Fix #1811: Assert in NullCoalescingInstruction.CheckInvariant after expression tree transform

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ExpressionTrees.cs

@@ -1038,7 +1038,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			await Task.Delay(100);
 			if (string.IsNullOrEmpty(str)) {
-#if ROSLYN
+#if CS70
 				if (int.TryParse(str, out int id)) {
 #else
 				int id;
@@ -1050,6 +1050,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				}
 			}
 		}
+
+		public void NullCoalescing()
+		{
+			Test<Func<string, string, string>>((string a, string b) => a ?? b, (string a, string b) => a ?? b);
+			Test<Func<int?, int>>((int? a) => a ?? 1, (int? a) => a ?? 1);
+		}
 	}
 
 	internal static class Extensions

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -604,7 +604,9 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			} else {
 				targetType = fallbackInstType;
 			}
-			return (new NullCoalescingInstruction(kind, trueInst, fallbackInst), targetType);
+			return (new NullCoalescingInstruction(kind, trueInst, fallbackInst) { 
+				UnderlyingResultType  = trueInstTypeNonNullable.GetStackType()
+			}, targetType);
 		}
 
 		(ILInstruction, IType) ConvertComparison(CallInstruction invocation, ComparisonKind kind)