Remove always-on SingleUnderscorePattern language feature flag

src/Compiler/FSComp.txt

@@ -37,7 +37,6 @@ buildUnexpectedTypeArgs,"The non-generic type '%s' does not expect any type argu
 returnUsedInsteadOfReturnBang,"Consider using 'return!' instead of 'return'."
 yieldUsedInsteadOfYieldBang,"Consider using 'yield!' instead of 'yield'."
 tupleRequiredInAbstractMethod,"\nA tuple type is required for one or more arguments. Consider wrapping the given arguments in additional parentheses or review the definition of the interface."
-10,parsUnexpectedSymbolDot,"Unexpected symbol '.' in member definition. Expected 'with', '=' or other token."
 201,tcNamespaceCannotContainValues,"Namespaces cannot contain values. Consider using a module to hold your value declarations."
 202,unsupportedAttribute,"This attribute is currently unsupported by the F# compiler. Applying it will not achieve its intended effect."
 203,buildInvalidWarningNumber,"Invalid warning number '%s'"
@@ -1563,7 +1562,6 @@ optsAlwaysInline,"Always inline 'inline' functions"
 nativeResourceFormatError,"Stream does not begin with a null resource and is not in '.RES' format."
 nativeResourceHeaderMalformed,"Resource header beginning at offset %s is malformed."
 formatDashItem," - %s"
-featureSingleUnderscorePattern,"single underscore pattern"
 featureWildCardInForLoop,"wild card in for loop"
 featureRelaxWhitespace,"whitespace relaxation"
 featureNameOf,"nameof"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -16,7 +16,6 @@ module internal FSharp.Compiler.Features
 
 [<RequireQualifiedAccess>]
 type LanguageFeature =
-    | SingleUnderscorePattern
     | WildCardInForLoop
     | RelaxWhitespace
     | RelaxWhitespace2
@@ -152,7 +151,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         dict
             [
                 // F# 4.7
-                LanguageFeature.SingleUnderscorePattern, languageVersion47
                 LanguageFeature.WildCardInForLoop, languageVersion47
                 LanguageFeature.RelaxWhitespace, languageVersion47
                 LanguageFeature.ImplicitYield, languageVersion47
@@ -364,7 +362,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
     /// Get a string name for the given feature.
     static member GetFeatureString feature =
         match feature with
-        | LanguageFeature.SingleUnderscorePattern -> FSComp.SR.featureSingleUnderscorePattern ()
         | LanguageFeature.WildCardInForLoop -> FSComp.SR.featureWildCardInForLoop ()
         | LanguageFeature.RelaxWhitespace -> FSComp.SR.featureRelaxWhitespace ()
         | LanguageFeature.RelaxWhitespace2 -> FSComp.SR.featureRelaxWhitespace2 ()

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -6,7 +6,6 @@ module internal FSharp.Compiler.Features
 /// LanguageFeature enumeration
 [<RequireQualifiedAccess>]
 type LanguageFeature =
-    | SingleUnderscorePattern
     | WildCardInForLoop
     | RelaxWhitespace
     | RelaxWhitespace2

src/Compiler/pars.fsy

@@ -2250,10 +2250,7 @@ opt_typ:
 
 atomicPatternLongIdent:
   | UNDERSCORE DOT pathOp
-     { if not (parseState.LexBuffer.SupportsFeature LanguageFeature.SingleUnderscorePattern) then
-          raiseParseErrorAt (rhs parseState 2) (FSComp.SR.parsUnexpectedSymbolDot())
-
-       let underscore = ident("_", rhs parseState 1)
+     { let underscore = ident("_", rhs parseState 1)
        let mDot = rhs parseState 2
        None, prependIdentInLongIdentWithTrivia (SynIdent(underscore, None)) mDot $3 }
 
@@ -2266,10 +2263,7 @@ atomicPatternLongIdent:
      { (None, $1) }
 
   | access UNDERSCORE DOT pathOp
-     { if not (parseState.LexBuffer.SupportsFeature LanguageFeature.SingleUnderscorePattern) then
-          raiseParseErrorAt (rhs parseState 3) (FSComp.SR.parsUnexpectedSymbolDot())
-
-       let underscore = ident("_", rhs parseState 2)
+     { let underscore = ident("_", rhs parseState 2)
        let mDot = rhs parseState 3
        Some($1), prependIdentInLongIdentWithTrivia (SynIdent(underscore, None)) mDot $4 }