Add new a.GetValueOrDefault(b) -> a ?? b transform for side-effect-free default values.

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -274,6 +274,18 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		protected internal override void VisitCall(Call inst)
 		{
+			if (NullableLiftingTransform.MatchGetValueOrDefault(inst, out var nullableValue, out var fallback)
+				&& SemanticHelper.IsPure(fallback.Flags))
+			{
+				context.Step("call Nullable{T}.GetValueOrDefault(a, b) -> a ?? b", inst);
+				var ldObj = new LdObj(nullableValue, inst.Method.DeclaringType);
+				var replacement = new NullCoalescingInstruction(NullCoalescingKind.NullableWithValueFallback, ldObj, fallback) {
+					UnderlyingResultType = fallback.ResultType
+				};
+				inst.ReplaceWith(replacement.WithILRange(inst));
+				replacement.AcceptVisitor(this);
+				return;
+			}
 			base.VisitCall(inst);
 			TransformAssignment.HandleCompoundAssign(inst, context);
 		}

ICSharpCode.Decompiler/IL/Transforms/NullableLiftingTransform.cs

@@ -16,7 +16,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -1047,6 +1046,24 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return true;
 		}
 
+		/// <summary>
+		/// Matches 'call nullableValue.GetValueOrDefault(fallback)'
+		/// </summary>
+		internal static bool MatchGetValueOrDefault(ILInstruction inst, out ILInstruction nullableValue, out ILInstruction fallback)
+		{
+			nullableValue = null;
+			fallback = null;
+			if (!(inst is Call call))
+				return false;
+			if (call.Method.Name != "GetValueOrDefault" || call.Arguments.Count != 2)
+				return false;
+			if (call.Method.DeclaringTypeDefinition?.KnownTypeCode != KnownTypeCode.NullableOfT)
+				return false;
+			nullableValue = call.Arguments[0];
+			fallback = call.Arguments[1];
+			return true;
+		}
+
 		/// <summary>
 		/// Matches 'call Nullable{T}.GetValueOrDefault(ldloca v)'
 		/// </summary>