[PATCH] c++: Fix ICE with structured binding initialized to incomplete array [PR97878]

[Jakub Jelinek]

Hi!

We ICE on the following testcase, for incomplete array a on auto [b] { a }; without
giving any kind of diagnostics, with auto [c] = a; during error-recovery.
The problem is that we get too far through check_initializer and e.g.
store_init_value -> constexpr stuff can't deal with incomplete array types.

As the type of the structured binding artificial variable is always deduced,
I think it is easiest to diagnose this early, even if they have array types
we'll need their deduced type to be complete rather than just its element
type.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

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

	PR c++/97878
	* decl.c (check_initializer): For structured bindings, require
	complete type even when they have array type.

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

--- 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)))
     {
       if (check_array_initializer (decl, type, init))
 	return NULL_TREE;
--- gcc/testsuite/g++.dg/cpp1z/decomp54.C.jj	2021-02-03 16:21:27.991237734 +0100
+++ gcc/testsuite/g++.dg/cpp1z/decomp54.C	2021-02-03 16:21:10.984428831 +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