[c++] Implement DR 976

[Nathan Sidwell <nathan@acm.org>]

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DR 976 affects conversion operator deduction, swapping reference stripping and 
cv-qual removal.  This allows 'Y::operator P const & ()' to deduce T against a 
call wanting plain A  (previously that would fail as 'P const' cannot be deduced 
from 'A').

It also affects deductions for array- or function-producing conversions, which I 
suspect is rarer.

pushed to trunk

nathan
-- 
Nathan Sidwell

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From 80f075b410125bddb31459428760645baba1a69f Mon Sep 17 00:00:00 2001
From: Nathan Sidwell <nathan@acm.org>
Date: Sat, 10 Jun 2023 12:42:17 -0400
Subject: [PATCH] c++: Adjust conversion deduction [PR61663][DR976]

Drop the return type's reference before doing cvqual and related decays.

	gcc/cp/
	PR c++/61663
	* pt.cc (maybe_adjust_types_for_deduction): Implement DR976.
	gcc/testsuite/
	* g++.dg/template/pr61663.C: New.
---
 gcc/cp/pt.cc                            | 11 +++--
 gcc/testsuite/g++.dg/template/pr61663.C | 63 +++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/template/pr61663.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 6b20c58ce66..6a2cf2c123f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -22725,10 +22725,16 @@ maybe_adjust_types_for_deduction (tree tparms,
       break;
 
     case DEDUCE_CONV:
+      /* [temp.deduct.conv] First remove a reference type on parm.
+	 DRs 322 & 976 affected this.  */
+      if (TYPE_REF_P (*parm))
+	*parm = TREE_TYPE (*parm);
+
       /* Swap PARM and ARG throughout the remainder of this
 	 function; the handling is precisely symmetric since PARM
 	 will initialize ARG rather than vice versa.  */
       std::swap (parm, arg);
+
       break;
 
     case DEDUCE_EXACT:
@@ -22795,11 +22801,6 @@ maybe_adjust_types_for_deduction (tree tparms,
       result |= UNIFY_ALLOW_OUTER_MORE_CV_QUAL;
     }
 
-  /* DR 322. For conversion deduction, remove a reference type on parm
-     too (which has been swapped into ARG).  */
-  if (strict == DEDUCE_CONV && TYPE_REF_P (*arg))
-    *arg = TREE_TYPE (*arg);
-
   return result;
 }
 
diff --git a/gcc/testsuite/g++.dg/template/pr61663.C b/gcc/testsuite/g++.dg/template/pr61663.C
new file mode 100644
index 00000000000..2964fa6c309
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/pr61663.C
@@ -0,0 +1,63 @@
+// { dg-do compile { target c++11 } }
+// PR c++/61663
+// DR 976, strip ref from conv op return type before doing
+// fn and ary decay or CV qual removal
+
+struct F 
+{
+  template<class T>
+  operator const T&();
+};
+
+void Foo () 
+{
+  F f;
+  int i = f;
+}
+
+template<typename T> struct X {};
+
+struct Y
+{
+  template<typename T> operator X<T> () &&; // #3
+  template<typename T> operator X<T> const & () const &; // #4
+};
+
+void Use (X<void>);
+Y Val ();
+Y const &Ref ();
+
+// { dg-final { scan-assembler "_Z5Frob3v:.*_ZNO1Ycv1XIT_EIvEEv.*_Z3Use1XIvE" } }
+void Frob3 ()
+{
+  Use (Val ()); // #3
+}
+
+// { dg-final { scan-assembler "_Z5Frob4v:.*_ZNKR1YcvRK1XIT_EIvEEv.*_Z3Use1XIvE" } }
+void Frob4 ()
+{
+  Use (Ref ()); // #4
+}
+
+struct Z 
+{
+  template<typename T> using FnRef = void (&) (T);
+  template<typename T> using AryRef = T (&)[];
+
+  template<typename T> operator FnRef<T> ();
+  template<typename T> operator AryRef<T> ();
+};
+
+// { dg-final { scan-assembler "_Z5Frob5R1Z:.*_ZN1ZcvRFvT_EIiEEv.*_ZN1ZcvRA_T_IiEEv" } }
+void Frob5 (Z &z)
+{
+  void (*fnptr)(int) = z;
+  int *iptr = z;
+}
+
+// { dg-final { scan-assembler "_Z5Frob6R1Z:.*_ZN1ZcvRFvT_EIfEEv.*_ZN1ZcvRA_T_IfEEv" } }
+void Frob6 (Z &z)
+{
+  void (&fnref)(float) = z;
+  float (&aryref)[] = z;
+}
-- 
2.40.1