Changing Span/Memory to return default on null instead of throwing. (#16186)

* Additional changes based on feedback (bounds checks)

* Adding remaining bounds checks

7 files changed, 98 insertions, 57 deletions

diff --git a/src/mscorlib/shared/System/Memory.cs b/src/mscorlib/shared/System/Memory.cs
index 4df630f764..8fc5b04ee4 100644
--- a/src/mscorlib/shared/System/Memory.cs
+++ b/src/mscorlib/shared/System/Memory.cs
@@ -40,14 +40,16 @@ namespace System
         /// Creates a new memory over the entirety of the target array.
         /// </summary>
         /// <param name="array">The target array.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
-        /// reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public Memory(T[] array)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                this = default;
+                return; // returns default
+            }
             if (default(T) == null && array.GetType() != typeof(T[]))
                 ThrowHelper.ThrowArrayTypeMismatchException();
 
@@ -63,8 +65,7 @@ namespace System
         /// <param name="array">The target array.</param>
         /// <param name="start">The index at which to begin the memory.</param>
         /// <param name="length">The number of items in the memory.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
-        /// reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> or end index is not in the range (&lt;0 or &gt;=Length).
@@ -73,7 +74,12 @@ namespace System
         public Memory(T[] array, int start, int length)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                this = default;
+                return; // returns default
+            }
             if (default(T) == null && array.GetType() != typeof(T[]))
                 ThrowHelper.ThrowArrayTypeMismatchException();
             if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
@@ -106,7 +112,7 @@ namespace System
         /// <summary>
         /// Defines an implicit conversion of an array to a <see cref="Memory{T}"/>
         /// </summary>
-        public static implicit operator Memory<T>(T[] array) => (array != null) ? new Memory<T>(array) : default;
+        public static implicit operator Memory<T>(T[] array) => new Memory<T>(array);
 
         /// <summary>
         /// Defines an implicit conversion of a <see cref="ArraySegment{T}"/> to a <see cref="Memory{T}"/>
diff --git a/src/mscorlib/shared/System/ReadOnlyMemory.cs b/src/mscorlib/shared/System/ReadOnlyMemory.cs
index 5a8a6fcc21..166a204edb 100644
--- a/src/mscorlib/shared/System/ReadOnlyMemory.cs
+++ b/src/mscorlib/shared/System/ReadOnlyMemory.cs
@@ -38,14 +38,16 @@ namespace System
         /// Creates a new memory over the entirety of the target array.
         /// </summary>
         /// <param name="array">The target array.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
-        /// reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public ReadOnlyMemory(T[] array)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                this = default;
+                return; // returns default
+            }
 
             _object = array;
             _index = 0;
@@ -59,8 +61,7 @@ namespace System
         /// <param name="array">The target array.</param>
         /// <param name="start">The index at which to begin the memory.</param>
         /// <param name="length">The number of items in the memory.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
-        /// reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> or end index is not in the range (&lt;0 or &gt;=Length).
@@ -69,7 +70,12 @@ namespace System
         public ReadOnlyMemory(T[] array, int start, int length)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                this = default;
+                return; // returns default
+            }
             if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 
@@ -97,7 +103,7 @@ namespace System
         /// <summary>
         /// Defines an implicit conversion of an array to a <see cref="ReadOnlyMemory{T}"/>
         /// </summary>
-        public static implicit operator ReadOnlyMemory<T>(T[] array) => (array != null) ? new ReadOnlyMemory<T>(array) : default;
+        public static implicit operator ReadOnlyMemory<T>(T[] array) => new ReadOnlyMemory<T>(array);
 
         /// <summary>
         /// Defines an implicit conversion of a <see cref="ArraySegment{T}"/> to a <see cref="ReadOnlyMemory{T}"/>
diff --git a/src/mscorlib/shared/System/ReadOnlySpan.Fast.cs b/src/mscorlib/shared/System/ReadOnlySpan.Fast.cs
index 5c627cc166..2a30b2a927 100644
--- a/src/mscorlib/shared/System/ReadOnlySpan.Fast.cs
+++ b/src/mscorlib/shared/System/ReadOnlySpan.Fast.cs
@@ -39,13 +39,16 @@ namespace System
         /// Creates a new read-only span over the entirety of the target array.
         /// </summary>
         /// <param name="array">The target array.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// reference (Nothing in Visual Basic).</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public ReadOnlySpan(T[] array)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                this = default;
+                return; // returns default
+            }
 
             _pointer = new ByReference<T>(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()));
             _length = array.Length;
@@ -58,7 +61,7 @@ namespace System
         /// <param name="array">The target array.</param>
         /// <param name="start">The index at which to begin the read-only span.</param>
         /// <param name="length">The number of items in the read-only span.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// reference (Nothing in Visual Basic).</exception>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> or end index is not in the range (&lt;0 or &gt;=Length).
@@ -67,7 +70,12 @@ namespace System
         public ReadOnlySpan(T[] array, int start, int length)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                this = default;
+                return; // returns default
+            }
             if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 
diff --git a/src/mscorlib/shared/System/ReadOnlySpan.cs b/src/mscorlib/shared/System/ReadOnlySpan.cs
index f0f022605e..4d747605ad 100644
--- a/src/mscorlib/shared/System/ReadOnlySpan.cs
+++ b/src/mscorlib/shared/System/ReadOnlySpan.cs
@@ -29,11 +29,11 @@ namespace System
 #if !FEATURE_PORTABLE_SPAN
             [NonVersionable]
 #endif // !FEATURE_PORTABLE_SPAN
-             get
-             {
-                 return _length;
-             }
-         }
+            get
+            {
+                return _length;
+            }
+        }
 
         /// <summary>
         /// Returns true if Length is 0.
@@ -83,13 +83,13 @@ namespace System
         /// <summary>
         /// Defines an implicit conversion of an array to a <see cref="ReadOnlySpan{T}"/>
         /// </summary>
-        public static implicit operator ReadOnlySpan<T>(T[] array) => array != null ? new ReadOnlySpan<T>(array) : default;
+        public static implicit operator ReadOnlySpan<T>(T[] array) => new ReadOnlySpan<T>(array);
 
         /// <summary>
         /// Defines an implicit conversion of a <see cref="ArraySegment{T}"/> to a <see cref="ReadOnlySpan{T}"/>
         /// </summary>
         public static implicit operator ReadOnlySpan<T>(ArraySegment<T> arraySegment)
-            => arraySegment.Array != null ? new ReadOnlySpan<T>(arraySegment.Array, arraySegment.Offset, arraySegment.Count) : default;
+            => new ReadOnlySpan<T>(arraySegment.Array, arraySegment.Offset, arraySegment.Count);
 
         /// <summary>
         /// Returns a 0-length read-only span whose base is the null pointer.
diff --git a/src/mscorlib/shared/System/Span.Fast.cs b/src/mscorlib/shared/System/Span.Fast.cs
index 1382e3d214..e0d4ec3227 100644
--- a/src/mscorlib/shared/System/Span.Fast.cs
+++ b/src/mscorlib/shared/System/Span.Fast.cs
@@ -39,14 +39,17 @@ namespace System
         /// Creates a new span over the entirety of the target array.
         /// </summary>
         /// <param name="array">The target array.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// reference (Nothing in Visual Basic).</exception>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public Span(T[] array)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                this = default;
+                return; // returns default
+            }
             if (default(T) == null && array.GetType() != typeof(T[]))
                 ThrowHelper.ThrowArrayTypeMismatchException();
 
@@ -61,7 +64,7 @@ namespace System
         /// <param name="array">The target array.</param>
         /// <param name="start">The index at which to begin the span.</param>
         /// <param name="length">The number of items in the span.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="array"/> is a null
+        /// <remarks>Returns default when <paramref name="array"/> is null.</remarks>
         /// reference (Nothing in Visual Basic).</exception>
         /// <exception cref="System.ArrayTypeMismatchException">Thrown when <paramref name="array"/> is covariant and array's type is not exactly T[].</exception>
         /// <exception cref="System.ArgumentOutOfRangeException">
@@ -71,7 +74,12 @@ namespace System
         public Span(T[] array, int start, int length)
         {
             if (array == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                this = default;
+                return; // returns default
+            }
             if (default(T) == null && array.GetType() != typeof(T[]))
                 ThrowHelper.ThrowArrayTypeMismatchException();
             if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
diff --git a/src/mscorlib/shared/System/Span.NonGeneric.cs b/src/mscorlib/shared/System/Span.NonGeneric.cs
index 9ce6acdff6..679e639c58 100644
--- a/src/mscorlib/shared/System/Span.NonGeneric.cs
+++ b/src/mscorlib/shared/System/Span.NonGeneric.cs
@@ -24,25 +24,29 @@ namespace System
     {
         /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+                return default;
 
             return new ReadOnlyMemory<char>(text, 0, text.Length);
         }
 
         /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
         /// </exception>
         public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text, int start)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+            {
+                if (start != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
+                return default;
+            }
 
             if ((uint)start > (uint)text.Length)
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
@@ -52,14 +56,18 @@ namespace System
 
         /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
         /// </exception>
         public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text, int start, int length)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
+                return default;
+            }
 
             if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
@@ -135,13 +143,13 @@ namespace System
         /// Creates a new readonly span over the portion of the target string.
         /// </summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         /// reference (Nothing in Visual Basic).</exception>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static ReadOnlySpan<char> AsReadOnlySpan(this string text)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+                return default;
 
             return new ReadOnlySpan<char>(ref text.GetRawStringData(), text.Length);
         }
@@ -150,9 +158,7 @@ namespace System
         /// Creates a new readonly span over the portion of the target string.
         /// </summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
-        /// reference (Nothing in Visual Basic).
-        /// </exception>
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
         /// </exception>
@@ -160,7 +166,11 @@ namespace System
         public static ReadOnlySpan<char> AsReadOnlySpan(this string text, int start)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+            {
+                if (start != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
+                return default;
+            }
 
             if ((uint)start > (uint)text.Length)
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
@@ -172,9 +182,7 @@ namespace System
         /// Creates a new readonly span over the portion of the target string.
         /// </summary>
         /// <param name="text">The target string.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
-        /// reference (Nothing in Visual Basic).
-        /// </exception>
+        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
         /// </exception>
@@ -182,21 +190,26 @@ namespace System
         public static ReadOnlySpan<char> AsReadOnlySpan(this string text, int start, int length)
         {
             if (text == null)
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+            {
+                if (start != 0 || length != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
+                return default;
+            }
 
             if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
 
             return new ReadOnlySpan<char>(ref Unsafe.Add(ref text.GetRawStringData(), start), length);
         }
-        
+
         internal static unsafe void ClearWithoutReferences(ref byte b, nuint byteLength)
         {
             if (byteLength == 0)
                 return;
-            
+
 #if CORECLR && (AMD64 || ARM64)
-            if (byteLength > 4096) goto PInvoke;
+            if (byteLength > 4096)
+                goto PInvoke;
             Unsafe.InitBlockUnaligned(ref b, 0, (uint)byteLength);
             return;
 #else
@@ -503,8 +516,8 @@ namespace System
 
             return;
 #endif
-            
-            PInvoke:
+
+        PInvoke:
             RuntimeImports.RhZeroMemory(ref b, byteLength);
         }
 
diff --git a/src/mscorlib/shared/System/Span.cs b/src/mscorlib/shared/System/Span.cs
index f5918110dd..106c0f7d5d 100644
--- a/src/mscorlib/shared/System/Span.cs
+++ b/src/mscorlib/shared/System/Span.cs
@@ -29,11 +29,11 @@ namespace System
 #if !FEATURE_PORTABLE_SPAN
             [NonVersionable]
 #endif // !FEATURE_PORTABLE_SPAN
-             get
-             {
-                 return _length;
-             }
-         }
+            get
+            {
+                return _length;
+            }
+        }
 
         /// <summary>
         /// Returns true if Length is 0.
@@ -84,13 +84,13 @@ namespace System
         /// <summary>
         /// Defines an implicit conversion of an array to a <see cref="Span{T}"/>
         /// </summary>
-        public static implicit operator Span<T>(T[] array) => array != null ? new Span<T>(array) : default;
+        public static implicit operator Span<T>(T[] array) => new Span<T>(array);
 
         /// <summary>
         /// Defines an implicit conversion of a <see cref="ArraySegment{T}"/> to a <see cref="Span{T}"/>
         /// </summary>
         public static implicit operator Span<T>(ArraySegment<T> arraySegment)
-            => arraySegment.Array != null ? new Span<T>(arraySegment.Array, arraySegment.Offset, arraySegment.Count) : default;
+            => new Span<T>(arraySegment.Array, arraySegment.Offset, arraySegment.Count);
 
         /// <summary>
         /// Returns an empty <see cref="Span{T}"/>