Improve specs for __traits(isSame) over two tuples

Related bug fix dlang/dmd#11373

Author: Nils Lankila

spec/traits.dd

@@ -1513,6 +1513,29 @@ void main()
     static assert(!__traits(isSame, (A a) => ++a.a, (B a) => ++a.a));
 }
 ---
+)
+
+        $(P If the two arguments are tuples then `isSame` returns `true` if the
+        two tuples, after expansion, have the same length and if each pair
+        of nth argument respects the constraints previously specified.)
+
+$(SPEC_RUNNABLE_EXAMPLE_COMPILE
+---
+import std.stdio;
+import std.meta;
+
+struct S { }
+
+void main()
+{
+    // true, like __traits(isSame(0,0)) && __traits(isSame(1,1))
+    writeln(__traits(isSame, AliasSeq!(0,1), AliasSeq!(0,1)));
+    // false, like __traits(isSame(S,std.meta)) && __traits(isSame(1,1))
+    writeln(__traits(isSame, AliasSeq!(S,1), AliasSeq!(std.meta,1)));
+    // false, the length of the sequences is different
+    writeln(__traits(isSame, AliasSeq!(1), AliasSeq!(1,2)));
+}
+---
 )
 
 $(H2 $(GNAME compiles))