Use til::some<T,N> to replace the SomeViewports class (#4174)

<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
## Summary of the Pull Request

Let's give it a test drive.

<!-- Other than the issue solved, is this relevant to any other issues/existing PRs? --> 
## References

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
* [x] Closes #4162 
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

<!-- Provide a more detailed description of the PR, other things fixed or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

Build and run it.

src/inc/til.h

@@ -3,23 +3,11 @@
 
 #pragma once
 
+#include "til/at.h"
 #include "til/some.h"
 
 namespace til // Terminal Implementation Library. Also: "Today I Learned"
 {
-    // The at function declares that you've already sufficiently checked that your array access
-    // is in range before retrieving an item inside it at an offset.
-    // This is to save double/triple/quadruple testing in circumstances where you are already
-    // pivoting on the length of a set and now want to pull elements out of it by offset
-    // without checking again.
-    // gsl::at will do the check again. As will .at(). And using [] will have a warning in audit.
-    template<class T>
-    constexpr auto at(T& cont, const size_t i) -> decltype(cont[cont.size()])
-    {
-#pragma warning(suppress : 26482) // Suppress bounds.2 check for indexing with constant expressions
-#pragma warning(suppress : 26446) // Suppress bounds.4 check for subscript operator.
-        return cont[i];
-    }
 }
 
 // These sit outside the namespace because they sit outside for WIL too.

src/inc/til/at.h

@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+#pragma once
+
+namespace til
+{
+    // The at function declares that you've already sufficiently checked that your array access
+    // is in range before retrieving an item inside it at an offset.
+    // This is to save double/triple/quadruple testing in circumstances where you are already
+    // pivoting on the length of a set and now want to pull elements out of it by offset
+    // without checking again.
+    // gsl::at will do the check again. As will .at(). And using [] will have a warning in audit.
+    template<class T>
+    constexpr auto at(T& cont, const size_t i) -> decltype(cont[cont.size()])
+    {
+#pragma warning(suppress : 26482) // Suppress bounds.2 check for indexing with constant expressions
+#pragma warning(suppress : 26446) // Suppress bounds.4 check for subscript operator.
+        return cont[i];
+    }
+}

src/inc/til/some.h

@@ -154,7 +154,7 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
                 _outOfRange();
             }
 
-            _array[_used] = val;
+            til::at(_array, _used) = val;
 
             ++_used;
         }
@@ -168,7 +168,7 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
 
             --_used;
 
-            _array[_used] = 0;
+            til::at(_array, _used) = 0;
         }
 
         [[noreturn]] void _invalidArg() const

src/types/inc/viewport.hpp

@@ -16,12 +16,16 @@ Author(s):
 
 namespace Microsoft::Console::Types
 {
-    struct SomeViewports;
+    class Viewport;
+
+    using SomeViewports = til::some<Viewport, 4>;
 
     class Viewport final
     {
     public:
         ~Viewport() {}
+        constexpr Viewport() noexcept :
+            _sr({ 0, 0, -1, -1 }){};
         Viewport(const Viewport& other) noexcept;
         Viewport(Viewport&&) = default;
         Viewport& operator=(const Viewport&) & = default;
@@ -125,28 +129,6 @@ namespace Microsoft::Console::Types
         friend class ViewportTests;
 #endif
     };
-
-    struct SomeViewports final
-    {
-        unsigned char used{ 0 };
-        std::array<Viewport, 4> viewports{ Viewport::Empty(), Viewport::Empty(), Viewport::Empty(), Viewport::Empty() };
-
-        // These two methods are to make this vaguely look like a std::vector.
-
-        // Size is the number of viewports that are valid inside this structure
-        size_t size() const noexcept { return used; }
-
-        // At retrieves a viewport at a particular index. If you retrieve beyond the valid size(),
-        // it will throw std::out_of_range
-        const Viewport& at(size_t index) const
-        {
-            if (index >= used)
-            {
-                throw std::out_of_range("Access attempted beyond valid size.");
-            }
-            return viewports.at(index);
-        }
-    };
 }
 
 inline COORD operator-(const COORD& a, const COORD& b) noexcept

src/types/viewport.cpp

@@ -18,7 +18,7 @@ Viewport::Viewport(const Viewport& other) noexcept :
 
 Viewport Viewport::Empty() noexcept
 {
-    return Viewport({ 0, 0, -1, -1 });
+    return Viewport();
 }
 
 Viewport Viewport::FromInclusive(const SMALL_RECT sr) noexcept
@@ -895,6 +895,7 @@ Viewport Viewport::ToOrigin() const noexcept
 //   that was covered by `main` before the regional area of `removeMe` was taken out.
 // - You must check that each viewport .IsValid() before using it.
 [[nodiscard]] SomeViewports Viewport::Subtract(const Viewport& original, const Viewport& removeMe) noexcept
+try
 {
     SomeViewports result;
 
@@ -907,7 +908,7 @@ Viewport Viewport::ToOrigin() const noexcept
     if (!intersection.IsValid())
     {
         // Just put the original rectangle into the results and return early.
-        result.viewports.at(result.used++) = original;
+        result.push_back(original);
     }
     // If the original rectangle matches the intersection, there is nothing to return.
     else if (original != intersection)
@@ -1003,27 +1004,28 @@ Viewport Viewport::ToOrigin() const noexcept
 
         if (top.IsValid())
         {
-            result.viewports.at(result.used++) = top;
+            result.push_back(top);
         }
 
         if (bottom.IsValid())
         {
-            result.viewports.at(result.used++) = bottom;
+            result.push_back(bottom);
         }
 
         if (left.IsValid())
         {
-            result.viewports.at(result.used++) = left;
+            result.push_back(left);
         }
 
         if (right.IsValid())
         {
-            result.viewports.at(result.used++) = right;
+            result.push_back(right);
         }
     }
 
     return result;
 }
+CATCH_FAIL_FAST()
 
 // Method Description:
 // - Returns true if the rectangle described by this Viewport has internal space