use unittest as an example

std/typecons.d

@@ -1216,40 +1216,6 @@ if (distinctFieldNames!(Specs))
          * $(TROW $(P `%(inner%|sep%)`), $(P The format `inner` is one format, that is applied
          *      on all fields of the `Tuple`. The inner format must be compatible to all
          *      of them.)))
-         *
-         * Example:
-         * ---
-         *  Tuple!(int, double)[3] tupList = [ tuple(1, 1.0), tuple(2, 4.0), tuple(3, 9.0) ];
-         *
-         *  // Default format
-         *  assert(format("%s", tuple("a", 1)) == `Tuple!(string, int)("a", 1)`);
-         *
-         *  // One Format for each individual component
-         *  assert(format("%(%#x v %.4f w %#x%)", tuple(1, 1.0, 10))         == `0x1 v 1.0000 w 0xa`);
-         *  assert(format(  "%#x v %.4f w %#x"  , tuple(1, 1.0, 10).expand)  == `0x1 v 1.0000 w 0xa`);
-         *
-         *  // One Format for all components
-         *  assert(format("%(>%s<%| & %)", tuple("abc", 1, 2.3, [4, 5])) == `>abc< & >1< & >2.3< & >[4, 5]<`);
-         *
-         *  // Array of Tuples
-         *  assert(format("%(%(f(%d) = %.1f%);  %)", tupList) == `f(1) = 1.0;  f(2) = 4.0;  f(3) = 9.0`);
-         *
-         *
-         *  // Error: %( %) missing.
-         *  assertThrown!FormatException(
-         *      format("%d, %f", tuple(1, 2.0)) == `1, 2.0`
-         *  );
-         *
-         *  // Error: %( %| %) missing.
-         *  assertThrown!FormatException(
-         *      format("%d", tuple(1, 2)) == `1, 2`
-         *  );
-         *
-         *  // Error: %d inadequate for double.
-         *  assertThrown!FormatException(
-         *      format("%(%d%|, %)", tuple(1, 2.0)) == `1, 2.0`
-         *  );
-         * ---
          */
         void toString(DG)(scope DG sink) const
         {
@@ -1257,6 +1223,48 @@ if (distinctFieldNames!(Specs))
             toString(sink, f);
         }
 
+        ///
+        static if (Types.length == 0)
+        @safe unittest
+        {
+            import std.format : format, FormatException;
+            import std.exception : assertThrown;
+
+            // enum tupStr = tuple(1, 1.0).toString; // toString is *impure* for double (issue 17628)
+            // static assert(tupStr == `Tuple!(int, double)(1, 1)`);
+
+            Tuple!(int, double)[3] tupList = [ tuple(1, 1.0), tuple(2, 4.0), tuple(3, 9.0) ];
+
+            // Default format
+            assert(format("%s", tuple("a", 1)) == `Tuple!(string, int)("a", 1)`);
+
+            // One Format for each individual component
+            assert(format("%(%#x v %.4f w %#x%)", tuple(1, 1.0, 10))         == `0x1 v 1.0000 w 0xa`);
+            assert(format(  "%#x v %.4f w %#x"  , tuple(1, 1.0, 10).expand)  == `0x1 v 1.0000 w 0xa`);
+
+            // One Format for all components
+            assert(format("%(>%s<%| & %)", tuple("abc", 1, 2.3, [4, 5])) == `>abc< & >1< & >2.3< & >[4, 5]<`);
+
+            // Array of Tuples
+            assert(format("%(%(f(%d) = %.1f%);  %)", tupList) == `f(1) = 1.0;  f(2) = 4.0;  f(3) = 9.0`);
+
+
+            // Error: %( %) missing.
+            assertThrown!FormatException(
+                format("%d, %f", tuple(1, 2.0)) == `1, 2.0`
+            );
+
+            // Error: %( %| %) missing.
+            assertThrown!FormatException(
+                format("%d", tuple(1, 2)) == `1, 2`
+            );
+
+            // Error: %d inadequate for double
+            assertThrown!FormatException(
+                format("%(%d%|, %)", tuple(1, 2.0)) == `1, 2.0`
+            );
+        }
+
         /// ditto
         void toString(DG, Char)(scope DG sink, const ref FormatSpec!Char fmt) const
         {
@@ -1929,39 +1937,8 @@ private template ReverseTupleSpecs(T...)
     import std.format : format, FormatException;
     import std.exception : assertThrown;
 
-    // enum tupStr = tuple(1, 1.0).toString; // toString is *impure*.
+    //enum tupStr = tuple(1, 1.0).toString; // toString is *impure*.
     //static assert(tupStr == `Tuple!(int, double)(1, 1)`);
-
-    Tuple!(int, double)[3] tupList = [ tuple(1, 1.0), tuple(2, 4.0), tuple(3, 9.0) ];
-
-    // Default format
-    assert(format("%s", tuple("a", 1)) == `Tuple!(string, int)("a", 1)`);
-
-    // One Format for each individual component
-    assert(format("%(%#x v %.4f w %#x%)", tuple(1, 1.0, 10))         == `0x1 v 1.0000 w 0xa`);
-    assert(format(  "%#x v %.4f w %#x"  , tuple(1, 1.0, 10).expand)  == `0x1 v 1.0000 w 0xa`);
-
-    // One Format for all components
-    assert(format("%(>%s<%| & %)", tuple("abc", 1, 2.3, [4, 5])) == `>abc< & >1< & >2.3< & >[4, 5]<`);
-
-    // Array of Tuples
-    assert(format("%(%(f(%d) = %.1f%);  %)", tupList) == `f(1) = 1.0;  f(2) = 4.0;  f(3) = 9.0`);
-
-
-    // Error: %( %) missing.
-    assertThrown!FormatException(
-        format("%d, %f", tuple(1, 2.0)) == `1, 2.0`
-    );
-
-    // Error: %( %| %) missing.
-    assertThrown!FormatException(
-        format("%d", tuple(1, 2)) == `1, 2`
-    );
-
-    // Error: %d inadequate for double
-    assertThrown!FormatException(
-        format("%(%d%|, %)", tuple(1, 2.0)) == `1, 2.0`
-    );
 }
 
 // Issue 17803, parte uno