Improve error reporting: Detect module declarations that incorrectly …

…end with `=` - fixes dotnet#1214

src/fsharp/CompileOps.fs

@@ -3235,8 +3235,12 @@ let PostParseModuleImpl (_i,defaultNamespace,isLastCompiland,filename,impl) =
 
     | ParsedImplFileFragment.AnonModule (defs,m)-> 
         let isLast, isExe = isLastCompiland 
-        if not (isLast && isExe ) && not (doNotRequireNamespaceOrModuleSuffixes |> List.exists (Filename.checkSuffix (String.lowercase filename))) then 
-            errorR(Error(FSComp.SR.buildMultiFileRequiresNamespaceOrModule(),trimRangeToLine m))
+        let lower = String.lowercase filename
+        if not (isLast && isExe) && not (doNotRequireNamespaceOrModuleSuffixes |> List.exists (Filename.checkSuffix lower)) then
+            match defs with
+            | SynModuleDecl.NestedModule(_) :: _ -> errorR(Error(FSComp.SR.noEqualSignAfterModule(),trimRangeToLine m))
+            | _ -> errorR(Error(FSComp.SR.buildMultiFileRequiresNamespaceOrModule(),trimRangeToLine m))
+
         let modname = ComputeAnonModuleName (nonNil defs) defaultNamespace filename (trimRangeToLine m)
         SynModuleOrNamespace(modname,true,defs,PreXmlDoc.Empty,[],None,m)
 
@@ -3259,8 +3263,12 @@ let PostParseModuleSpec (_i,defaultNamespace,isLastCompiland,filename,intf) =
 
     | ParsedSigFileFragment.AnonModule (defs,m) -> 
         let isLast, isExe = isLastCompiland
-        if not (isLast && isExe) && not (doNotRequireNamespaceOrModuleSuffixes |> List.exists (Filename.checkSuffix (String.lowercase filename))) then 
-            errorR(Error(FSComp.SR.buildMultiFileRequiresNamespaceOrModule(),m))
+        let lower = String.lowercase filename
+        if not (isLast && isExe) && not (doNotRequireNamespaceOrModuleSuffixes |> List.exists (Filename.checkSuffix lower)) then 
+            match defs with
+            | SynModuleSigDecl.NestedModule(_) :: _ -> errorR(Error(FSComp.SR.noEqualSignAfterModule(),m))
+            | _ -> errorR(Error(FSComp.SR.buildMultiFileRequiresNamespaceOrModule(),m))
+
         let modname = ComputeAnonModuleName (nonNil defs) defaultNamespace filename (trimRangeToLine m)
         SynModuleOrNamespaceSig(modname,true,defs,PreXmlDoc.Empty,[],None,m)
 

src/fsharp/FSComp.txt

@@ -42,6 +42,7 @@ buildCouldNotReadVersionInfoFromMscorlib,"Could not read version from mscorlib.d
 220,buildAssemblyResolutionFailed,"Assembly resolution failure at or near this location"
 221,buildImplicitModuleIsNotLegalIdentifier,"The declarations in this file will be placed in an implicit module '%s' based on the file name '%s'. However this is not a valid F# identifier, so the contents will not be accessible from other files. Consider renaming the file or adding a 'module' or 'namespace' declaration at the top of the file."
 222,buildMultiFileRequiresNamespaceOrModule,"Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'. Only the last source file of an application may omit such a declaration."
+222,noEqualSignAfterModule,"Files in libraries or multiple-file applications must begin with a namespace or module declaration. When using a module declaration at the start of a file the '=' sign is not allowed. If this is a top-level module, consider removing the = to resolve this error."
 223,buildMultipleToplevelModules,"This file contains multiple declarations of the form 'module SomeNamespace.SomeModule'. Only one declaration of this form is permitted in a file. Change your file to use an initial namespace declaration and/or use 'module ModuleName = ...' to define your modules."
 224,buildOptionRequiresParameter,"Option requires parameter: %s"
 225,buildCouldNotFindSourceFile,"Source file '%s' could not be found"

tests/fsharpqa/Source/Conformance/ImplementationFilesAndSignatureFiles/NamespacesFragmentsAndImplementationFiles/basic/E_NoNamespaceModuleDec02a.fs

@@ -2,7 +2,7 @@
 // This testcase will emit a warning even though it contains a module, becase
 // it is a nested module and not a 'real' module
 
-//<Expects status="error" span="(7,1)" id="FS0222">Files in libraries or multiple-file applications must begin with a namespace or module declaration, e\.g\. 'namespace SomeNamespace\.SubNamespace' or 'module SomeNamespace\.SomeModule'\. Only the last source file of an application may omit such a declaration\.$</Expects>
+//<Expects status="error" span="(7,1)" id="FS0222">Files in libraries or multiple-file applications must begin with a namespace or module declaration. When using a module declaration at the start of a file the '=' sign is not allowed. If this is a top-level module, consider removing the = to resolve this error.</Expects>
 
 module ANestedModule =
 

tests/fsharpqa/Source/Warnings/ModuleWithoutNamespace.fs

@@ -0,0 +1,7 @@
+// #Warnings
+//<Expects status="error" span="(4,1)" id="FS0222">Files in libraries or multiple-file applications must begin with a namespace or module declaration. When using a module declaration at the start of a file the '=' sign is not allowed. If this is a top-level module, consider removing the = to resolve this error.</Expects>
+
+module X =
+  let x = 1
+
+0

tests/fsharpqa/Source/Warnings/env.lst

@@ -7,6 +7,7 @@
 	SOURCE=ElseBranchHasWrongType.fs # ElseBranchHasWrongType.fs
 	SOURCE=MissingExpressionAfterLet.fs # MissingExpressionAfterLet.fs
 	SOURCE=AssignmentOnImmutable.fs # AssignmentOnImmutable.fs
+	SOURCE=ModuleWithoutNamespace.fs # ModuleWithoutNamespace.fs
 	SOURCE=UpcastInsteadOfDowncast.fs # UpcastInsteadOfDowncast.fs
 	SOURCE=UpcastFunctionInsteadOfDowncast.fs # UpcastFunctionInsteadOfDowncast.fs
 	SOURCE=DowncastInsteadOfUpcast.fs # DowncastInsteadOfUpcast.fs