List patterns: Slice value is assumed to be never null #57457

alrz · 2021-10-28T23:32:19Z

Spec is updated as follows:

The member that makes the type sliceable is assumed to be well-behaved, that is, the return value is never null and that it is a proper subslice of the containing list.

~~In a nullable-enabled context, the annotation on the output type is ignored to avoid producing nonsensical warnings.~~
~~(no longer needed to ignore, we're emitting the test but assuming the false branch is unreachable)~~ not an option.

Updated LDM decision (ended up agreeing with this approach): dotnet/csharplang#5599

Test plan: #51289

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

jcouv · 2021-10-30T07:40:23Z

I'm not following. The LDM decision was to honor the annotation on Slice method. The current behavior is correct, no?

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTestBase.cs

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder.cs

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

jcouv

Done with review pass (iteration 9)

jcouv

LGTM Thanks (iteration 17)

jcouv · 2022-01-11T18:07:50Z

@333fred @AlekseyTs @dotnet/roslyn-compiler for second review. Thanks

333fred

Done review pass (commit 17)

333fred · 2022-01-12T01:23:18Z

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

+            Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "slice").WithLocation(12, 13),
+            // (14,13): warning CS8602: Dereference of a possibly null reference.
+            //             list.ToString();
+            Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "list").WithLocation(14, 13)


I'm not sure I understand this one. Shouldn't the [] pattern say that it's a non-null slice, since it has to dereference the slice to determine if the slice is empty? #Closed

Makes sense. I'd defer to @jcouv for that. My solution was to ignore and report the annotation on the return type.

If we consider slice.Length evaluation (or any other) as a dereference with no warning on forthcoming derefs, shouldn't we inform the user about it?

I think we can do something in between: ignore and report only if there's a dereference i.e. when the result is being used - that would exclude var patterns. Not sure how that would play out with Nullable<T> as the result.

Thanks for spotting this. There's indeed a good question here.
From the LDM notes: "We will adjust our expectations around slice patterns to assume that Slice will never return a null value, and we will not insert a null test into the reachability or codegen DAGs."

I think it should follow that both the .. var slice and .. [] slice2 cases should yield not-null state for those variables.

@jcouv Can you please take another look? thanks.

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

AlekseyTs · 2022-01-12T16:03:47Z

Done with review pass (commit 17)

alrz · 2022-01-19T19:15:16Z

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

                                        addToTempMap(output, outputSlot, type.Type);
+                                        this.State[outputSlot] = NullableFlowState.NotNull; // Slice value is assumed to be never null


❓ Should we also strip the annotation on the return type for semantic model?

I am not sure if hovering over [.. var variable] shows a nullable type (probably shouldn't?)

The var will definitely have a ?, because it always does. Theoretically, though, it should say that variable is considered not null here.

333fred · 2022-01-19T19:29:33Z

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

@@ -3880,12 +3880,12 @@ public void M()
            rest.ToString(); // 1

        if (new C<int?>() is [1, ..var rest2])
-            rest2.Value.ToString(); // 2
+            rest2.Value.ToString(); // 2 (assumed not-null)


This isn't quite what I meant: the numbers are wrong, because warning 2 is no longer there.

I renumbered the one you commented on. Will update the rest in the next commit.

jcouv

LGTM Thanks (iteration 18).
Nit: some line-number comments in tests could be adjusted, if no longer producing a diagnostic

333fred

LGTM (commit 19)

jcouv · 2022-01-20T06:11:38Z

Thanks @alrz! :-)

* upstream/main: (1035 commits) Add missing header Mark IVSTypeScriptFormattingServiceImplementation as optional, but require it in the constructor Fix Go To Base for a symbol with a single metadata location (dotnet#58965) [EnC] Store entire spans instead of line deltas (dotnet#58831) Delete CodeAnalysisRules.ruleset (dotnet#58968) Allow xml docs to still be created when 'emit metadata only' is on. (dotnet#57667) Fix ParseVBErrorOrWarning (dotnet#47833) Update parameter nullability to match implementation Ensure CSharpUseParameterNullCheckingDiagnosticAnalyzer works with nullable parameters Add tests for issues fixed by previous PR (dotnet#58764) Update src/Features/CSharp/Portable/Completion/CompletionProviders/ExplicitInterfaceMemberCompletionProvider.CompletionSymbolDisplay.cs Disallow null checking discard parameters (dotnet#58952) Add extension method Escape type arguments Few fixes Update tests. Add Analyzers layer to CODEOWNERS Add formatting analyzer test for param nullchecking (dotnet#58936) Move reading HideAdvancedMembers option up (dotnet#58747) List patterns: Slice value is assumed to be never null (dotnet#57457) ...

ghost added the Community The pull request was submitted by a contributor who is not a Microsoft employee. label Oct 28, 2021

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alrz commented Oct 28, 2021

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Update list-patterns.md dotnet/csharplang#5364

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Slice value is always assumed to be not null

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Flaky test: Roslyn.VisualStudio.IntegrationTests.CSharp.CSharpFindReferences.FindReferencesToCtor #57552

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alrz commented Nov 4, 2021

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alrz marked this pull request as ready for review November 4, 2021 09:37

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jcouv added the Feature - List Patterns label Nov 8, 2021

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alrz requested a review from jcouv November 10, 2021 19:06

The annotation on the output type is ignored

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alrz changed the title ~~Slice value is always assumed to be not null~~ List patterns: Slice value is assumed to be never null Nov 10, 2021

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List patterns: Slice value is assumed to be never null #57457

List patterns: Slice value is assumed to be never null #57457

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		addToTempMap(output, outputSlot, type.Type);
		this.State[outputSlot] = NullableFlowState.NotNull; // Slice value is assumed to be never null

List patterns: Slice value is assumed to be never null #57457

List patterns: Slice value is assumed to be never null #57457

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