Type extensions should throw a more descriptive error (#3248)

src/fsharp/FSComp.txt

@@ -789,7 +789,7 @@ tcTypeAbbreviationHasTypeParametersMissingOnType,"This type abbreviation has one
 954,tcTypeDefinitionIsCyclicThroughInheritance,"This type definition involves an immediate cyclic reference through a struct field or inheritance relation"
 tcReservedSyntaxForAugmentation,"The syntax 'type X with ...' is reserved for augmentations. Types whose representations are hidden but which have members are now declared in signatures using 'type X = ...'. You may also need to add the '[<Sealed>] attribute to the type definition in the signature"
 956,tcMembersThatExtendInterfaceMustBePlacedInSeparateModule,"Members that extend interface, delegate or enum types must be placed in a module separate to the definition of the type. This module must either have the AutoOpen attribute or be opened explicitly by client code to bring the extension members into scope."
-957,tcDeclaredTypeParametersForExtensionDoNotMatchOriginal,"The declared type parameters for this type extension do not match the declared type parameters on the original type '%s'"
+957,tcDeclaredTypeParametersForExtensionDoNotMatchOriginal,"One or more of the declared type parameters for this type extension have a missing or wrong type constraint not matching the original type constraints on '%s'"
 959,tcTypeDefinitionsWithImplicitConstructionMustHaveOneInherit,"Type definitions may only have one 'inherit' specification and it must be the first declaration"
 960,tcTypeDefinitionsWithImplicitConstructionMustHaveLocalBindingsBeforeMembers,"'let' and 'do' bindings must come before member and interface definitions in type definitions"
 961,tcInheritDeclarationMissingArguments,"This 'inherit' declaration specifies the inherited type but no arguments. Consider supplying arguments, e.g. 'inherit BaseType(args)'."

tests/fsharp/typecheck/sigs/neg22.bsl

@@ -33,8 +33,8 @@ but given a
     'float<kg>'    
 The unit of measure 'm' does not match the unit of measure 'kg'
 
-neg22.fs(28,12,28,18): typecheck error FS0957: The declared type parameters for this type extension do not match the declared type parameters on the original type 'LibGen<_>'
+neg22.fs(28,12,28,18): typecheck error FS0957: One or more of the declared type parameters for this type extension have a missing or wrong type constraint not matching the original type constraints on 'LibGen<_>'
 
-neg22.fs(40,12,40,18): typecheck error FS0957: The declared type parameters for this type extension do not match the declared type parameters on the original type 'LibGen<_>'
+neg22.fs(40,12,40,18): typecheck error FS0957: One or more of the declared type parameters for this type extension have a missing or wrong type constraint not matching the original type constraints on 'LibGen<_>'
 
 neg22.fs(33,17,33,19): typecheck error FS0341: The signature and implementation are not compatible because the type parameter 'T' has a constraint of the form 'T :> System.ValueType but the implementation does not. Either remove this constraint from the signature or add it to the implementation.