Fix handling of curly braces in FormattableStrings

src/Compiler/Checking/CheckFormatStrings.fs

@@ -48,14 +48,21 @@ let newInfo () =
     addZeros       = false
     precision      = false}
 
+let escapeDotnetFormatString str =
+    str
+    // We need to double '{' and '}', because even if they were escaped in the
+    // original string, extra curly braces were stripped away by the F# lexer.
+    |> Seq.collect (fun x -> if x = '{' || x = '}' then [x;x] else [x])
+    |> System.String.Concat
+
 let parseFormatStringInternal
         (m: range)
         (fragRanges: range list)
         (g: TcGlobals)
         isInterpolated
         isFormattableString
         (context: FormatStringCheckContext option)
-        fmt
+        (fmt: string)
         printerArgTy
         printerResidueTy = 
 
@@ -86,6 +93,8 @@ let parseFormatStringInternal
     // there are no accurate intra-string ranges available for exact error message locations within the string.
     // The 'm' range passed as an input is however accurate and covers the whole string.
     //
+    let escapeFormatStringEnabled = g.langVersion.SupportsFeature Features.LanguageFeature.EscapeDotnetFormattableStrings
+
     let fmt, fragments = 
 
         //printfn "--------------------" 
@@ -175,7 +184,7 @@ let parseFormatStringInternal
         | _ -> 
             // Don't muck with the fmt when there is no source code context to go get the original
             // source code (i.e. when compiling or background checking)
-            fmt, [ (0, 1, m) ]
+            (if escapeFormatStringEnabled then escapeDotnetFormatString fmt else fmt), [ (0, 1, m) ]
 
     let len = fmt.Length
 

src/Compiler/FSComp.txt

@@ -1662,3 +1662,4 @@ reprStateMachineInvalidForm,"The state machine has an unexpected form"
 3548,matchNotAllowedForUnionCaseWithNoData,"Pattern discard is not allowed for union case that takes no data."
 3549,tcSynTypeOrInvalidInDeclaration,"SynType.Or is not permitted in this declaration"
 3550,chkDuplicatedMethodParameter,"Duplicate parameter. The parameter '%s' has been used more that once in this method."
+featureEscapeBracesInFormattableString,"Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -57,6 +57,7 @@ type LanguageFeature =
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
     | ErrorForNonVirtualMembersOverrides
+    | EscapeDotnetFormattableStrings
     | ArithmeticInLiterals
 
 /// LanguageVersion management
@@ -131,6 +132,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.MatchNotAllowedForUnionCaseWithNoData, previewVersion
                 LanguageFeature.CSharpExtensionAttributeNotRequired, previewVersion
                 LanguageFeature.ErrorForNonVirtualMembersOverrides, previewVersion
+                LanguageFeature.EscapeDotnetFormattableStrings, previewVersion
                 LanguageFeature.ArithmeticInLiterals, previewVersion
 
             ]
@@ -242,6 +244,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.MatchNotAllowedForUnionCaseWithNoData -> FSComp.SR.featureMatchNotAllowedForUnionCaseWithNoData ()
         | LanguageFeature.CSharpExtensionAttributeNotRequired -> FSComp.SR.featureCSharpExtensionAttributeNotRequired ()
         | LanguageFeature.ErrorForNonVirtualMembersOverrides -> FSComp.SR.featureErrorForNonVirtualMembersOverrides ()
+        | LanguageFeature.EscapeDotnetFormattableStrings -> FSComp.SR.featureEscapeBracesInFormattableString ()
         | LanguageFeature.ArithmeticInLiterals -> FSComp.SR.featureArithmeticInLiterals ()
 
     /// Get a version string associated with the given feature.

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -47,6 +47,7 @@ type LanguageFeature =
     | MatchNotAllowedForUnionCaseWithNoData
     | CSharpExtensionAttributeNotRequired
     | ErrorForNonVirtualMembersOverrides
+    | EscapeDotnetFormattableStrings
     | ArithmeticInLiterals
 
 /// LanguageVersion management

src/Compiler/xlf/FSComp.txt.cs.xlf

@@ -192,6 +192,11 @@
         <target state="translated">chyba při zastaralém přístupu konstruktoru s atributem RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">více typů podporuje měrné jednotky</target>

src/Compiler/xlf/FSComp.txt.de.xlf

@@ -192,6 +192,11 @@
         <target state="translated">Beim veralteten Zugriff auf das Konstrukt mit dem RequireQualifiedAccess-Attribut wird ein Fehler ausgegeben.</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">Maßeinheitenunterstützung durch weitere Typen</target>

src/Compiler/xlf/FSComp.txt.es.xlf

@@ -192,6 +192,11 @@
         <target state="translated">error en el acceso en desuso de la construcción con el atributo RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">más tipos admiten las unidades de medida</target>

src/Compiler/xlf/FSComp.txt.fr.xlf

@@ -192,6 +192,11 @@
         <target state="translated">donner une erreur sur l’accès déconseillé de la construction avec l’attribut RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">d'autres types prennent en charge les unités de mesure</target>

src/Compiler/xlf/FSComp.txt.it.xlf

@@ -192,6 +192,11 @@
         <target state="translated">errore durante l'accesso deprecato del costrutto con l'attributo RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">più tipi supportano le unità di misura</target>

src/Compiler/xlf/FSComp.txt.ja.xlf

@@ -192,6 +192,11 @@
         <target state="translated">RequireQualifiedAccess 属性を持つコンストラクトの非推奨アクセスでエラーが発生しました</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">単位をサポートするその他の型</target>

src/Compiler/xlf/FSComp.txt.ko.xlf

@@ -192,6 +192,11 @@
         <target state="translated">RequireQualifiedAccess 특성을 사용하여 사용되지 않는 구문 액세스에 대한 오류 제공</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">더 많은 형식이 측정 단위를 지원함</target>

src/Compiler/xlf/FSComp.txt.pl.xlf

@@ -192,6 +192,11 @@
         <target state="translated">wskazywanie błędu w przypadku przestarzałego dostępu do konstrukcji z atrybutem RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">więcej typów obsługuje jednostki miary</target>

src/Compiler/xlf/FSComp.txt.pt-BR.xlf

@@ -192,6 +192,11 @@
         <target state="translated">fornecer erro no acesso preterido do constructo com o atributo RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">mais tipos dão suporte para unidades de medida</target>

src/Compiler/xlf/FSComp.txt.ru.xlf

@@ -192,6 +192,11 @@
         <target state="translated">выдать ошибку при устаревшем доступе к конструкции с атрибутом RequireQualifiedAccess</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">другие типы поддерживают единицы измерения</target>

src/Compiler/xlf/FSComp.txt.tr.xlf

@@ -192,6 +192,11 @@
         <target state="translated">RequireQualifiedAccess özniteliğine sahip yapının kullanım dışı erişiminde hata</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">tür daha ölçü birimlerini destekler</target>

src/Compiler/xlf/FSComp.txt.zh-Hans.xlf

@@ -192,6 +192,11 @@
         <target state="translated">对具有 RequireQualifiedAccess 属性的构造进行弃用的访问时出错</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">更多类型支持度量单位</target>

src/Compiler/xlf/FSComp.txt.zh-Hant.xlf

@@ -192,6 +192,11 @@
         <target state="translated">對具有 RequireQualifiedAccess 屬性的建構的已取代存取發出錯誤</target>
         <note />
       </trans-unit>
+      <trans-unit id="featureEscapeBracesInFormattableString">
+        <source>Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</source>
+        <target state="new">Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString</target>
+        <note />
+      </trans-unit>
       <trans-unit id="featureExpandedMeasurables">
         <source>more types support units of measure</source>
         <target state="translated">更多支援測量單位的類型</target>

tests/FSharp.Compiler.ComponentTests/Language/InterpolatedStringsTests.fs

@@ -32,8 +32,19 @@ let c: System.IFormattable = $"string"
         |> compile
         |> shouldSucceed
 
+    [<Fact>]
+    let ``Interpolated string literal typed as FormattableString handles double braces correctly`` () =
+        Fsx """
+let a = $"{{hello}} world" : System.FormattableString
+printf $"{a.Format}"
+        """
+        |> withLangVersionPreview
+        |> compileExeAndRun
+        |> shouldSucceed
+        |> withStdOutContains "{{hello}} world"
+
     [<Fact>]
     let ``Percent sign characters in interpolated strings`` () =
         Assert.Equal("%", $"%%")
         Assert.Equal("42%", $"{42}%%")
-        Assert.Equal("% 42", $"%%%3d{42}")
+        Assert.Equal("% 42", $"%%%3d{42}")