Fix #21562 - Trying to fill a void[] causes dmd to segfault

Author: ntrel

compiler/src/dmd/expressionsem.d

@@ -11635,7 +11635,14 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             t1.isStaticOrDynamicArray() &&
             t1.nextOf().toBasetype().ty == Tvoid)
         {
-            if (t2.nextOf().implicitConvTo(t1.nextOf()))
+            auto t2n = t2.nextOf();
+            if (!t2n)
+            {
+                // filling not allowed
+                error(exp.loc, "cannot copy `%s` to `%s`",
+                    t2.toChars(), t1.toChars());
+            }
+            else if (t2n.implicitConvTo(t1.nextOf()))
             {
                 if (sc.setUnsafe(false, exp.loc, "copying `%s` to `%s`", t2, t1))
                     return setError();

compiler/test/fail_compilation/test15704.d

@@ -1,9 +1,11 @@
 /*
  * TEST_OUTPUT:
 ---
-fail_compilation/test15704.d(17): Error: copying `void[]` to `void[]` is not allowed in a `@safe` function
-fail_compilation/test15704.d(18): Error: copying `const(void)[]` to `void[]` is not allowed in a `@safe` function
-fail_compilation/test15704.d(19): Deprecation: copying `int[]` to `void[]` will become `@system` in a future release
+fail_compilation/test15704.d(19): Error: copying `void[]` to `void[]` is not allowed in a `@safe` function
+fail_compilation/test15704.d(20): Error: copying `const(void)[]` to `void[]` is not allowed in a `@safe` function
+fail_compilation/test15704.d(21): Deprecation: copying `int[]` to `void[]` will become `@system` in a future release
+fail_compilation/test15704.d(22): Error: cannot implicitly convert expression `cast(byte)0` of type `byte` to `void[]`
+fail_compilation/test15704.d(22): Error: cannot copy `byte` to `void[]`
 ---
  */
 
@@ -17,4 +19,5 @@ void main() @safe {
     arr1[] = arr2[];  // overwrites pointers with arbitrary ints
     arr1[] = new const(void)[3];
     arr1[] = [5];
+    arr1[] = byte(); // filling not allowed
 }