In case of l/rvalue or cv-qual mismatch during reference binding, we try
to give more helpful diagnostics by attempting a bad conversion that
ignores the mismatch. But in doing so, we may end up instantiating an
ill-formed conversion function, something that would otherwise be
avoided if we didn't try for the better diagnostic in the first place.
We could just give up on producing a good diagnostic in this case, but
ideally we could keep the good diagnostics while avoiding unnecessary
template instantiations.
To that end, this patch adapts the bad conversion shortcutting mechanism
from r12-3346-g47543e5f9d1fc5 to handle this situation as well. The main
observation from there applies here as well: when there's a strictly
viable candidate, we don't care about the distinction between an unviable
and non-strictly viable candidate. And in turn it also means we don't
really care about the distinction between an invalid and bad conversion.
Of course, we don't know whether there's a strictly viable candidate until
after the fact, so we still need to keep track of when we "shortcutted"
distinguishing between an invalid and bad conversion. This patch adds a
new kind of conversion, ck_shortcutted, to represent such conversions.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/94894
PR c++/105766
PR c++/106201
gcc/cp/ChangeLog:
* call.cc (enum conversion_kind): Add ck_shortcutted enumerator.
(has_next): Return false for it.
(reference_binding): Return a ck_shortcutted conversion instead
of an actual bad conversion when LOOKUP_SHORTCUT_BAD_CONVS is set.
Remove now obsolete early exit for the incomplete target type case.
(implicit_conversion_1): Don't mask out LOOKUP_SHORTCUT_BAD_CONVS.
(add_function_candidate): Set LOOKUP_SHORTCUT_BAD_CONVS iff
shortcut_bad_convs.
(missing_conversion_p): Return true for a ck_shortcutted
conversion.
* cp-tree.h (LOOKUP_SHORTCUT_BAD_CONVS): Define.
gcc/testsuite/ChangeLog:
* g++.dg/conversion/ref8.C: New test.
* g++.dg/conversion/ref9.C: New test.
---
gcc/cp/call.cc | 87 ++++++++++++++++----------
gcc/cp/cp-tree.h | 5 ++
gcc/testsuite/g++.dg/conversion/ref8.C | 22 +++++++
gcc/testsuite/g++.dg/conversion/ref9.C | 21 +++++++
4 files changed, 103 insertions(+), 32 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/conversion/ref8.C
create mode 100644 gcc/testsuite/g++.dg/conversion/ref9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index fc98552fda2..ca2f5fbca39 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -59,7 +59,8 @@ enum conversion_kind {
ck_ambig,
ck_list,
ck_aggr,
- ck_rvalue
+ ck_rvalue,
+ ck_shortcutted,
};
/* The rank of the conversion. Order of the enumerals matters; better
@@ -775,7 +776,8 @@ has_next (conversion_kind code)
return !(code == ck_identity
|| code == ck_ambig
|| code == ck_list
- || code == ck_aggr);
+ || code == ck_aggr
+ || code == ck_shortcutted);
}
static conversion *
@@ -1912,18 +1914,38 @@ reference_binding (tree rto, tree rfrom, tree expr,
bool c_cast_p, int flags,
difference in top-level cv-qualification is subsumed by the
initialization itself and does not constitute a conversion. */
+ bool maybe_valid_p = true;
+
/* [dcl.init.ref]
Otherwise, the reference shall be an lvalue reference to a
non-volatile const type, or the reference shall be an rvalue
- reference.
+ reference. */
+ if (!CP_TYPE_CONST_NON_VOLATILE_P (to) && !TYPE_REF_IS_RVALUE (rto))
+ maybe_valid_p = false;
- We try below to treat this as a bad conversion to improve diagnostics,
- but if TO is an incomplete class, we need to reject this conversion
- now to avoid unnecessary instantiation. */
- if (!CP_TYPE_CONST_NON_VOLATILE_P (to) && !TYPE_REF_IS_RVALUE (rto)
- && !COMPLETE_TYPE_P (to))
- return NULL;
+ /* [dcl.init.ref]
+
+ Otherwise, a temporary of type "cv1 T1" is created and
+ initialized from the initializer expression using the rules for a
+ non-reference copy initialization. If T1 is reference-related to
+ T2, cv1 must be the same cv-qualification as, or greater
+ cv-qualification than, cv2; otherwise, the program is ill-formed. */
+ if (related_p && !at_least_as_qualified_p (to, from))
+ maybe_valid_p = false;
+
+ /* We try below to treat an invalid reference binding as a bad conversion
+ to improve diagnostics, but doing so may cause otherwise unnecessary
+ instantiations that can lead to hard error. So during the first pass
+ of overload resolution wherein we shortcut bad conversions, instead just
+ produce a special conversion that we'll use to indicate we might need to
+ perform a second pass if there's no strictly viable candidate. */
+ if (!maybe_valid_p && (flags & LOOKUP_SHORTCUT_BAD_CONVS))
+ {
+ conv = alloc_conversion (ck_shortcutted);
+ conv->bad_p = true;
+ return conv;
+ }
/* We're generating a temporary now, but don't bind any more in the
conversion (specifically, don't slice the temporary returned by a
@@ -1967,7 +1989,9 @@ reference_binding (tree rto, tree rfrom, tree expr, bool
c_cast_p, int flags,
sflags, complain);
if (!new_second)
return NULL;
- return merge_conversion_sequences (t, new_second);
+ conv = merge_conversion_sequences (t, new_second);
+ gcc_assert (maybe_valid_p || conv->bad_p);
+ return conv;
}
}
@@ -1976,24 +2000,7 @@ reference_binding (tree rto, tree rfrom, tree expr, bool c_cast_p, int flags,
creation of a temporary. */
conv->need_temporary_p = true;
conv->rvaluedness_matches_p = TYPE_REF_IS_RVALUE (rto);
-
- /* [dcl.init.ref]
-
- Otherwise, the reference shall be an lvalue reference to a
- non-volatile const type, or the reference shall be an rvalue
- reference. */
- if (!CP_TYPE_CONST_NON_VOLATILE_P (to) && !TYPE_REF_IS_RVALUE (rto))
- conv->bad_p = true;
-
- /* [dcl.init.ref]
-
- Otherwise, a temporary of type "cv1 T1" is created and
- initialized from the initializer expression using the rules for a
- non-reference copy initialization. If T1 is reference-related to
- T2, cv1 must be the same cv-qualification as, or greater
- cv-qualification than, cv2; otherwise, the program is ill-formed. */
- if (related_p && !at_least_as_qualified_p (to, from))
- conv->bad_p = true;
+ conv->bad_p |= !maybe_valid_p;
return conv;
}
@@ -2015,7 +2022,8 @@ implicit_conversion_1 (tree to, tree from, tree expr,
bool c_cast_p,
resolution, or after we've chosen one. */
flags &= (LOOKUP_ONLYCONVERTING|LOOKUP_NO_CONVERSION|LOOKUP_COPY_PARM
|LOOKUP_NO_TEMP_BIND|LOOKUP_NO_RVAL_BIND|LOOKUP_PREFER_RVALUE
- |LOOKUP_NO_NARROWING|LOOKUP_PROTECT|LOOKUP_NO_NON_INTEGRAL);
+ |LOOKUP_NO_NARROWING|LOOKUP_PROTECT|LOOKUP_NO_NON_INTEGRAL
+ |LOOKUP_SHORTCUT_BAD_CONVS);
/* FIXME: actually we don't want warnings either, but we can't just
have 'complain &= ~(tf_warning|tf_error)' because it would cause
@@ -2433,6 +2441,11 @@ add_function_candidate (struct z_candidate **candidates,
if (! viable)
goto out;
+ if (shortcut_bad_convs)
+ flags |= LOOKUP_SHORTCUT_BAD_CONVS;
+ else
+ flags &= ~LOOKUP_SHORTCUT_BAD_CONVS;
+
/* Third, for F to be a viable function, there shall exist for each
argument an implicit conversion sequence that converts that argument
to the corresponding parameter of F. */
@@ -6038,14 +6051,24 @@ perfect_candidate_p (z_candidate *cand)
return true;
}
-/* True iff one of CAND's argument conversions is NULL. */
+/* True iff one of CAND's argument conversions is missing. */
static bool
missing_conversion_p (const z_candidate *cand)
{
for (unsigned i = 0; i < cand->num_convs; ++i)
- if (!cand->convs[i])
- return true;
+ {
+ conversion *conv = cand->convs[i];
+ if (!conv)
+ return true;
+ if (conv->kind == ck_shortcutted)
+ {
+ /* We don't know whether this conversion is outright invalid or
+ just bad, so be conservative. */
+ gcc_checking_assert (conv->bad_p);
+ return true;
+ }
+ }
return false;
}
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index bec98aa2ac3..f3b7da2549c 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5874,6 +5874,11 @@ enum overload_flags { NO_SPECIAL = 0, DTOR_FLAG,
TYPENAME_FLAG };
#define LOOKUP_REVERSED (LOOKUP_REWRITTEN << 1)
/* We're initializing an aggregate from a parenthesized list of values. */
#define LOOKUP_AGGREGATE_PAREN_INIT (LOOKUP_REVERSED << 1)
+/* We're computing conversions as part of a first pass of overload resolution
+ wherein we don't try to distinguish an unviable candidate from a
+ non-strictly viable candidate and thus can avoid computing unnecessary
+ conversions. */
+#define LOOKUP_SHORTCUT_BAD_CONVS (LOOKUP_AGGREGATE_PAREN_INIT << 1)
/* These flags are used by the conversion code.
CONV_IMPLICIT : Perform implicit conversions (standard and
user-defined).
diff --git a/gcc/testsuite/g++.dg/conversion/ref8.C
b/gcc/testsuite/g++.dg/conversion/ref8.C
new file mode 100644
index 00000000000..e27f642690c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/ref8.C
@@ -0,0 +1,22 @@
+// PR c++/105766
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct baz {
+ baz() = default;
+ baz(int) requires __is_constructible(T, int);
+};
+
+struct foo;
+
+struct bar {
+ bar() = default;
+ bar(foo&);
+ bar(int);
+};
+
+struct foo {
+ baz<bar> m_bars;
+};
+
+foo a;
diff --git a/gcc/testsuite/g++.dg/conversion/ref9.C
b/gcc/testsuite/g++.dg/conversion/ref9.C
new file mode 100644
index 00000000000..e6dfc03cd7f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/conversion/ref9.C
@@ -0,0 +1,21 @@
+// PR c++/106201
+// { dg-do compile { target c++11 } }
+
+struct A {
+ template<class T, class = decltype(f(*(T*)nullptr))>
+ A(const T&);
+};
+
+struct B {
+ template<class T> B(const T&);
+};
+
+void f(A&);
+void f(B);
+
+struct C { };
+
+int main() {
+ C c;
+ f(c);
+}
