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c++: Fix wrong conversion error with non-viable overload [PR94124]

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This is a bad interaction between sharing a constructor for an array
and stripping its trailing zero-initializers.  Here we reuse a ctor
and then strip its 0s.  This breaks overload resolution in this test:
D can be initialized from {} but not from {0}, so if we truncate the
constructor not to include the zero, the F(D) overload becomes valid
and then we get the ambiguous conversion error.

	PR c++/94124 - wrong conversion error with non-viable overload.
	* decl.c (reshape_init_array_1): Unshare a constructor if we
	stripped trailing zero-initializers.

	* g++.dg/cpp0x/initlist-overload1.C: New test.
This commit is contained in:
Marek Polacek 2020-03-10 18:55:42 -04:00
parent 481fcfe6fe
commit e11d05c1ed
4 changed files with 33 additions and 0 deletions
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6
gcc/cp/ChangeLog
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@ -1,3 +1,9 @@
2020-03-10 Marek Polacek <polacek@redhat.com>
PR c++/94124 - wrong conversion error with non-viable overload.
* decl.c (reshape_init_array_1): Unshare a constructor if we
stripped trailing zero-initializers.
2020-03-10 Jason Merrill <jason@redhat.com>
PR c++/93901

7
gcc/cp/decl.c
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@ -6062,6 +6062,13 @@ reshape_init_array_1 (tree elt_type, tree max_index, reshape_iter *d,
else if (last_nonzero < nelts - 1)
nelts = last_nonzero + 1;
/* Sharing a stripped constructor can get in the way of
overload resolution. E.g., initializing a class from
{{0}} might be invalid while initializing the same class
from {{}} might be valid. */
if (reuse)
new_init = unshare_constructor (new_init);
vec_safe_truncate (CONSTRUCTOR_ELTS (new_init), nelts);
}

5
gcc/testsuite/ChangeLog
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@ -1,3 +1,8 @@
2020-03-10 Marek Polacek <polacek@redhat.com>
PR c++/94124 - wrong conversion error with non-viable overload.
* g++.dg/cpp0x/initlist-overload1.C: New test.
2020-03-10 Jiufu Guo <guojiufu@linux.ibm.com>
PR target/93709

15
gcc/testsuite/g++.dg/cpp0x/initlist-overload1.C Normal file
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@ -0,0 +1,15 @@
// PR c++/94124 - wrong conversion error with non-viable overload.
// { dg-do compile { target c++11 } }
template <int N> struct A { typedef int _Type[N]; };
template <int N> struct B { typename A<N>::_Type _M_elems; };
class C { };
struct D {
D(C);
};
struct F {
F(B<2>);
F(D); // This overload should not be viable.
};
F fn1() { return {{{0}}}; }