c++: Explicit constructor called in copy-initialization [PR90320]

This test is rejected with a bogus "use of deleted function" error
starting with r225705 whereby convert_like_real/ck_base no longer
sets LOOKUP_ONLYCONVERTING for user_conv_p conversions.  This does
not seem to be always correct.  To recap, when we have something like
T t = x where T is a class type and the type of x is not T or derived
from T, we perform copy-initialization, something like:
  1. choose a user-defined conversion to convert x to T, the result is
     a prvalue,
  2. use this prvalue to direct-initialize t.

In the second step, explicit constructors should be considered, since
we're direct-initializing.  This is what r225705 fixed.

In this PR we are dealing with the first step, I think, where explicit
constructors should be skipped.  [over.match.copy] says "The converting
constructors of T are candidate functions" which clearly eliminates
explicit constructors.  But we also have to copy-initialize the argument
we are passing to such a converting constructor, and here we should
disregard explicit constructors too.

In this testcase we have

  V v = m;

and we choose V::V(M) to convert m to V.  But we wrongly choose
the explicit M::M<M&>(M&) to copy-initialize the argument; it's
a better match for a non-const lvalue than the implicit M::M(const M&)
but because it's explicit, we shouldn't use it.

When convert_like is processing the ck_user conversion -- the convfn is
V::V(M) -- it can see that cand->flags contains LOOKUP_ONLYCONVERTING,
but then when we're in build_over_call for this convfn, we have no way
to pass the flag to convert_like for the argument 'm', because convert_like
doesn't take flags.  Fixed by creating a new conversion flag, copy_init_p,
set in ck_base/ck_rvalue to signal that explicit constructors should be
skipped.

LOOKUP_COPY_PARM looks relevant, but again, it's a LOOKUP_* flag, so
can't pass it to convert_like.  DR 899 also seemed related, but that
deals with direct-init contexts only.

	PR c++/90320
	* call.c (struct conversion): Add copy_init_p.
	(standard_conversion): Set copy_init_p in ck_base and ck_rvalue
	if FLAGS demands LOOKUP_ONLYCONVERTING.
	(convert_like_real) <case ck_base>: If copy_init_p is set, or
	LOOKUP_ONLYCONVERTING into FLAGS.

	* g++.dg/cpp0x/explicit13.C: New test.
	* g++.dg/cpp0x/explicit14.C: New test.

gcc/cp/ChangeLog

@@ -1,3 +1,12 @@
+2020-04-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/90320
+	* call.c (struct conversion): Add copy_init_p.
+	(standard_conversion): Set copy_init_p in ck_base and ck_rvalue
+	if FLAGS demands LOOKUP_ONLYCONVERTING.
+	(convert_like_real) <case ck_base>: If copy_init_p is set, or
+	LOOKUP_ONLYCONVERTING into FLAGS.
+
 2020-04-26  Iain Sandoe  <iain@sandoe.co.uk>
 
 	PR c++/94752

gcc/cp/call.c

@@ -92,7 +92,7 @@ struct conversion {
      language standards, e.g. disregarding pointer qualifiers or
      converting integers to pointers.  */
   BOOL_BITFIELD bad_p : 1;
-  /* If KIND is ck_ref_bind ck_base_conv, true to indicate that a
+  /* If KIND is ck_ref_bind or ck_base, true to indicate that a
      temporary should be created to hold the result of the
      conversion.  If KIND is ck_ambig or ck_user, true means force
      copy-initialization.  */
@@ -111,6 +111,10 @@ struct conversion {
   /* Whether check_narrowing should only check TREE_CONSTANTs; used
      in build_converted_constant_expr.  */
   BOOL_BITFIELD check_narrowing_const_only: 1;
+  /* True if this conversion is taking place in a copy-initialization context
+     and we should only consider converting constructors.  Only set in
+     ck_base and ck_rvalue.  */
+  BOOL_BITFIELD copy_init_p : 1;
   /* The type of the expression resulting from the conversion.  */
   tree type;
   union {
@@ -1252,6 +1256,10 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
       if (flags & LOOKUP_PREFER_RVALUE)
 	/* Tell convert_like_real to set LOOKUP_PREFER_RVALUE.  */
 	conv->rvaluedness_matches_p = true;
+      /* If we're performing copy-initialization, remember to skip
+	 explicit constructors.  */
+      if (flags & LOOKUP_ONLYCONVERTING)
+	conv->copy_init_p = true;
     }
 
    /* Allow conversion between `__complex__' data types.  */
@@ -1528,6 +1536,10 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
       if (flags & LOOKUP_PREFER_RVALUE)
 	/* Tell convert_like_real to set LOOKUP_PREFER_RVALUE.  */
 	conv->rvaluedness_matches_p = true;
+      /* If we're performing copy-initialization, remember to skip
+	 explicit constructors.  */
+      if (flags & LOOKUP_ONLYCONVERTING)
+	conv->copy_init_p = true;
     }
   else
     return NULL;
@@ -7653,12 +7665,16 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
 	 type is the same class as, or a derived class of, the class of the
 	 destination [is treated as direct-initialization].  [dcl.init] */
       flags = LOOKUP_NORMAL;
+      /* This conversion is being done in the context of a user-defined
+	 conversion (i.e. the second step of copy-initialization), so
+	 don't allow any more.  */
       if (convs->user_conv_p)
-	/* This conversion is being done in the context of a user-defined
-	   conversion (i.e. the second step of copy-initialization), so
-	   don't allow any more.  */
 	flags |= LOOKUP_NO_CONVERSION;
-      else
+      /* We might be performing a conversion of the argument
+	 to the user-defined conversion, i.e., not a conversion of the
+	 result of the user-defined conversion.  In which case we skip
+	 explicit constructors.  */
+      if (convs->copy_init_p)
 	flags |= LOOKUP_ONLYCONVERTING;
       if (convs->rvaluedness_matches_p)
 	/* standard_conversion got LOOKUP_PREFER_RVALUE.  */

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/90320
+	* g++.dg/cpp0x/explicit13.C: New test.
+	* g++.dg/cpp0x/explicit14.C: New test.
+
 2020-04-27  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR d/89418

gcc/testsuite/g++.dg/cpp0x/explicit13.C

@@ -0,0 +1,14 @@
+// PR c++/90320
+// { dg-do compile { target c++11 } }
+
+struct M {
+  M() = default;
+  template <typename T> explicit M(T&&) = delete;
+};
+
+struct V {
+  V(M m);
+};
+
+M m;
+V v = m;

gcc/testsuite/g++.dg/cpp0x/explicit14.C

@@ -0,0 +1,16 @@
+// PR c++/90320
+// { dg-do compile { target c++11 } }
+
+struct B { };
+
+struct M : B {
+  M() = default;
+  template <typename T> explicit M(T&&) = delete;
+};
+
+struct V {
+  V(B);
+};
+
+M m;
+V v = m;