////////////////////////////////////////////////////////////////////////////////
//
// Helper methods
//
////////////////////////////////////////////////////////////////////////////////
//
// In error recovery and reporting scenarios we sometimes end up in a situation
// like this:
//
// x.Foo( y=>
//
// and the question is, "is Foo a valid extension method of x?" If Foo is
// generic, then Foo will be something like:
//
// static Blah Foo<T>(this Bar<T> bar, Func<T, T> f){ ... }
//
// What we would like to know is: given _only_ the expression x, can we infer
// what T is in Bar<T> ? If we can, then for error recovery and reporting
// we can provisionally consider Foo to be an extension method of x. If we
// cannot deduce this just from x then we should consider Foo to not be an
// extension method of x, at least until we have more information.
//
// Clearly it is pointless to run multiple phases
public static ImmutableArray<TypeSymbol> InferTypeArgumentsFromFirstArgument(
ConversionsBase conversions,
MethodSymbol method,
ImmutableArray<BoundExpression> arguments,
ref HashSet<DiagnosticInfo> useSiteDiagnostics)
{
Debug.Assert((object)method != null);
Debug.Assert(method.Arity > 0);
Debug.Assert(!arguments.IsDefault);
// We need at least one formal parameter type and at least one argument.
if ((method.ParameterCount < 1) || (arguments.Length < 1))
{
return default(ImmutableArray<TypeSymbol>);
}
Debug.Assert(!method.ParameterTypes[0].IsDynamic());
var constructedFromMethod = method.ConstructedFrom;
var inferrer = new MethodTypeInferrer(
conversions,
constructedFromMethod.TypeParameters,
constructedFromMethod.ContainingType,
constructedFromMethod.GetParameterTypes(),
constructedFromMethod.ParameterRefKinds,
arguments);
if (!inferrer.InferTypeArgumentsFromFirstArgument(ref useSiteDiagnostics))
{
return default(ImmutableArray<TypeSymbol>);
}
return inferrer.GetInferredTypeArguments();
}
public static MethodTypeInferenceResult Infer(
Binder binder,
ImmutableArray<TypeParameterSymbol> methodTypeParameters,
// We are attempting to build a map from method type parameters
// to inferred type arguments.
NamedTypeSymbol constructedContainingTypeOfMethod,
ImmutableArray<TypeSymbol> formalParameterTypes,
// We have some unusual requirements for the types that flow into the inference engine.
// Consider the following inference problems:
//
// Scenario one:
//
// class C<T>
// {
// delegate Y FT<X, Y>(T t, X x);
// static void M<U, V>(U u, FT<U, V> f);
// ...
// C<double>.M(123, (t,x)=>t+x);
//
// From the first argument we infer that U is int. How now must we make an inference on
// the second argument? The *declared* type of the formal is C<T>.FT<U,V>. The
// actual type at the time of inference is known to be C<double>.FT<int, something>
// where "something" is to be determined by inferring the return type of the
// lambda by determine the type of "double + int".
//
// Therefore when we do type inference, if a formal parameter type is a generic delegate
// then *its* formal parameter types must be the formal parameter types of the
// *constructed* generic delegate C<double>.FT<...>, not C<T>.FT<...>.
//
// One would therefore suppose that we'd expect the formal parameter types to here
// be passed in with the types constructed with the information known from the
// call site, not the declared types.
//
// Contrast that with this scenario:
//
// Scenario Two:
//
// interface I<T>
// {
// void M<U>(T t, U u);
// }
// ...
// static void Foo<V>(V v, I<V> iv)
// {
// iv.M(v, "");
// }
//
// Obviously inference should succeed here; it should infer that U is string.
//
// But consider what happens during the inference process on the first argument.
// The first thing we will do is say "what's the type of the argument? V. What's
// the type of the corresponding formal parameter? The first formal parameter of
// I<V>.M<whatever> is of type V. The inference engine will then say "V is a
// method type parameter, and therefore we have an inference from V to V".
// But *V* is not one of the method type parameters being inferred; the only
// method type parameter being inferred here is *U*.
//
// This is perhaps some evidence that the formal parameters passed in should be
// the formal parameters of the *declared* method; in this case, (T, U), not
// the formal parameters of the *constructed* method, (V, U).
//
// However, one might make the argument that no, we could just add a check
// to ensure that if we see a method type parameter as a formal parameter type,
// then we only perform the inference if the method type parameter is a type
// parameter of the method for which inference is being performed.
//
// Unfortunately, that does not work either:
//
// Scenario three:
//
// class C<T>
// {
// static void M<U>(T t, U u)
// {
// ...
// C<U>.M(u, 123);
// ...
// }
// }
//
// The *original* formal parameter types are (T, U); the *constructed* formal parameter types
// are (U, U), but *those are logically two different U's*. The first U is from the outer caller;
// the second U is the U of the recursive call.
//
// That is, suppose someone called C<string>.M<double>(string, double). The recursive call should be to
// C<double>.M<int>(double, int). We should absolutely not make an inference on the first argument
// from U to U just because C<U>.M<something>'s first formal parameter is of type U. If we did then
// inference would fail, because we'd end up with two bounds on 'U' -- 'U' and 'int'. We only want
// the latter bound.
//
// What these three scenarios show is that for a "normal" inference we need to have the
// formal parameters of the *original* method definition, but when making an inference from a lambda
// to a delegate, we need to have the *constructed* method signature in order that the formal
// parameters *of the delegate* be correct.
//
// How to solve this problem?
//
// We solve it by passing in the formal parameters in their *original* form, but also getting
// the *fully constructed* type that the method call is on. When constructing the fixed
// delegate type for inference from a lambda, we do the appropriate type substitution on
// the delegate.
ImmutableArray<RefKind> formalParameterRefKinds, // Optional; assume all value if missing.
ImmutableArray<BoundExpression> arguments,// Required; in scenarios like method group conversions where there are
// no arguments per se we cons up some fake arguments.
ref HashSet<DiagnosticInfo> useSiteDiagnostics)
{
Debug.Assert(!methodTypeParameters.IsDefault);
Debug.Assert(methodTypeParameters.Length > 0);
Debug.Assert(!formalParameterTypes.IsDefault);
Debug.Assert(formalParameterRefKinds.IsDefault || formalParameterRefKinds.Length == formalParameterTypes.Length);
Debug.Assert(!arguments.IsDefault);
// Early out: if the method has no formal parameters then we know that inference will fail.
if (formalParameterTypes.Length == 0)
{
return new MethodTypeInferenceResult(false, default(ImmutableArray<TypeSymbol>));
// UNDONE: OPTIMIZATION: We could check to see whether there is a type
// UNDONE: parameter which is never used in any formal parameter; if
// UNDONE: so then we know ahead of time that inference will fail.
}
var inferrer = new MethodTypeInferrer(
binder.Conversions,
methodTypeParameters,
constructedContainingTypeOfMethod,
formalParameterTypes,
formalParameterRefKinds,
arguments);
return inferrer.InferTypeArgs(binder, ref useSiteDiagnostics);
}