C++ PATCH for c++/92032 - DR 1601: Promotion of enum with fixed underlying type

[Marek Polacek <polacek at redhat dot com>]

I've been messing with compare_ics recently and noticed that we don't
implement CWG 1601, which should be fairly easy.

The motivating example is 

  enum E : char { e };
  void f(char);
  void f(int);
  void g() {
    f(e);
  }

where the call to f was ambiguous but we should choose f(char).

Currently we give f(int) cr_promotion in standard_conversion, while
f(char) remains cr_std, which is worse than cr_promotion.  So I thought
I'd give it cr_promotion also and then add a tiebreaker to compare_ics.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-10-08  Marek Polacek  <polacek@redhat.com>

	PR c++/92032 - DR 1601: Promotion of enum with fixed underlying	type.
	* call.c (standard_conversion): When converting an enumeration with
	a fixed underlying type to the underlying type, give it the cr_promotion
	rank.
	(compare_ics): Implement a tiebreaker as per CWG 1601.

	* g++.dg/cpp0x/scoped_enum10.C: New test.
	* g++.dg/cpp0x/scoped_enum11.C: New test.

diff --git gcc/cp/call.c gcc/cp/call.c
index 6c9acac4614..10172855d4d 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -1484,8 +1484,18 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
 
       conv = build_conv (ck_std, to, conv);
 
-      /* Give this a better rank if it's a promotion.  */
-      if (same_type_p (to, type_promotes_to (from))
+      tree underlying_type = NULL_TREE;
+      if (TREE_CODE (from) == ENUMERAL_TYPE
+	  && ENUM_FIXED_UNDERLYING_TYPE_P (from))
+	underlying_type = ENUM_UNDERLYING_TYPE (from);
+
+      /* Give this a better rank if it's a promotion.
+
+	 To handle CWG 1601, also bump the rank if we are converting
+	 an enumeration with a fixed underlying type to the underlying
+	 type.  */
+      if ((same_type_p (to, type_promotes_to (from))
+	   || (underlying_type && same_type_p (to, underlying_type)))
 	  && next_conversion (conv)->rank <= cr_promotion)
 	conv->rank = cr_promotion;
     }
@@ -10506,6 +10516,31 @@ compare_ics (conversion *ics1, conversion *ics2)
 	}
     }
 
+  /* [over.ics.rank]
+
+     Per CWG 1601:
+     -- A conversion that promotes an enumeration whose underlying type
+     is fixed to its underlying type is better than one that promotes to
+     the promoted underlying type, if the two are different.  */
+  if (ics1->rank == cr_promotion
+      && ics2->rank == cr_promotion
+      && UNSCOPED_ENUM_P (from_type1)
+      && ENUM_FIXED_UNDERLYING_TYPE_P (from_type1)
+      && same_type_p (from_type1, from_type2))
+    {
+      tree utype = ENUM_UNDERLYING_TYPE (from_type1);
+      tree prom = type_promotes_to (from_type1);
+      if (!same_type_p (utype, prom))
+	{
+	  if (same_type_p (to_type1, utype)
+	      && same_type_p (to_type2, prom))
+	    return 1;
+	  else if (same_type_p (to_type2, utype)
+		   && same_type_p (to_type1, prom))
+	    return -1;
+	}
+    }
+
   /* Neither conversion sequence is better than the other.  */
   return 0;
 }
diff --git gcc/testsuite/g++.dg/cpp0x/scoped_enum10.C gcc/testsuite/g++.dg/cpp0x/scoped_enum10.C
new file mode 100644
index 00000000000..b588581cd3e
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/scoped_enum10.C
@@ -0,0 +1,37 @@
+// PR c++/92032 - DR 1601: Promotion of enumeration with fixed underlying type.
+// { dg-do compile { target c++11 } }
+
+enum E : char { e };
+enum F : int { f };
+enum G : long { g };
+enum H : unsigned { h };
+
+int f1(char);
+void f1(int);
+
+void f2(int);
+int f2(char);
+
+int f3(int);
+void f3(short);
+
+int f4(long);
+void f4(int);
+
+void f5(unsigned);
+int f5(int);
+
+int f6(unsigned);
+void f6(int);
+
+void
+test ()
+{
+  int r = 0;
+  r += f1 (e);
+  r += f2 (e);
+  r += f3 (f);
+  r += f4 (g);
+  r += f5 (f);
+  r += f6 (h);
+}
diff --git gcc/testsuite/g++.dg/cpp0x/scoped_enum11.C gcc/testsuite/g++.dg/cpp0x/scoped_enum11.C
new file mode 100644
index 00000000000..e6dcfbac9d8
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/scoped_enum11.C
@@ -0,0 +1,35 @@
+// PR c++/92032 - DR 1601: Promotion of enumeration with fixed underlying type.
+// { dg-do compile { target c++11 } }
+
+enum E1 : long { e1 };
+enum E2 : short { e2 };
+
+int f1(short);
+void f1(int);
+
+void f2(int);
+int f2(short);
+
+void f3(int);
+int f3(long);
+
+int f4(short);
+void f4(long);
+
+int f5(int);
+void f5(long);
+
+int f6(unsigned int); // { dg-message "candidate" }
+void f6(long); // { dg-message "candidate" }
+
+void
+fn ()
+{
+  int r = 0;
+  r += f1 (e2);
+  r += f2 (e2);
+  r += f3 (e1);
+  r += f4 (e2);
+  r += f5 (e2);
+  r += f6 (e2); // { dg-error "ambiguous" }
+}