Disallow null checking discard parameters (#58952)

src/Analyzers/CSharp/Tests/UseParameterNullChecking/UseParameterNullCheckingTests.cs

@@ -1230,6 +1230,57 @@ class C
  }.RunAsync();
  }
 
+ [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIsNullCheck)]
+ public async Task TestDiscard1()
+ {
+ await new VerifyCS.Test()
+ {
+ TestCode = @"using System;
+
+class C
+{
+ Action<string> lambda = _ =>
+ {
+ [|if (_ is null)
+ throw new ArgumentNullException(nameof(_));|]
+ };
+}
+",
+ FixedCode = @"using System;
+
+class C
+{
+ Action<string> lambda = _!! =>
+ {
+ };
+}
+",
+ LanguageVersion = LanguageVersionExtensions.CSharpNext
+ }.RunAsync();
+ }
+
+ [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIsNullCheck)]
+ public async Task TestDiscard2()
+ {
+ var testCode = @"
+using System;
+
+class C
+{
+ Action<string, string> lambda = (_, _) =>
+ {
+ if ({|CS0103:_|} is null)
+ throw new ArgumentNullException(nameof({|CS0103:_|}));
+ };
+}";
+ await new VerifyCS.Test()
+ {
+ TestCode = testCode,
+ FixedCode = testCode,
+ LanguageVersion = LanguageVersionExtensions.CSharpNext
+ }.RunAsync();
+ }
+
  [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseIsNullCheck)]
  public async Task TestAnonymousMethod()
  {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6169,8 +6169,8 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
  <data name="ERR_DuplicateNullSuppression" xml:space="preserve">
  <value>Duplicate null suppression operator ('!')</value>
  </data>
- <data name="ERR_IncorrectNullCheckSyntax" xml:space="preserve">
- <value>Incorrect parameter null checking syntax. Should be '!!'.</value>
+ <data name="ERR_DiscardCannotBeNullChecked" xml:space="preserve">
+ <value>Discard parameter cannot be null-checked.</value>
  </data>
  <data name="ERR_MustNullCheckInImplementation" xml:space="preserve">
  <value>Parameter '{0}' can only have exclamation-point null checking in implementation methods.</value>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2020,7 +2020,7 @@ internal enum ErrorCode
  ERR_StructHasInitializersAndNoDeclaredConstructor = 8983,
  ERR_EncUpdateFailedDelegateTypeChanged = 8984,
 
- ERR_IncorrectNullCheckSyntax = 8990,
+ ERR_DiscardCannotBeNullChecked = 8990,
  ERR_MustNullCheckInImplementation = 8991,
  ERR_NonNullableValueTypeIsNullChecked = 8992,
  WRN_NullCheckedHasDefaultNull = 8993,

src/Compilers/CSharp/Portable/Symbols/Source/ParameterHelpers.cs

@@ -805,6 +805,10 @@ internal static void ReportParameterNullCheckingErrors(DiagnosticBag diagnostics
  {
  diagnostics.Add(useSiteInfo.DiagnosticInfo, location);
  }
+ if (parameter.IsDiscard)
+ {
+ diagnostics.Add(ErrorCode.ERR_DiscardCannotBeNullChecked, location);
+ }
  if (parameter.TypeWithAnnotations.NullableAnnotation.IsAnnotated()
  || parameter.Type.IsNullableTypeOrTypeParameter())
  {