c++: Fix sizeof VLA lambda capture.

sizeof a VLA type is not a constant in C or the GNU C++ extension, so we
need to capture the VLA even in unevaluated context.  For PR60855 we stopped
looking through a previous capture, but we also need to capture the first
time the variable is mentioned.

	PR c++/86216
	* semantics.c (process_outer_var_ref): Capture VLAs even in
	unevaluated context.

gcc/cp/ChangeLog

 2020-01-31  Jason Merrill  <jason@redhat.com>
 
+	PR c++/86216
+	* semantics.c (process_outer_var_ref): Capture VLAs even in
+	unevaluated context.
+
 	PR c++/14179
 	* decl.c (reshape_init_array_1): Reuse a single CONSTRUCTOR with
 	non-aggregate elements.

gcc/cp/semantics.c

@@ -3524,8 +3524,15 @@ tree
 process_outer_var_ref (tree decl, tsubst_flags_t complain, bool odr_use)
 {
   if (cp_unevaluated_operand)
-    /* It's not a use (3.2) if we're in an unevaluated context.  */
-    return decl;
+    {
+      tree type = TREE_TYPE (decl);
+      if (!dependent_type_p (type)
+	  && variably_modified_type_p (type, NULL_TREE))
+	/* VLAs are used even in unevaluated context.  */;
+      else
+	/* It's not a use (3.2) if we're in an unevaluated context.  */
+	return decl;
+    }
   if (decl == error_mark_node)
     return decl;
 

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-vla5.C

+// PR c++/86216
+// { dg-do compile { target c++11 } }
+// { dg-additional-options -Wno-vla }
+
+template <typename T> void b(int n, T arg) {
+  int buffer[arg];
+  int buffer2[arg][arg];
+  [&] {
+    n = sizeof(buffer);
+    n = sizeof(buffer2);	// { dg-bogus "sorry" "" { xfail *-*-* } }
+  }();
+}
+int main() { b(2, 3); }