PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.

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.

gcc/cp/ChangeLog
2020-02-26  Marek Polacek  <polacek@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.

gcc/testsuite/ChangeLog
2020-02-26  Marek Polacek  <polacek@redhat.com>

	PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
	* g++.dg/cpp0x/overload-conv-4.C: New test.

4 files changed, 36 insertions, 1 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 260f38f..1b5e2bb 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2020-02-26  Marek Polacek  <polacek@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.
+
 2020-02-26  Jason Merrill  <jason@redhat.com>
 
 	PR c++/86521 - C++17 copy elision in initialization by constructor.
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 3b0b216..22de5c7 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10240,7 +10240,9 @@ 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 a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 4c94167..440e35a 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/90998 - ICE with copy elision in init by ctor and -Wconversion.
+	* g++.dg/cpp0x/overload-conv-4.C: New test.
+
 2020-02-25  Alexandre Oliva <aoliva@redhat.com>
 
 	PR c++/86747
diff --git a/gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C b/gcc/testsuite/g++.dg/cpp0x/overload-conv-4.C
new file mode 100644
index 0000000..6fcdbba
--- /dev/null
+++ b/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 }
+}