C++ PATCH for c++/90998 - ICE with copy elision in init by ctor and -Wconversion

[Marek Polacek]

After r269667 which introduced joust_maybe_elide_copy, in C++17 we can elide
a constructor if it uses a conversion function that returns a prvalue, and
use the conversion function in its stead.

This eliding means that if we have a candidate that previously didn't have
->second_conv, it can have it after the elision.  This confused the
-Wconversion warning because it was assuming that if cand1->second_conv is
non-null, so is cand2->second_conv.  Here cand1->second_conv was non-null
but cand2->second_conv remained null, so it crashed in compare_ics.

I checked with clang that both compilers call A::operator B() in C++17 and
B::B(A const &) otherwise.

Bootstrapped/regtested on x86_64-linux, ok for trunk and 9?

2019-10-25  Marek Polacek  <pola...@redhat.com>

        PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
        * call.c (joust): Don't attempt to warn if ->second_conv is null.

        * g++.dg/cpp0x/overload-conv-4.C: New test.

diff --git gcc/cp/call.c gcc/cp/call.c
index cbd1fe8a0a4..b0c6370107d 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -10870,7 +10870,9 @@ joust (struct z_candidate *cand1, struct z_candidate 
*cand2, bool warn,
      either between a constructor and a conversion op, or between two
      conversion ops.  */
   if ((complain & tf_warning)
-      && winner && warn_conversion && cand1->second_conv
+      /* In C++17, the constructor might have been elided, which means that
+        an originally null ->second_conv could become non-null.  */
+      && winner && warn_conversion && cand1->second_conv && cand2->second_conv
       && (!DECL_CONSTRUCTOR_P (cand1->fn) || !DECL_CONSTRUCTOR_P (cand2->fn))
       && winner != compare_ics (cand1->second_conv, cand2->second_conv))
     {
diff --git gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C 
gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C
new file mode 100644
index 00000000000..6fcdbbaa6a4
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C
@@ -0,0 +1,23 @@
+// PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wconversion" }
+
+struct B;
+
+struct A {
+    operator B();
+};
+
+struct B {
+    B(A const &rs);
+    B(B const &rs);
+};
+
+B
+f (A x)
+{
+  // C++14: we call B::B(A const &)
+  // C++17: we call A::operator B()
+  return B(x); // { dg-warning "choosing .A::operator B\\(\\). over 
.B::B\\(const A&\\)" "" { target c++17 } }
+  // { dg-warning "for conversion from .A. to .B." "" { target c++17 } .-1 }
+}