[sv-bc] Erratatum/proposal for 7.14 (structure expression default values)

Using structure expressions as default values without an
explicit cast can lead to unintended results. As there's no
particular benefit to omitting the cast, I propose to add the
following sentence to 7.14 after the first example of using
the default keyword.

  "The expression after the 'default' keyword shall not itself
   be a structure expression or structure literal unless an
   explicit cast is used."

Consider the following example (from Yong Xiao)

  module m;
    parameter a = 1, b = 2;
    typedef struct { byte a, b;} F1;
    typedef struct { F1 f1; byte f2 [10:0];} S1;
    initial begin
      S1 y = {default:{a:20, b:4, default:'0}};
    end
   endmodule

which would apparently (???) be equivalent to

   y.f1.a = 20;
   y.f1.b = 4;
   y.f2[a] = 20;
   y.f2[b] = 4;
   y.f2[...other_idx...] = 0;

and is also vulnerable to a change in the parameter names a and b.

-- Brad

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Received on Sun Apr 11 13:28:52 2004