c++: Diagnose references to void in structured bindings [PR99650]

We ICE on the following testcase, because std::tuple_element<...,...>::type
is void and for structured bindings we therefore need to create
void & or void && which is invalid.  We created such REFERENCE_TYPE and
later ICEd in the middle-end.
The following patch fixes it by diagnosing that.

2021-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99650
	* decl.c (cp_finish_decomp): Diagnose void initializers when
	using tuple_element and get.

	* g++.dg/cpp1z/decomp55.C: New test.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 8e8f37d060e6f72bf68a22d53f155f68921d1da0..b1d8e444b66ed2ac1a3e2e38d2cd3f63b14dcf9b 100644 (file)
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -8629,6 +8629,12 @@ cp_finish_decomp (tree decl, tree first, unsigned int count)
                         : get_tuple_element_type (type, i));
          input_location = sloc;
 
+         if (VOID_TYPE_P (eltype))
+           {
+             error ("%<std::tuple_element<%u, %T>::type%> is %<void%>",
+                    i, type);
+             eltype = error_mark_node;
+           }
          if (init == error_mark_node || eltype == error_mark_node)
            {
              inform (dloc, "in initialization of structured binding "

diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp55.C b/gcc/testsuite/g++.dg/cpp1z/decomp55.C
new file mode 100644 (file)
index 0000000..bb4bf75
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp55.C
@@ -0,0 +1,19 @@
+// PR c++/99650
+// { dg-do compile { target c++17 } }
+
+namespace std {
+  template<typename T> struct tuple_size;
+  template<int, typename> struct tuple_element;
+}
+
+struct A {
+  int i;
+  template <int I> void get() { }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 2; };
+template<int I> struct std::tuple_element<I,A> { using type = void; };
+
+A a = { 42 };
+auto [ x, y ] = a;     // { dg-error ".std::tuple_element<0, A>::type. is .void." }
+// { dg-message "in initialization of structured binding variable 'x'" "" { target *-*-* } .-1 }