Merge pull request #3118 from n8sh/backport-phobos-7490

Port more efficient Filter & staticMap from dlang/phobos#7490
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

Author: dlang-bot

src/core/internal/traits.d

@@ -466,6 +466,26 @@ template Filter(alias pred, TList...)
         else
             alias Filter = AliasSeq!();
     }
+    /* The next case speeds up compilation by reducing
+     * the number of Filter instantiations
+     */
+    else static if (TList.length == 2)
+    {
+        static if (pred!(TList[0]))
+        {
+            static if (pred!(TList[1]))
+                alias Filter = AliasSeq!(TList[0], TList[1]);
+            else
+                alias Filter = AliasSeq!(TList[0]);
+        }
+        else
+        {
+            static if (pred!(TList[1]))
+                alias Filter = AliasSeq!(TList[1]);
+            else
+                alias Filter = AliasSeq!();
+        }
+    }
     else
     {
         alias Filter =
@@ -486,6 +506,37 @@ template staticMap(alias F, T...)
     {
         alias staticMap = AliasSeq!(F!(T[0]));
     }
+    /* Cases 2 to 8 improve compile performance by reducing
+     * the number of recursive instantiations of staticMap
+     */
+    else static if (T.length == 2)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]));
+    }
+    else static if (T.length == 3)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]));
+    }
+    else static if (T.length == 4)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]), F!(T[3]));
+    }
+    else static if (T.length == 5)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]), F!(T[3]), F!(T[4]));
+    }
+    else static if (T.length == 6)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]), F!(T[3]), F!(T[4]), F!(T[5]));
+    }
+    else static if (T.length == 7)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]), F!(T[3]), F!(T[4]), F!(T[5]), F!(T[6]));
+    }
+    else static if (T.length == 8)
+    {
+        alias staticMap = AliasSeq!(F!(T[0]), F!(T[1]), F!(T[2]), F!(T[3]), F!(T[4]), F!(T[5]), F!(T[6]), F!(T[7]));
+    }
     else
     {
         alias staticMap =