Better error reporting for let bindings.

src/Compiler/FSComp.txt

@@ -679,7 +679,7 @@ tcUnnamedArgumentsDoNotFormPrefix,"The unnamed arguments do not form a prefix of
 824,tcAttributesAreNotPermittedOnLetBindings,"Attributes are not permitted on 'let' bindings in expressions"
 825,tcDefaultValueAttributeRequiresVal,"The 'DefaultValue' attribute may only be used on 'val' declarations"
 826,tcConditionalAttributeRequiresMembers,"The 'ConditionalAttribute' attribute may only be used on members"
-827,tcInvalidActivePatternName,"This is not a valid name for an active pattern"
+827,tcInvalidActivePatternName,"'%s' is not a valid method name. Active patterns may only be defined as let-bound module or class functions."
 828,tcEntryPointAttributeRequiresFunctionInModule,"The 'EntryPointAttribute' attribute may only be used on function definitions in modules"
 829,tcMutableValuesCannotBeInline,"Mutable values cannot be marked 'inline'"
 830,tcMutableValuesMayNotHaveGenericParameters,"Mutable values cannot have generic parameters"
@@ -1782,3 +1782,4 @@ featureEmptyBodiedComputationExpressions,"Support for computation expressions wi
 featureAllowAccessModifiersToAutoPropertiesGettersAndSetters,"Allow access modifiers to auto properties getters and setters"
 3871,tcAccessModifiersNotAllowedInSRTPConstraint,"Access modifiers cannot be applied to an SRTP constraint."
 featureAllowObjectExpressionWithoutOverrides,"Allow object expressions without overrides"
+3872,tcPartialActivePattern,"Multi-case partial active patterns are not supported. Consider using a single-case partial active pattern or a full active pattern."

src/Compiler/SyntaxTree/PrettyNaming.fs

@@ -963,6 +963,15 @@ type ActivePatternInfo =
 
     member x.ActiveTagsWithRanges = let (APInfo(_, tags, _)) = x in tags
 
+    // FIXME
+    member x.LogicalName =
+        let (APInfo(isTotal, tags, _)) = x
+
+        tags
+        |> List.map fst
+        |> String.concat "|"
+        |> (fun s -> if isTotal then "(|" + s + "|)" else "(|" + s + "|_|)")
+
     member x.Range = let (APInfo(_, _, m)) = x in m
 
 let ActivePatternInfoOfValName nm (m: range) =

src/Compiler/SyntaxTree/PrettyNaming.fsi

@@ -228,6 +228,7 @@ type internal ActivePatternInfo =
 
     member ActiveTags: string list
     member ActiveTagsWithRanges: (string * range) list
+    member LogicalName: string
     member IsTotal: bool
     member Range: range