System.Activator.CreateInstance<T>() -> new T() transform is only valid if T has new() constraint.

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

@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+using System;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
@@ -76,6 +77,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return new T();
 		}
 
+		public T NotNew<T>()
+		{
+			return Activator.CreateInstance<T>();
+		}
+
 		public bool IsNull<T>(T t)
 		{
 			return t == null;

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Generics.il

@@ -237,6 +237,20 @@
     IL_0026:  ret
   } // end of method Generics::New
 
+  .method public hidebysig instance !!T  NotNew<T>() cil managed
+  {
+    // Code size       11 (0xb)
+    .maxstack  1
+    .locals init (!!T V_0)
+    IL_0000:  nop
+    IL_0001:  call       !!0 [mscorlib]System.Activator::CreateInstance<!!0>()
+    IL_0006:  stloc.0
+    IL_0007:  br.s       IL_0009
+
+    IL_0009:  ldloc.0
+    IL_000a:  ret
+  } // end of method Generics::NotNew
+
   .method public hidebysig instance bool 
           IsNull<T>(!!T t) cil managed
   {

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Generics.opt.il

@@ -188,6 +188,14 @@
     IL_001f:  ret
   } // end of method Generics::New
 
+  .method public hidebysig instance !!T  NotNew<T>() cil managed
+  {
+    // Code size       6 (0x6)
+    .maxstack  8
+    IL_0000:  call       !!0 [mscorlib]System.Activator::CreateInstance<!!0>()
+    IL_0005:  ret
+  } // end of method Generics::NotNew
+
   .method public hidebysig instance bool 
           IsNull<T>(!!T t) cil managed
   {

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Generics.opt.roslyn.il

@@ -179,6 +179,14 @@
     IL_0005:  ret
   } // end of method Generics::New
 
+  .method public hidebysig instance !!T  NotNew<T>() cil managed
+  {
+    // Code size       6 (0x6)
+    .maxstack  8
+    IL_0000:  call       !!0 [mscorlib]System.Activator::CreateInstance<!!0>()
+    IL_0005:  ret
+  } // end of method Generics::NotNew
+
   .method public hidebysig instance bool 
           IsNull<T>(!!T t) cil managed
   {

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Generics.roslyn.il

@@ -231,6 +231,20 @@
     IL_000a:  ret
   } // end of method Generics::New
 
+  .method public hidebysig instance !!T  NotNew<T>() cil managed
+  {
+    // Code size       11 (0xb)
+    .maxstack  1
+    .locals init (!!T V_0)
+    IL_0000:  nop
+    IL_0001:  call       !!0 [mscorlib]System.Activator::CreateInstance<!!0>()
+    IL_0006:  stloc.0
+    IL_0007:  br.s       IL_0009
+
+    IL_0009:  ldloc.0
+    IL_000a:  ret
+  } // end of method Generics::NotNew
+
   .method public hidebysig instance bool 
           IsNull<T>(!!T t) cil managed
   {

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -104,7 +104,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					}
 					break;
 				case "System.Activator.CreateInstance":
-					if (method.TypeArguments.Count == 1 && arguments.Length == 0 && method.TypeArguments[0].Kind == TypeKind.TypeParameter) {
+					if (arguments.Length == 0 && method.TypeArguments.Count == 1 && IsInstantiableTypeParameter(method.TypeArguments[0])) {
 						invocationExpression.ReplaceWith(new ObjectCreateExpression(context.TypeSystemAstBuilder.ConvertType(method.TypeArguments.First())));
 					}
 					break;
@@ -201,6 +201,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return;
 		}
 
+		bool IsInstantiableTypeParameter(IType type)
+		{
+			return type is ITypeParameter tp && tp.HasDefaultConstructorConstraint;
+		}
+
 		Expression WrapInParens(Expression expression)
 		{
 			if (expression is ConditionalExpression)