/// <summary>
/// Determine whether there is any substitution of type parameters that will
/// make two types identical.
/// </summary>
public static bool CanUnify(TypeSymbol t1, TypeSymbol t2)
{
MutableTypeMap substitution = null;
bool result = CanUnifyHelper(t1, t2, ref substitution);
Debug.Assert(!result || (substitution == null && t1 == t2) ||
substitution.SubstituteType(t1) == substitution.SubstituteType(t2));
return(result);
}
/// <summary>
/// Determine whether there is any substitution of type parameters that will
/// make two types identical.
/// </summary>
/// <param name="t1">LHS</param>
/// <param name="t2">RHS</param>
/// <param name="substitution">
/// Substitutions performed so far (or null for none).
/// Keys are type parameters, values are types (possibly type parameters).
/// Will be updated with new subsitutions by the callee.
/// Should be ignored when false is returned.
/// </param>
/// <returns>True if there exists a type map such that Map(LHS) == Map(RHS).</returns>
/// <remarks>
/// Derived from Dev10's BSYMMGR::UnifyTypes.
/// Two types will not unify if they have different custom modifiers.
/// </remarks>
private static bool CanUnifyHelper(TypeSymbol t1, TypeSymbol t2, ref MutableTypeMap substitution)
{
if (ReferenceEquals(t1, t2))
{
return(true);
}
else if ((object)t1 == null || (object)t2 == null)
{
// Can't both be null or they would have been ReferenceEquals
return(false);
}
if (substitution != null)
{
t1 = substitution.SubstituteType(t1);
t2 = substitution.SubstituteType(t2);
}
// If one of the types is a type parameter, then the substitution could make them ReferenceEquals.
if (ReferenceEquals(t1, t2))
{
return(true);
}
// We can avoid a lot of redundant checks if we ensure that we only have to check
// for type parameters on the LHS
if (!t1.IsTypeParameter() && t2.IsTypeParameter())
{
TypeSymbol tmp = t1;
t1 = t2;
t2 = tmp;
}
// If t1 is not a type parameter, then neither is t2
Debug.Assert(t1.IsTypeParameter() || !t2.IsTypeParameter());
switch (t1.Kind)
{
case SymbolKind.ArrayType:
{
if (t2.TypeKind != t1.TypeKind)
{
return(false);
}
ArrayTypeSymbol at1 = (ArrayTypeSymbol)t1;
ArrayTypeSymbol at2 = (ArrayTypeSymbol)t2;
if (at1.Rank != at2.Rank || !at1.CustomModifiers.SequenceEqual(at2.CustomModifiers))
{
return(false);
}
return(CanUnifyHelper(at1.ElementType, at2.ElementType, ref substitution));
}
case SymbolKind.PointerType:
{
if (t2.TypeKind != t1.TypeKind)
{
return(false);
}
PointerTypeSymbol pt1 = (PointerTypeSymbol)t1;
PointerTypeSymbol pt2 = (PointerTypeSymbol)t2;
if (!pt1.CustomModifiers.SequenceEqual(pt2.CustomModifiers))
{
return(false);
}
return(CanUnifyHelper(pt1.PointedAtType, pt2.PointedAtType, ref substitution));
}
case SymbolKind.NamedType:
case SymbolKind.ErrorType:
{
if (t2.TypeKind != t1.TypeKind)
{
return(false);
}
NamedTypeSymbol nt1 = (NamedTypeSymbol)t1;
NamedTypeSymbol nt2 = (NamedTypeSymbol)t2;
if (!nt1.IsGenericType)
{
return(!nt2.IsGenericType && nt1 == nt2);
}
else if (!nt2.IsGenericType)
{
return(false);
}
int arity = nt1.Arity;
if (nt2.Arity != arity || nt2.OriginalDefinition != nt1.OriginalDefinition)
{
return(false);
}
for (int i = 0; i < arity; i++)
{
if (!CanUnifyHelper(nt1.TypeArgumentsNoUseSiteDiagnostics[i], nt2.TypeArgumentsNoUseSiteDiagnostics[i], ref substitution))
{
return(false);
}
}
// Note: Dev10 folds this into the loop since GetTypeArgsAll includes type args for containing types
return((object)nt1.ContainingType == null || CanUnifyHelper(nt1.ContainingType, nt2.ContainingType, ref substitution));
}
case SymbolKind.TypeParameter:
{
// These substitutions are not allowed in C#
if (t2.TypeKind == TypeKind.Pointer || t2.SpecialType == SpecialType.System_Void)
{
return(false);
}
TypeParameterSymbol tp1 = (TypeParameterSymbol)t1;
// Perform the "occurs check" - i.e. ensure that t2 doesn't contain t1 to avoid recursive types
// Note: t2 can't be the same type param - we would have caught that with ReferenceEquals above
if (Contains(t2, tp1))
{
return(false);
}
if (substitution == null)
{
substitution = new MutableTypeMap();
}
// MutableTypeMap.Add will throw if the key has already been added. However,
// if t1 was already in the substitution, it would have been substituted at the
// start of this method and we wouldn't be here.
substitution.Add(tp1, t2);
return(true);
}
default:
{
return(t1 == t2);
}
}
}
private static void AppendMapping(MutableTypeMap customTypeSubstitution, NamedTypeSymbol namedTypeSymbol, bool invert = false)
{
for (var i = 0; i < namedTypeSymbol.TypeParameters.Length; i++)
{
var typeParameterSymbol = namedTypeSymbol.TypeParameters[i];
var typeArgument = namedTypeSymbol.TypeArguments[i];
if (!ReferenceEquals(typeParameterSymbol, typeArgument))
{
if (invert)
{
Debug.Assert(typeArgument is TypeParameterSymbol, "TypeParameterSymbol is required");
customTypeSubstitution.Add(typeArgument as TypeParameterSymbol, customTypeSubstitution.SubstituteType(typeParameterSymbol));
}
else
{
customTypeSubstitution.Add(typeParameterSymbol, typeArgument);
}
}
}
}