Refactor: access SumType value by index, not type

Previously, code that already knew the desired index was forced to
compute a type to pass to get!T or memberName!T, just for get!T or
memberName!T to turn that type back into an index. Removing this
unnecessary round trip simplifies the code.

Additionally, since memberName is no longer dependent on SumType.Types,
it can be moved to module scope, and its instantiations can be shared
across different SumType instances.

Author: pbackus

std/sumtype.d

@@ -254,6 +254,8 @@ private enum hasPostblit(T) = __traits(hasPostblit, T);
 
 private enum isInout(T) = is(T == inout);
 
+private enum memberName(size_t tid) = "values_" ~ toCtString!tid;
+
 /**
  * A [tagged union](https://en.wikipedia.org/wiki/Tagged_union) that can hold a
  * single value from any of a specified set of types.
@@ -290,45 +292,42 @@ private:
 
     union Storage
     {
-        // Workaround for https://issues.dlang.org/show_bug.cgi?id=20068
-        template memberName(T)
-        if (IndexOf!(T, Types) >= 0)
-        {
-            enum tid = IndexOf!(T, Types);
-            mixin("enum memberName = `values_", toCtString!tid, "`;");
-        }
 
-        static foreach (T; Types)
+        static foreach (tid, T; Types)
         {
-            mixin("T ", memberName!T, ";");
+            /+
+            Giving these fields individual names makes it possible to use brace
+            initialization for Storage.
+            +/
+            mixin("T ", memberName!tid, ";");
         }
     }
 
     Storage storage;
     Tag tag;
 
-    /* Accesses the value stored in a SumType.
+    /* Accesses the value stored in a SumType by its index.
      *
      * This method is memory-safe, provided that:
      *
      *   1. A SumType's tag is always accurate.
-     *   2. A SumType cannot be assigned to in @safe code if that assignment
-     *      could cause unsafe aliasing.
+     *   2. A SumType's value cannot be unsafely aliased in @safe code.
      *
      * All code that accesses a SumType's tag or storage directly, including
      * @safe code in this module, must be manually checked to ensure that it
      * does not violate either of the above requirements.
      */
     @trusted
-    ref inout(T) get(T)() inout
-    if (IndexOf!(T, Types) >= 0)
+    // Explicit return type omitted
+    // Workaround for https://github.com/dlang/dmd/issues/20549
+    ref get(size_t tid)() inout
+    if (tid < Types.length)
     {
-        enum tid = IndexOf!(T, Types);
         assert(tag == tid,
-            "This `" ~ SumType.stringof ~
-            "` does not contain a(n) `" ~ T.stringof ~ "`"
+            "This `" ~ SumType.stringof ~ "`" ~
+            "does not contain a(n) `" ~ Types[tid].stringof ~ "`"
         );
-        return __traits(getMember, storage, Storage.memberName!T);
+        return storage.tupleof[tid];
     }
 
 public:
@@ -363,11 +362,11 @@ public:
             static if (isCopyable!T)
             {
                 // Workaround for https://issues.dlang.org/show_bug.cgi?id=21542
-                __traits(getMember, storage, Storage.memberName!T) = __ctfe ? value : forward!value;
+                storage.tupleof[tid] = __ctfe ? value : forward!value;
             }
             else
             {
-                __traits(getMember, storage, Storage.memberName!T) = forward!value;
+                storage.tupleof[tid] = forward!value;
             }
 
             tag = tid;
@@ -380,7 +379,7 @@ public:
                 /// ditto
                 this(const(T) value) const
                 {
-                    __traits(getMember, storage, Storage.memberName!T) = value;
+                    storage.tupleof[tid] = value;
                     tag = tid;
                 }
             }
@@ -397,7 +396,7 @@ public:
                 /// ditto
                 this(immutable(T) value) immutable
                 {
-                    __traits(getMember, storage, Storage.memberName!T) = value;
+                    storage.tupleof[tid] = value;
                     tag = tid;
                 }
             }
@@ -415,7 +414,7 @@ public:
                 this(Value)(Value value) inout
                 if (is(Value == DeducedParameterType!(inout(T))))
                 {
-                    __traits(getMember, storage, Storage.memberName!T) = value;
+                    storage.tupleof[tid] = value;
                     tag = tid;
                 }
             }
@@ -442,10 +441,9 @@ public:
                 storage = other.match!((ref value) {
                     alias OtherTypes = Map!(InoutOf, Types);
                     enum tid = IndexOf!(typeof(value), OtherTypes);
-                    alias T = Types[tid];
 
                     mixin("inout(Storage) newStorage = { ",
-                        Storage.memberName!T, ": value",
+                        memberName!tid, ": value",
                     " };");
 
                     return newStorage;
@@ -462,10 +460,10 @@ public:
                 this(ref SumType other)
                 {
                     storage = other.match!((ref value) {
-                        alias T = typeof(value);
+                        enum tid = IndexOf!(typeof(value), Types);
 
                         mixin("Storage newStorage = { ",
-                            Storage.memberName!T, ": value",
+                            memberName!tid, ": value",
                         " };");
 
                         return newStorage;
@@ -487,10 +485,9 @@ public:
                     storage = other.match!((ref value) {
                         alias OtherTypes = Map!(ConstOf, Types);
                         enum tid = IndexOf!(typeof(value), OtherTypes);
-                        alias T = Types[tid];
 
                         mixin("const(Storage) newStorage = { ",
-                            Storage.memberName!T, ": value",
+                            memberName!tid, ": value",
                         " };");
 
                         return newStorage;
@@ -512,10 +509,9 @@ public:
                     storage = other.match!((ref value) {
                         alias OtherTypes = Map!(ImmutableOf, Types);
                         enum tid = IndexOf!(typeof(value), OtherTypes);
-                        alias T = Types[tid];
 
                         mixin("immutable(Storage) newStorage = { ",
-                            Storage.memberName!T, ": value",
+                            memberName!tid, ": value",
                         " };");
 
                         return newStorage;
@@ -637,13 +633,13 @@ public:
                 {
                     // Workaround for https://issues.dlang.org/show_bug.cgi?id=21542
                     mixin("Storage newStorage = { ",
-                        Storage.memberName!T, ": __ctfe ? rhs : forward!rhs",
+                        memberName!tid, ": __ctfe ? rhs : forward!rhs",
                     " };");
                 }
                 else
                 {
                     mixin("Storage newStorage = { ",
-                        Storage.memberName!T, ": forward!rhs",
+                        memberName!tid, ": forward!rhs",
                     " };");
                 }
 
@@ -1146,7 +1142,7 @@ version (D_BetterC) {} else
     alias MySum = SumType!(ubyte, void*[2]);
 
     MySum x = [null, cast(void*) 0x12345678];
-    void** p = &x.get!(void*[2])[1];
+    void** p = &x.get!1[1];
     x = ubyte(123);
 
     assert(*p != cast(void*) 0x12345678);
@@ -1178,8 +1174,8 @@ version (D_BetterC) {} else
     catch (Exception e) {}
 
     assert(
-        (x.tag == 0 && x.get!A.value == 123) ||
-        (x.tag == 1 && x.get!B.value == 456)
+        (x.tag == 0 && x.get!0.value == 123) ||
+        (x.tag == 1 && x.get!1.value == 456)
     );
 }
 
@@ -1238,8 +1234,8 @@ version (D_BetterC) {} else
     SumType!(S[1]) x = [S(0)];
     SumType!(S[1]) y = x;
 
-    auto xval = x.get!(S[1])[0].n;
-    auto yval = y.get!(S[1])[0].n;
+    auto xval = x.get!0[0].n;
+    auto yval = y.get!0[0].n;
 
     assert(xval != yval);
 }
@@ -1324,8 +1320,8 @@ version (D_BetterC) {} else
     SumType!S y;
     y = x;
 
-    auto xval = x.get!S.n;
-    auto yval = y.get!S.n;
+    auto xval = x.get!0.n;
+    auto yval = y.get!0.n;
 
     assert(xval != yval);
 }
@@ -1399,8 +1395,8 @@ version (D_BetterC) {} else
     SumType!S x = S();
     SumType!S y = x;
 
-    auto xval = x.get!S.n;
-    auto yval = y.get!S.n;
+    auto xval = x.get!0.n;
+    auto yval = y.get!0.n;
 
     assert(xval != yval);
 }
@@ -1872,10 +1868,10 @@ private template matchImpl(Flag!"exhaustive" exhaustive, handlers...)
              * argument's tag, so there's no need for TagTuple.
              */
             enum handlerArgs(size_t caseId) =
-                "args[0].get!(SumTypes[0].Types[" ~ toCtString!caseId ~ "])()";
+                "args[0].get!(" ~ toCtString!caseId ~ ")()";
 
             alias valueTypes(size_t caseId) =
-                typeof(args[0].get!(SumTypes[0].Types[caseId])());
+                typeof(args[0].get!(caseId)());
 
             enum numCases = SumTypes[0].Types.length;
         }
@@ -1901,9 +1897,7 @@ private template matchImpl(Flag!"exhaustive" exhaustive, handlers...)
 
                 template getType(size_t i)
                 {
-                    enum tid = tags[i];
-                    alias T = SumTypes[i].Types[tid];
-                    alias getType = typeof(args[i].get!T());
+                    alias getType = typeof(args[i].get!(tags[i])());
                 }
 
                 alias valueTypes = Map!(getType, Iota!(tags.length));
@@ -2128,8 +2122,7 @@ private template handlerArgs(size_t caseId, typeCounts...)
     {
         handlerArgs = AliasSeq!(
             handlerArgs,
-            "args[" ~ toCtString!i ~ "].get!(SumTypes[" ~ toCtString!i ~ "]" ~
-            ".Types[" ~ toCtString!(tags[i]) ~ "])(), "
+            "args[" ~ toCtString!i ~ "].get!(" ~ toCtString!(tags[i]) ~ ")(), "
         );
     }
 }
@@ -2393,7 +2386,7 @@ version (D_Exceptions)
         (ref double d) { d *= 2; }
     );
 
-    assert(value.get!double.isClose(6.28));
+    assert(value.get!1.isClose(6.28));
 }
 
 // Unreachable handlers