[gcc r11-7776] c++: Diagnose references to void in structured bindings [PR99650]

[Jakub Jelinek]

https://gcc.gnu.org/g:3e07e7a6a7f34f0ec2f1a3e50ebc52b77de11a30

commit r11-7776-g3e07e7a6a7f34f0ec2f1a3e50ebc52b77de11a30
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 23 10:23:42 2021 +0100

    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:
---
 gcc/cp/decl.c                         |  6 ++++++
 gcc/testsuite/g++.dg/cpp1z/decomp55.C | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 8e8f37d060e..b1d8e444b66 100644
--- 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
index 00000000000..bb4bf754abb
--- /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 }