allow _ in use bindings by checking for special case of TPat_wild i…

…n TcLetBinding implements fsharp/fslang-suggestions#881 and https://github.com/fsharp/fslang-design/blob/6c1ba3425c6129f1b6cf22ac8ddec30761136a12/RFCs/FS-1102-discards-on-use-bindings.md has basic test UseBindingDiscard01, that ensures, that `use _ = new Disposable()` type checks
dotnet · Jun 4, 2021 · a96d7ce · a96d7ce
1 parent afe8830
commit a96d7ce
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Showing 4 changed files with 33 additions and 1 deletion.
diff --git a/src/fsharp/CheckExpressions.fs b/src/fsharp/CheckExpressions.fs
@@ -9770,7 +9770,12 @@ and TcLetBinding cenv isUse env containerInfo declKind tpenv (synBinds, synBinds
 
                 let tmp, _ = mkCompGenLocal m "patternInput" (generalizedTypars +-> tauTy)
 
-                if isUse || isFixed then
+                if isUse then
+                    let isDiscarded = match checkedPat with TPat_wild _ -> true | _ -> false
+                    if not isDiscarded then
+                        errorR(Error(FSComp.SR.tcInvalidUseBinding(), m))
+
+                elif isFixed then
                     errorR(Error(FSComp.SR.tcInvalidUseBinding(), m))
 
                 // If the overall declaration is declaring statics or a module value, then force the patternInputTmp to also
@@ -9798,6 +9803,8 @@ and TcLetBinding cenv isUse env containerInfo declKind tpenv (synBinds, synBinds
         // Add the dispose of any "use x = ..." to bodyExpr
         let mkCleanup (bodyExpr, bodyExprTy) =
             if isUse && not isFixed then
+                let isDiscarded = match checkedPat2 with TPat_wild _ -> true | _ -> false
+                let allValsDefinedByPattern = if isDiscarded then [patternInputTmp] else allValsDefinedByPattern
                 (allValsDefinedByPattern, (bodyExpr, bodyExprTy)) ||> List.foldBack (fun v (bodyExpr, bodyExprTy) ->
                     AddCxTypeMustSubsumeType ContextInfo.NoContext denv cenv.css v.Range NoTrace cenv.g.system_IDisposable_ty v.Type
                     let cleanupE = BuildDisposableCleanup cenv env m v

diff --git a/...FSharp.Compiler.ComponentTests/Conformance/DeclarationElements/LetBindings/UseBindings.fs b/...FSharp.Compiler.ComponentTests/Conformance/DeclarationElements/LetBindings/UseBindings.fs
@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.ComponentTests.Conformance.DeclarationElements.LetBindings
+
+open Xunit
+open FSharp.Test.Utilities.Compiler
+open FSharp.Test.Utilities.Xunit.Attributes
+
+module UseBindings =
+
+    //<Expects status="success"></Expects>
+    [<Theory; Directory(__SOURCE_DIRECTORY__ + "/../../../resources/tests/Conformance/DeclarationElements/LetBindings", Includes=[|"UseBindingDiscard01.fs"|])>]
+    let ``UseBindings - UseBindingDiscard01.fs - `` compilation =
+        compilation
+        |> asFsx
+        |> compile
+        |> shouldSucceed
+        |> ignore
diff --git a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
@@ -66,6 +66,7 @@
     <Compile Include="Conformance\BasicTypeAndModuleDefinitions\RecordTypes.fs" />
     <Compile Include="Conformance\BasicTypeAndModuleDefinitions\UnionTypes.fs" />
     <Compile Include="Conformance\DeclarationElements\LetBindings\TypeFunctions.fs" />
+    <Compile Include="Conformance\DeclarationElements\LetBindings\UseBindings.fs" />
     <Compile Include="Conformance\Expressions\ApplicationExpressions\BasicApplication.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\PatternMatching.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\SequenceIteration.fs" />

diff --git a/...tTests/resources/tests/Conformance/DeclarationElements/LetBindings/UseBindingDiscard01.fs b/...tTests/resources/tests/Conformance/DeclarationElements/LetBindings/UseBindingDiscard01.fs
@@ -0,0 +1,6 @@
+// #DeclarationElements #LetBindings 
+//<Expects status="success"></Expects>
+
+let answer =
+    use _ = new System.IO.MemoryStream()
+    42