[PR c++/84943] mark function as used when taking its address

fn[0]() ICEd because we would fold the INDIRECT_REF used for the
array indexing while building the address for the call, after not
finding the decl hiding there at first.  But the decl would be exposed
by the folding, and then lower layers would complain we had the decl,
after all, but it wasn't one of the artificial or special functions
that could be called without being marked as used.

This patch arranges for a FUNCTION_DECL to be marked as used when
taking its address, just like we already did when taking the address
of a static function to call it as a member function (i.e. using the
obj.fn() notation).  However, we shouldn't mark functions as used when
just performing overload resolution, lest we might instantiate
templates we shouldn't, as in g++.dg/overload/template1.C, so we
adjust mark_used to return early when testing conversions.


for  gcc/cp/ChangeLog

	PR c++/84943
	* typeck.c (cp_build_addr_expr_1): Mark FUNCTION_DECL as
	used.
	* decl2.c (mark_used): Return without effects if tf_conv.

for  gcc/testsuite/ChangeLog

	PR c++/84943
	* g++.dg/pr84943.C: New.
	* g++.dg/pr84943-2.C: New.

git-svn-id: https://gcc.gnu.org/svn/gcc/trunk@259067 138bc75d-0d04-0410-961f-82ee72b054a4

6 files changed, 94 insertions, 0 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 4a9f9995d24..46f05c8661e 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2018-04-04  Alexandre Oliva <aoliva@redhat.com>
+
+	PR c++/84943
+	* typeck.c (cp_build_addr_expr_1): Mark FUNCTION_DECL as
+	used.
+	* decl2.c (mark_used): Return without effects if tf_conv.
+
 2018-04-03  Jason Merrill  <jason@redhat.com>
 
 	PR c++/85092 - C++17 ICE with unused list constructor.
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index fa753749e1a..6ae6cef78dd 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -5201,6 +5201,12 @@ maybe_instantiate_decl (tree decl)
 bool
 mark_used (tree decl, tsubst_flags_t complain)
 {
+  /* If we're just testing conversions or resolving overloads, we
+     don't want any permanent effects like forcing functions to be
+     output or instantiating templates.  */
+  if ((complain & tf_conv))
+    return true;
+
   /* If DECL is a BASELINK for a single function, then treat it just
      like the DECL for the function.  Otherwise, if the BASELINK is
      for an overloaded function, we don't know which function was
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index e33f2c34c7f..2b7a771bbeb 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -5971,6 +5971,9 @@ cp_build_addr_expr_1 (tree arg, bool strict_lvalue, tsubst_flags_t complain)
      so we can just form an ADDR_EXPR with the correct type.  */
   if (processing_template_decl || TREE_CODE (arg) != COMPONENT_REF)
     {
+      if (TREE_CODE (arg) == FUNCTION_DECL
+	  && !mark_used (arg, complain) && !(complain & tf_error))
+	return error_mark_node;
       val = build_address (arg);
       if (TREE_CODE (arg) == OFFSET_REF)
 	PTRMEM_OK_P (val) = PTRMEM_OK_P (arg);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index de14495bf9a..95cd7c8392d 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2018-04-04  Alexandre Oliva <aoliva@redhat.com>
+
+	PR c++/84943
+	* g++.dg/pr84943.C: New.
+	* g++.dg/pr84943-2.C: New.
+
 2018-04-03  Jakub Jelinek  <jakub@redhat.com>
 
 	PR rtl-optimization/85167
diff --git a/gcc/testsuite/g++.dg/pr84943-2.C b/gcc/testsuite/g++.dg/pr84943-2.C
new file mode 100644
index 00000000000..d1ef012b915
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr84943-2.C
@@ -0,0 +1,64 @@
+// { dg-do run }
+
+// Avoid -pedantic-error default
+// { dg-options "" }
+
+// Make sure the functions referenced by various forms of
+// address-taking are marked as used and compiled in.
+
+static void ac() {}
+void a() {
+  ac[0](); // { dg-warning "arithmetic" }
+}
+
+static void bc() {}
+void b() {
+  (&*&*&*&bc)();
+}
+
+template <typename U> U cc() {}
+void (*c())() {
+  return cc;
+}
+
+template <typename T>
+struct x {
+  void a(int);
+  template <typename U> static U a(x*) {}
+  static void a(long) {}
+  static void a(void *) {}
+  static void a() {
+    void (*p0)(void*) = x().a;
+    p0(0);
+    void (*p1)(long) = a;
+    p1(0);
+    void (*p2)() = a;
+    p2();
+    void (*p3)(x*) = a;
+    p3(0);
+  }
+};
+
+struct z {
+  void a(int);
+  template <typename U> static U a(z*) {}
+  static void a(long) {}
+  static void a(void *) {}
+  static void a() {
+    void (*p0)(void*) = z().a;
+    p0(0);
+    void (*p1)(long) = a;
+    p1(0);
+    void (*p2)() = a;
+    p2();
+    void (*p3)(z*) = a;
+    p3(0);
+  }
+};
+
+int main(int argc, char *argv[]) {
+  if (argc > 1) {
+    x<void>().a();
+    z().a();
+  }
+}
diff --git a/gcc/testsuite/g++.dg/pr84943.C b/gcc/testsuite/g++.dg/pr84943.C
new file mode 100644
index 00000000000..36f75a16411
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr84943.C
@@ -0,0 +1,8 @@
+// { dg-do compile }
+
+// Avoid -pedantic-error default
+// { dg-options "" }
+
+void a() {
+  a[0](); // { dg-warning "arithmetic" }
+}