check the equivalence of the bodies of functions yet.

	PR c++/118049

gcc/cp/ChangeLog:

	* module.cc (trees_in::is_matching_decl): Propagate
	FNDECL_USED_AUTO as well.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/auto-5_a.C: New test.
	* g++.dg/modules/auto-5_b.C: New test.
	* g++.dg/modules/auto-5_c.C: New test.
	* g++.dg/modules/auto-6_a.H: New test.
	* g++.dg/modules/auto-6_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

6 files changed, 44 insertions, 0 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 61116fe..6fe64bb 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -11902,8 +11902,13 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
 	{
 	  dump (dumper::MERGE)
 	    && dump ("Propagating deduced return type to %N", existing);
+	  FNDECL_USED_AUTO (e_inner) = true;
+	  DECL_SAVED_AUTO_RETURN_TYPE (existing) = TREE_TYPE (e_type);
 	  TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);
 	}
+      else if (type_uses_auto (d_ret)
+	       && !same_type_p (TREE_TYPE (d_type), TREE_TYPE (e_type)))
+	goto mismatch;
     }
   else if (is_typedef)
     {
diff --git a/gcc/testsuite/g++.dg/modules/auto-5_a.C b/gcc/testsuite/g++.dg/modules/auto-5_a.C
new file mode 100644
index 0000000..fcab6f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-5_a.C
@@ -0,0 +1,10 @@
+// PR c++/118049
+// { dg-additional-options "-fmodules -Wno-global-module" }
+// { dg-module-cmi A }
+
+module;
+template <typename T> struct S {
+  auto foo() {}
+};
+export module A;
+template struct S<char>;
diff --git a/gcc/testsuite/g++.dg/modules/auto-5_b.C b/gcc/testsuite/g++.dg/modules/auto-5_b.C
new file mode 100644
index 0000000..f75ed2d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-5_b.C
@@ -0,0 +1,14 @@
+// PR c++/118049
+// { dg-additional-options "-fmodules -Wno-global-module" }
+// { dg-module-cmi B }
+
+module;
+template <typename T> struct S {
+  auto foo() {}
+};
+template struct S<char>;
+export module B;
+import A;
+template <typename> void x() {
+  S<char>{}.foo();
+}
diff --git a/gcc/testsuite/g++.dg/modules/auto-5_c.C b/gcc/testsuite/g++.dg/modules/auto-5_c.C
new file mode 100644
index 0000000..f351c2b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-5_c.C
@@ -0,0 +1,4 @@
+// PR c++/118049
+// { dg-additional-options "-fmodules -fno-module-lazy -fdump-lang-module-alias" }
+
+import B;
diff --git a/gcc/testsuite/g++.dg/modules/auto-6_a.H b/gcc/testsuite/g++.dg/modules/auto-6_a.H
new file mode 100644
index 0000000..3ad08ab
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-6_a.H
@@ -0,0 +1,5 @@
+// { dg-additional-options "-fmodule-header" }
+
+inline auto foo() {
+  return 1;
+}
diff --git a/gcc/testsuite/g++.dg/modules/auto-6_b.C b/gcc/testsuite/g++.dg/modules/auto-6_b.C
new file mode 100644
index 0000000..aab7be4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/auto-6_b.C
@@ -0,0 +1,6 @@
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+
+inline auto foo() {  // { dg-error "conflicting" }
+  return 1.0;
+}
+import "auto-6_a.H";