Bugfix for underscore dot lambda: Forbidding expressions not based on the implied _ arg

src/Compiler/FSComp.txt

@@ -1725,3 +1725,4 @@ featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged
 3579,alwaysUseTypedStringInterpolation,"Interpolated string contains untyped identifiers. Adding typed format specifiers is recommended."
 3580,tcUnexpectedFunTypeInUnionCaseField,"Unexpected function type in union case field definition. If you intend the field to be a function, consider wrapping the function signature with parens, e.g. | Case of a -> b into | Case of (a -> b)."
 3582,tcInfoIfFunctionShadowsUnionCase,"This is a function definition that shadows a union case. If this is what you want, ignore or suppress this warning. If you want it to be a union case deconstruction, add parentheses."
+3583,tcDotLambdaAtNotSupportedExpression,"Shorthand lambda syntax is only supported for atomic expressions, such as method, property, field or indexer on the implied '_' argument. For example: 'let f = _.Length'."

src/Compiler/SyntaxTree/SyntaxTreeOps.fs

@@ -97,7 +97,9 @@ let rec pushUnaryArg expr arg =
     | SynExpr.DotGet (synExpr, rangeOfDot, synLongIdent, range) -> SynExpr.DotGet(pushUnaryArg synExpr arg, rangeOfDot, synLongIdent, range)
     | SynExpr.DotIndexedGet (objectExpr, indexArgs, dotRange, range) ->
         SynExpr.DotIndexedGet(pushUnaryArg objectExpr arg, indexArgs, dotRange, range)
-    | _ -> expr
+    | _ ->
+        errorR (Error(FSComp.SR.tcDotLambdaAtNotSupportedExpression (), expr.Range))
+        expr
 
 let (|SynSingleIdent|_|) x =
     match x with