compiler: fix evaluation order of LHS index expressions

    
    The spec says that when an index expression appears on the left hand
    side of an assignment, the operands should be evaluated. The
    gofrontend code was assuming that that only referred to the index
    operand. But discussion of https://golang.org/issue/23188 has
    clarified that this means both the slice/map/string operand and the
    index operand. Adjust the gofrontend code accordingly.
    
    Fixes golang/go#23188
    
    Reviewed-on: https://go-review.googlesource.com/123155

From-SVN: r262554

gcc/go/gofrontend/MERGE

-8ad67a72a4fa59efffc891e73ecf10020e3c565d
+ea7ac7784791dca517b6681a02c39c11bf136755
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -10898,6 +10898,20 @@ Array_index_expression::do_check_types(Gogo*)
     }
 }
 
+// The subexpressions of an array index must be evaluated in order.
+// If this is indexing into an array, rather than a slice, then only
+// the index should be evaluated.  Since this is called for values on
+// the left hand side of an assigment, evaluating the array, meaning
+// copying the array, will cause a different array to be modified.
+
+bool
+Array_index_expression::do_must_eval_subexpressions_in_order(
+    int* skip) const
+{
+  *skip = this->array_->type()->is_slice_type() ? 0 : 1;
+  return true;
+}
+
 // Flatten array indexing by using temporary variables for slices and indexes.
 
 Expression*

gcc/go/gofrontend/expressions.h

@@ -2771,12 +2771,10 @@ class Index_expression : public Parser_expression
 				this->location());
   }
 
+  // This shouldn't be called--we don't know yet.
   bool
-  do_must_eval_subexpressions_in_order(int* skip) const
-  {
-    *skip = 1;
-    return true;
-  }
+  do_must_eval_subexpressions_in_order(int*) const
+  { go_unreachable(); }
 
   void
   do_dump_expression(Ast_dump_context*) const;
@@ -2882,11 +2880,7 @@ class Array_index_expression : public Expression
   }
 
   bool
-  do_must_eval_subexpressions_in_order(int* skip) const
-  {
-    *skip = 1;
-    return true;
-  }
+  do_must_eval_subexpressions_in_order(int* skip) const;
 
   bool
   do_is_addressable() const;
@@ -2965,11 +2959,8 @@ class String_index_expression : public Expression
   }
 
   bool
-  do_must_eval_subexpressions_in_order(int* skip) const
-  {
-    *skip = 1;
-    return true;
-  }
+  do_must_eval_subexpressions_in_order(int*) const
+  { return true; }
 
   Bexpression*
   do_get_backend(Translate_context*);
@@ -3052,11 +3043,8 @@ class Map_index_expression : public Expression
   }
 
   bool
-  do_must_eval_subexpressions_in_order(int* skip) const
-  {
-    *skip = 1;
-    return true;
-  }
+  do_must_eval_subexpressions_in_order(int*) const
+  { return true; }
 
   // A map index expression is an lvalue but it is not addressable.