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

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.

So this patch makes is_matching_decl propagate the deduced return
type alongside our propagation of the exception specification.
Another option would be to propagate it later when installing the
imported definition from read_function_def, but it seems preferable
to do 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.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/module.cc

@@ -11537,6 +11537,15 @@ 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))
+	{
+	  dump (dumper::MERGE)
+	    && dump ("Propagating deduced return type to %N", existing);
+	  TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);
+	}
     }
   else if (is_typedef)
     {

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 T(); }
+};
+
+template<class T>
+void g() {
+  A<int> a;
+  a.f();
+}

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 }
+}