compiler: check slice to pointer-to-array conversion element type

When checking a slice to pointer-to-array conversion, I forgot to
verify that the elements types are identical.

For golang/go#395

Change-Id: I533ac52c0b390af96fce78a8c468ae9d8ad79da9
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/339329
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

go/expressions.cc

@@ -3962,7 +3962,10 @@ Type_conversion_expression::do_lower(Gogo*, Named_object*,
   if (type->points_to() != NULL
       && type->points_to()->array_type() != NULL
       && !type->points_to()->is_slice_type()
-      && val->type()->is_slice_type())
+      && val->type()->is_slice_type()
+      && Type::are_identical(type->points_to()->array_type()->element_type(),
+			     val->type()->array_type()->element_type(),
+			     0, NULL))
     {
       Temporary_statement* val_temp = NULL;
       if (!val->is_multi_eval_safe())

go/types.cc

@@ -846,7 +846,9 @@ Type::are_convertible(const Type* lhs, const Type* rhs, std::string* reason)
   if (rhs->is_slice_type()
       && lhs->points_to() != NULL
       && lhs->points_to()->array_type() != NULL
-      && !lhs->points_to()->is_slice_type())
+      && !lhs->points_to()->is_slice_type()
+      && Type::are_identical(lhs->points_to()->array_type()->element_type(),
+			     rhs->array_type()->element_type(), 0, reason))
     return true;
 
   // An unsafe.Pointer type may be converted to any pointer type or to