[gcc r11-10525] c++: fix ICE in joust_maybe_elide_copy [PR106675]

[gcc r11-10525] c++: fix ICE in joust_maybe_elide_copy [PR106675]

[Marek Polacek]

https://gcc.gnu.org/g:53512f0055186d5556e9859c1f8beb25d2d28530

commit r11-10525-g53512f0055186d5556e9859c1f8beb25d2d28530
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Feb 10 17:26:57 2023 -0500

    c++: fix ICE in joust_maybe_elide_copy [PR106675]
    
    joust_maybe_elide_copy checks that the last conversion in the ICS for
    the first argument is ck_ref_bind, which is reasonable, because we've
    checked that we're dealing with a copy/move constructor.  But it can
    also happen that we couldn't figure out which conversion function is
    better to convert the argument, as in this testcase: joust couldn't
    decide if we should go with
    
      operator foo &()
    
    or
    
      operator foo const &()
    
    so we get a ck_ambig, which then upsets joust_maybe_elide_copy.  Since
    a ck_ambig can validly occur, I think we should just return early, as
    in the patch below.
    
            PR c++/106675
    
    gcc/cp/ChangeLog:
    
            * call.c (joust_maybe_elide_copy): Return false for ck_ambig.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/overload-conv-5.C: New test.
    
    (cherry picked from commit cce62625025380c2ea2a220deb10f8f355f83abf)

Diff:
---
 gcc/cp/call.c                                |  2 ++
 gcc/testsuite/g++.dg/cpp0x/overload-conv-5.C | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 46c2a066499..7c3ce1693e1 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -11593,6 +11593,8 @@ joust_maybe_elide_copy (z_candidate *&cand)
   if (!DECL_COPY_CONSTRUCTOR_P (fn) && !DECL_MOVE_CONSTRUCTOR_P (fn))
     return false;
   conversion *conv = cand->convs[0];
+  if (conv->kind == ck_ambig)
+    return false;
   gcc_checking_assert (conv->kind == ck_ref_bind);
   conv = next_conversion (conv);
   if (conv->kind == ck_user && !TYPE_REF_P (conv->type))
diff --git a/gcc/testsuite/g++.dg/cpp0x/overload-conv-5.C b/gcc/testsuite/g++.dg/cpp0x/overload-conv-5.C
new file mode 100644
index 00000000000..b1e7766e42b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/overload-conv-5.C
@@ -0,0 +1,21 @@
+// PR c++/106675
+// { dg-do compile { target c++11 } }
+
+struct foo {
+    int n_;
+    foo(int n) : n_(n) {}
+};
+
+struct bar {
+    int n_;
+
+    operator foo() const {
+        return foo(n_);
+    }
+    operator foo &() { return *reinterpret_cast<foo *>(n_); }
+    operator foo const &() = delete;
+
+    void crashgcc() {
+        foo tmp(*this); // { dg-error "ambiguous" }
+    }
+};