is it regression?

[drug]

https://run.dlang.io/is/HJxtvw

```
import std.stdio, std.typecons, std.math;
void main()
{
     auto foo = nullable(2.0);
     auto bar = nullable(2.0);

     assert (foo.approxEqual(bar));
}
```
Comiling gives the following:
Up to      2.067.1: Failure with output:
-----
onlineapp.d(4): Error: undefined identifier nullable, did you mean 
struct Nullable(T)?
onlineapp.d(5): Error: undefined identifier nullable, did you mean 
struct Nullable(T)?
-----

2.068.2 to 2.072.2: Failure with output:
-----
onlineapp.d(4): Error: undefined identifier 'nullable', did you mean 
struct 'Nullable(T)'?
onlineapp.d(5): Error: undefined identifier 'nullable', did you mean 
struct 'Nullable(T)'?
-----

2.073.2 to 2.077.1: Success and no output
Since      2.078.1: Failure with output:
-----
/path/to/dmd.linux/dmd2/linux/bin64/../../src/phobos/std/math.d(7575): 
Error: template std.math.approxEqual cannot deduce function from 
argument types !()(Nullable!double, Nullable!double, double, double), 
candidates are:
/path/to/dmd.linux/dmd2/linux/bin64/../../src/phobos/std/math.d(7499): 
      std.math.approxEqual(T, U, V)(T lhs, U rhs, V maxRelDiff, V 
maxAbsDiff = 1e-05) if ((isNumeric!T || isInputRange!T && 
isNumeric!(ElementType!T)) && (isNumeric!U || isInputRange!U && 
isNumeric!(ElementType!U)) && isNumeric!V)
/path/to/dmd.linux/dmd2/linux/bin64/../../src/phobos/std/math.d(7573): 
      std.math.approxEqual(T, U)(T lhs, U rhs)
onlineapp.d(7): Error: template instance 
std.math.approxEqual!(Nullable!double, Nullable!double) error instantiating
-----
tldr - std.math.approxEqual stops deduced its args type when Nullable is 
used.