c++: function NTTP argument considered unused [PR53164, PR105848]

Here at parse time the template argument f (an OVERLOAD) in A<f> gets
resolved ahead of time to the FUNCTION_DECL f<int>, and we defer marking
f<int> as used until instantiation (of g) as usual.

Later when instantiating g the type A<f> (where f has already been
resolved) is non-dependent, so tsubst_aggr_type avoids re-processing its
template arguments, and we end up never actually marking f<int> as used
(which means we never instantiate it) even though A<f>::h() later calls
it, leading to a link error.

This patch works around this issue by looking through ADDR_EXPR when
calling mark_used on the substituted callee of a CALL_EXPR.

	PR c++/53164
	PR c++/105848

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Look through an
	ADDR_EXPR callee when calling mark_used.

gcc/testsuite/ChangeLog:

	* g++.dg/template/fn-ptr3.C: New test.

1 files changed, 16 insertions, 4 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 59b9431..dcacba5 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -20884,10 +20884,22 @@ tsubst_copy_and_build (tree t,
 	  }
 
 	/* Remember that there was a reference to this entity.  */
-	if (function != NULL_TREE
-	    && DECL_P (function)
-	    && !mark_used (function, complain) && !(complain & tf_error))
-	  RETURN (error_mark_node);
+	if (function != NULL_TREE)
+	  {
+	    tree inner = function;
+	    if (TREE_CODE (inner) == ADDR_EXPR
+		&& TREE_CODE (TREE_OPERAND (inner, 0)) == FUNCTION_DECL)
+	      /* We should already have called mark_used when taking the
+		 address of this function, but do so again anyway to make
+		 sure it's odr-used: at worst this is a no-op, but if we
+		 obtained this FUNCTION_DECL as part of ahead-of-time overload
+		 resolution then that call to mark_used wouldn't have marked it
+		 odr-used yet (53164).  */
+	      inner = TREE_OPERAND (inner, 0);
+	    if (DECL_P (inner)
+		&& !mark_used (inner, complain) && !(complain & tf_error))
+	      RETURN (error_mark_node);
+	  }
 
 	if (!maybe_fold_fn_template_args (function, complain))
 	  return error_mark_node;