Better std.algorithm.comparison.clamp error messages

changelog/clamp_assert_message.dd

@@ -0,0 +1,8 @@
+Better static assert messages for `std.algorithm.comparison.clamp`
+
+Until now, `clamp` used a template constraint to check if the passed types
+could be used. If they were not, it was very tedious to figure out why.
+
+As the template constraint is not used for overload resolution
+the constrains are moved into static asserts with expressive error
+messages.

std/algorithm/comparison.d

@@ -577,19 +577,20 @@ Returns:
     and `T3` are different.
 */
 T1 clamp(T1, T2, T3)(T1 val, T2 lower, T3 upper)
-if (is(typeof(val.lessThan(lower) ? lower : val.greaterThan(upper) ? upper : val))
-    && (is(T2 : T1) && is(T3 : T1)))
-// cannot use :
-// `if (is(typeof(val.lessThan(lower) ? lower : val.greaterThan(upper) ? upper : val) : T1))
-// because of https://issues.dlang.org/show_bug.cgi?id=16235.
-// Once that is fixed, we can simply use the ternary in both the template constraint
-// and the template body
-in
 {
+    static assert(is(T2 : T1), "T2 of type '", T2.stringof
+            , "' must be implicitly convertible to type of T1 '"
+            , T1.stringof, "'");
+    static assert(is(T3 : T1), "T3 of type '", T3.stringof
+            , "' must be implicitly convertible to type of T1 '"
+            , T1.stringof, "'");
+
     assert(!lower.greaterThan(upper), "Lower can't be greater than upper.");
-}
-do
-{
+
+    // `if (is(typeof(val.lessThan(lower) ? lower : val.greaterThan(upper) ? upper : val) : T1))
+    // because of https://issues.dlang.org/show_bug.cgi?id=16235.
+    // Once that is fixed, we can simply use the ternary in both the template constraint
+    // and the template body
     if (val.lessThan(lower))
         return lower;
     else if (val.greaterThan(upper))