gcc/testsuite/gdc.test/compilable/test7815.d

   1 // REQUIRED_ARGS: -o-
   2 /*
   3 TEST_OUTPUT:
   4 ---
   5 ---
   6 */
   7
   8 mixin template Helpers()
   9 {
  10     static if (is(Flags!Move))
  11     {
  12         Flags!Move flags;
  13     }
  14     else
  15     {
  16         pragma(msg, "X: ", __traits(derivedMembers, Flags!Move));
  17     }
  18 }
  19
  20 template Flags(T)
  21 {
  22     mixin({
  23         int defs = 1;
  24         foreach (name; __traits(derivedMembers, Move))
  25         {
  26             defs++;
  27         }
  28         if (defs)
  29         {
  30             return "struct Flags { bool x; }";
  31         }
  32         else
  33         {
  34             return "";
  35         }
  36     }());
  37 }
  38
  39 struct Move
  40 {
  41     int a;
  42     mixin Helpers!();
  43 }
  44
  45 enum a7815 = Move.init.flags;
  46
  47 /+
  48 This originally was an invalid case:
  49 When the Move struct member is analyzed:
  50 1. mixin Helpers!() is instantiated.
  51 2. In Helpers!(), static if and its condition is(Flags!Move)) evaluated.
  52 3. In Flags!Move, string mixin evaluates and CTFE lambda.
  53 4. __traits(derivedMembers, Move) tries to see the member of Move.
  54    4a. mixin Helpers!() member is analyzed.
  55    4b. `static if (is(Flags!Move))` in Helpers!() is evaluated
  56    4c. The Flags!Move instantiation is already in progress, so it cannot be resolved.
  57    4d. `static if` fails because Flags!Move cannot be determined as a type.
  58 5. __traits(derivedMembers, Move) returns a 1-length tuple("a").
  59 6. The lambda in Flags!Move returns a string "struct Flags {...}", then
  60    Flags!Move is instantiated to a new struct Flags.
  61 7. Finally Move struct does not have flags member, then the `enum a7815`
  62    definition will fail in its initializer.
  63
  64 Now, static if will behave like a string mixin: it is invisible during its own expansion.
  65 +/