On 2/4/21 12:47 PM, Jakub Jelinek wrote:
On Thu, Feb 04, 2021 at 11:03:10AM -0500, Jason Merrill wrote:
--- gcc/cp/decl.c.jj    2021-01-28 16:13:04.000000000 +0100
+++ gcc/cp/decl.c       2021-02-03 15:57:16.980557836 +0100
@@ -6837,7 +6837,8 @@ check_initializer (tree decl, tree init,
       /* We will have already complained.  */
       return NULL_TREE;
-  if (TREE_CODE (type) == ARRAY_TYPE)
+  if (TREE_CODE (type) == ARRAY_TYPE
+      && (!DECL_DECOMPOSITION_P (decl) || DECL_DECOMP_BASE (decl)))

This needs a comment.  Or handle this inside check_array_initializer, where
we could give a more helpful diagnostic?

So like this?

2021-02-04  Jakub Jelinek  <ja...@redhat.com>

        PR c++/97878
        * decl.c (check_array_initializer): For structured bindings, require
        the array type to be complete.

        * g++.dg/cpp1z/decomp54.C: New test.

--- gcc/cp/decl.c.jj    2021-02-03 17:13:58.567872007 +0100
+++ gcc/cp/decl.c       2021-02-04 18:42:35.347374198 +0100
@@ -6768,6 +6768,19 @@ check_array_initializer (tree decl, tree
  {
    tree element_type = TREE_TYPE (type);
+ /* Structured binding when initialized with an array type needs
+     to have complete type.  */
+  if (decl
+      && DECL_DECOMPOSITION_P (decl)
+      && !DECL_DECOMP_BASE (decl)
+      && !COMPLETE_TYPE_P (type))
+    {
+      error_at (DECL_SOURCE_LOCATION (decl),
+               "structured binding has incomplete type");

Let's print the type.  OK with that change.

+      TREE_TYPE (decl) = error_mark_node;
+      return true;
+    }
+
    /* The array type itself need not be complete, because the
       initializer may tell us how many elements are in the array.
       But, the elements of the array must be complete.  */
--- gcc/testsuite/g++.dg/cpp1z/decomp54.C.jj    2021-02-04 18:33:34.829393825 
+0100
+++ gcc/testsuite/g++.dg/cpp1z/decomp54.C       2021-02-04 18:33:34.829393825 
+0100
@@ -0,0 +1,17 @@
+// PR c++/97878
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+extern int a[];
+auto [b] { a };        // { dg-error "has incomplete type" }
+               // { dg-warning "only available with" "" { target c++14_down } 
.-1 }
+auto [c] = a;  // { dg-error "has incomplete type" }
+               // { dg-warning "only available with" "" { target c++14_down } 
.-1 }
+extern int d[0];
+auto [e] { d };        // { dg-error "too many initializers for" }
+               // { dg-error "1 name provided for structured binding" "" { 
target *-*-* } .-1 }
+               // { dg-message "decomposes into 0 elements" "" { target *-*-* 
} .-2 }
+               // { dg-warning "only available with" "" { target c++14_down } 
.-3 }
+auto [f] = d;  // { dg-error "1 name provided for structured binding" }
+               // { dg-message "decomposes into 0 elements" "" { target *-*-* 
} .-1 }
+               // { dg-warning "only available with" "" { target c++14_down } 
.-2 }


        Jakub


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