[C++ PATCH] PR c++/87051

[Ville Voutilainen]

Howdy! The tricky details of copy constructors, part 76.
In this approach, I let the const-copy "dominate"; that is, if
a const-copy was found, a non-const-copy will not turn off triviality
of the copy constructor, and a const-copy will reinstate triviality
of the copy constructor even if a non-const copy removed said
triviality.

I suppose we could try moving this knowledge from the class level
to the level of special member function decl, but I wonder whether
we have the bits to do that. This approach seems to solve the
problem, certainly.

This has been tested manually on Linux-x64, running full suite
on Linux-PPC64.

Thoughts? OK for trunk if the full suite passes?

2018-09-13  Ville Voutilainen  <ville.voutilai...@gmail.com>

    gcc/cp

     PR c++/87051
    * decl.c (grok_special_member_properties): Don't mark the class
    as having a non-trivial copy constructor if the copy constructor
    we're looking at is not a const-copy and a const-copy was already
    found, reset the copy constructor triviality if we found a
    trivial const-copy.

    testsuite/

     PR c++/87051
    * g++.dg/ext/is_trivially_constructible7.C: New.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 50b60e8..1b09721 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -13211,8 +13211,11 @@ grok_special_member_properties (tree decl)
 	     are no other parameters or else all other parameters have
 	     default arguments.  */
 	  TYPE_HAS_COPY_CTOR (class_type) = 1;
-	  if (user_provided_p (decl))
-	    TYPE_HAS_COMPLEX_COPY_CTOR (class_type) = 1;
+	  if (user_provided_p (decl)
+	      && (ctor > 1 || !TYPE_HAS_CONST_COPY_CTOR (class_type)))
+		TYPE_HAS_COMPLEX_COPY_CTOR (class_type) = 1;
+	  else if (ctor > 1)
+	    TYPE_HAS_COMPLEX_COPY_CTOR (class_type) = 0;
 	  if (ctor > 1)
 	    TYPE_HAS_CONST_COPY_CTOR (class_type) = 1;
 	}
diff --git a/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C b/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C
new file mode 100644
index 0000000..177eb2e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_trivially_constructible7.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+
+// PR c++/87051
+
+struct M {
+  M(const M&) = default;
+  M(M&);
+};
+
+struct M2 {
+  M2(M2&);
+  M2(const M2&) = default;
+};
+
+static_assert( __is_trivially_constructible(M, M&&), "");
+static_assert( __is_trivially_constructible(M2, M2&&), "");