where is generic nullable type constraint

whereisgenericnullable.ipynb

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   "source": [
    "#nullable enable\n",
    "class A();\n",
    "T? GetNull<T>(in T? t = default) where T : struct => t;\n",
    "T? GetNull<T>(in T? t = default) where T : class => t;\n",
    "new\n",
    "{\n",
    "    a = GetNull<A>(),\n",
    "    b = GetNull<int>(),\n",
    "    d = GetNull<(int a, int b)>(),\n",
    "}"
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    {
     "ename": "Error",
     "evalue": "(4,4): error CS0111: Type 'Submission#6' already defines a member called 'GetNull2' with the same parameter types",
     "output_type": "error",
     "traceback": [
      "(4,4): error CS0111: Type 'Submission#6' already defines a member called 'GetNull2' with the same parameter types"
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   "source": [
    "#nullable enable\n",
    "class A();\n",
    "T? GetNull2<T>() where T : struct => default;\n",
    "T? GetNull2<T>() where T : class => default;\n",
    "new\n",
    "{\n",
    "    a = GetNull2<A>(),\n",
    "    b = GetNull2<int>(),\n",
    "    d = GetNull2<(int a, int b)>(),\n",
    "}"
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   "source": [
    "#nullable enable\n",
    "class A();\n",
    "Func<T>? GetNull3<T>() { return null; }\n",
    "\n",
    "new\n",
    "{\n",
    "    a = GetNull3<A>()?.Invoke(),\n",
    "    b = GetNull3<int>()?.Invoke(),\n",
    "    d = GetNull3<(int a, int b)>()?.Invoke(),\n",
    "}"
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   "outputs": [
    {
     "ename": "Error",
     "evalue": "(4,41): error CS8978: 'T' cannot be made nullable.",
     "output_type": "error",
     "traceback": [
      "(4,41): error CS8978: 'T' cannot be made nullable."
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   "source": [
    "#nullable enable\n",
    "class A();\n",
    "Func<T>? GetNull3<T>() { return null; }\n",
    "T? GetNull4<T>() { return GetNull3<T>()?.Invoke(); }\n",
    "new\n",
    "{\n",
    "    a = GetNull4<A>(),\n",
    "    b = GetNull4<int>(),\n",
    "    d = GetNull4<(int a, int b)>(),\n",
    "}"
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