compiler: implement interface assertions

This is a lot harder than 'regular' type assertions as the actual
methods need to be checked.

compiler.go

@@ -426,7 +426,12 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 		}
 		rangeValue := llvm.ConstNamedStruct(rangeType, rangeValues)
 		ranges = append(ranges, rangeValue)
+		methods := make([]*types.Selection, 0, len(meta.Methods))
 		for _, method := range meta.Methods {
+			methods = append(methods, method)
+		}
+		c.ir.SortMethods(methods)
+		for _, method := range methods {
 			f := c.ir.GetFunction(program.MethodValue(method))
 			if f.llvmFn.IsNil() {
 				return errors.New("cannot find function: " + f.LinkName())
@@ -440,6 +445,25 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 		startIndex += len(meta.Methods)
 	}
 
+	interfaceTypes := c.ir.AllInterfaces()
+	interfaceLengths := make([]llvm.Value, len(interfaceTypes))
+	interfaceMethods := make([]llvm.Value, 0)
+	for i, itfType := range interfaceTypes {
+		if itfType.Type.NumMethods() > 0xff {
+			return errors.New("too many methods for interface " + itfType.Type.String())
+		}
+		interfaceLengths[i] = llvm.ConstInt(llvm.Int8Type(), uint64(itfType.Type.NumMethods()), false)
+		funcs := make([]*types.Func, itfType.Type.NumMethods())
+		for i := range funcs {
+			funcs[i] = itfType.Type.Method(i)
+		}
+		c.ir.SortFuncs(funcs)
+		for _, f := range funcs {
+			id := llvm.ConstInt(llvm.Int16Type(), uint64(c.ir.MethodNum(f)), false)
+			interfaceMethods = append(interfaceMethods, id)
+		}
+	}
+
 	if len(ranges) >= 1<<16 {
 		return errors.New("method call numbers do not fit in a 16-bit integer")
 	}
@@ -469,8 +493,24 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 	signatureArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(signatureArrayNewGlobal, signatureArrayOldGlobal.Type()))
 	signatureArrayOldGlobal.EraseFromParentAsGlobal()
 	signatureArrayNewGlobal.SetName("runtime.methodSetSignatures")
+	interfaceLengthsArray := llvm.ConstArray(llvm.Int8Type(), interfaceLengths)
+	interfaceLengthsArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceLengthsArray.Type(), "runtime.interfaceLengths.tmp")
+	interfaceLengthsArrayNewGlobal.SetInitializer(interfaceLengthsArray)
+	interfaceLengthsArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
+	interfaceLengthsArrayOldGlobal := c.mod.NamedGlobal("runtime.interfaceLengths")
+	interfaceLengthsArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(interfaceLengthsArrayNewGlobal, interfaceLengthsArrayOldGlobal.Type()))
+	interfaceLengthsArrayOldGlobal.EraseFromParentAsGlobal()
+	interfaceLengthsArrayNewGlobal.SetName("runtime.interfaceLengths")
+	interfaceMethodsArray := llvm.ConstArray(llvm.Int16Type(), interfaceMethods)
+	interfaceMethodsArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceMethodsArray.Type(), "runtime.interfaceMethods.tmp")
+	interfaceMethodsArrayNewGlobal.SetInitializer(interfaceMethodsArray)
+	interfaceMethodsArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
+	interfaceMethodsArrayOldGlobal := c.mod.NamedGlobal("runtime.interfaceMethods")
+	interfaceMethodsArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(interfaceMethodsArrayNewGlobal, interfaceMethodsArrayOldGlobal.Type()))
+	interfaceMethodsArrayOldGlobal.EraseFromParentAsGlobal()
+	interfaceMethodsArrayNewGlobal.SetName("runtime.interfaceMethods")
 
-	c.mod.NamedGlobal("runtime.firstInterfaceNum").SetInitializer(llvm.ConstInt(llvm.Int16Type(), uint64(c.ir.FirstDynamicType()), false))
+	c.mod.NamedGlobal("runtime.firstTypeWithMethods").SetInitializer(llvm.ConstInt(llvm.Int16Type(), uint64(c.ir.FirstDynamicType()), false))
 
 	// see: https://reviews.llvm.org/D18355
 	c.mod.AddNamedMetadataOperand("llvm.module.flags",
@@ -2238,25 +2278,43 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		if err != nil {
 			return llvm.Value{}, err
 		}
-		if _, ok := expr.AssertedType.Underlying().(*types.Interface); ok {
+
-			// TODO: check whether the type implements the interface.
-			return llvm.Value{}, errors.New("todo: assert on interface")
-		}
 		assertedType, err := c.getLLVMType(expr.AssertedType)
 		if err != nil {
 			return llvm.Value{}, err
 		}
-		assertedTypeNum, typeExists := c.ir.TypeNum(expr.AssertedType)
+		valueNil, err := getZeroValue(assertedType)
-		if !typeExists {
+		if err != nil {
-			// Static analysis has determined this type assert will never apply.
+			return llvm.Value{}, err
-			return llvm.ConstStruct([]llvm.Value{llvm.Undef(assertedType), llvm.ConstInt(llvm.Int1Type(), 0, false)}, false), nil
 		}
-		if assertedTypeNum >= 1<<16 {
-			return llvm.Value{}, errors.New("interface typecodes do not fit in a 16-bit integer")
-		}
-		actualTypeNum := c.builder.CreateExtractValue(itf, 0, "interface.type")
 
-		commaOk := c.builder.CreateICmp(llvm.IntEQ, llvm.ConstInt(llvm.Int16Type(), uint64(assertedTypeNum), false), actualTypeNum, "")
+		actualTypeNum := c.builder.CreateExtractValue(itf, 0, "interface.type")
+		commaOk := llvm.Value{}
+		if itf, ok := expr.AssertedType.Underlying().(*types.Interface); ok {
+			// Type assert on interface type.
+			// This is slightly non-trivial: at runtime the list of methods
+			// needs to be checked to see whether it implements the interface.
+			// At the same time, the interface value itself is unchanged.
+			itfTypeNum := c.ir.InterfaceNum(itf)
+			itfTypeNumValue := llvm.ConstInt(llvm.Int16Type(), uint64(itfTypeNum), false)
+			fn := c.mod.NamedFunction("runtime.interfaceImplements")
+			commaOk = c.builder.CreateCall(fn, []llvm.Value{actualTypeNum, itfTypeNumValue}, "")
+
+		} else {
+			// Type assert on concrete type.
+			// This is easy: just compare the type number.
+			assertedTypeNum, typeExists := c.ir.TypeNum(expr.AssertedType)
+			if !typeExists {
+				// Static analysis has determined this type assert will never apply.
+				return llvm.ConstStruct([]llvm.Value{valueNil, llvm.ConstInt(llvm.Int1Type(), 0, false)}, false), nil
+			}
+			if assertedTypeNum >= 1<<16 {
+				return llvm.Value{}, errors.New("interface typecodes do not fit in a 16-bit integer")
+			}
+
+			assertedTypeNumValue := llvm.ConstInt(llvm.Int16Type(), uint64(assertedTypeNum), false)
+			commaOk = c.builder.CreateICmp(llvm.IntEQ, assertedTypeNumValue, actualTypeNum, "")
+		}
 
 		// Add 2 new basic blocks (that should get optimized away): one for the
 		// 'ok' case and one for all instructions following this type assert.
@@ -2269,11 +2327,6 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		// typeassert should return a zero value, not an incorrectly casted
 		// value.
 
-		valueNil, err := getZeroValue(assertedType)
-		if err != nil {
-			return llvm.Value{}, err
-		}
-
 		prevBlock := c.builder.GetInsertBlock()
 		okBlock := c.ctx.AddBasicBlock(frame.fn.llvmFn, "typeassert.ok")
 		nextBlock := c.ctx.AddBasicBlock(frame.fn.llvmFn, "typeassert.next")
@@ -2282,29 +2335,37 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		// Retrieve the value from the interface if the type assert was
 		// successful.
 		c.builder.SetInsertPointAtEnd(okBlock)
-		valuePtr := c.builder.CreateExtractValue(itf, 1, "typeassert.value.ptr")
 		var valueOk llvm.Value
-		if c.targetData.TypeAllocSize(assertedType) > c.targetData.TypeAllocSize(c.i8ptrType) {
+		if _, ok := expr.AssertedType.Underlying().(*types.Interface); ok {
-			// Value was stored in an allocated buffer, load it from there.
+			// Type assert on interface type. Easy: just return the same
-			valuePtrCast := c.builder.CreateBitCast(valuePtr, llvm.PointerType(assertedType, 0), "")
+			// interface value.
-			valueOk = c.builder.CreateLoad(valuePtrCast, "typeassert.value.ok")
+			valueOk = itf
 		} else {
-			// Value was stored directly in the interface.
+			// Type assert on concrete type. Extract the underlying type from
-			switch assertedType.TypeKind() {
+			// the interface (but only after checking it matches).
-			case llvm.IntegerTypeKind:
+			valuePtr := c.builder.CreateExtractValue(itf, 1, "typeassert.value.ptr")
-				valueOk = c.builder.CreatePtrToInt(valuePtr, assertedType, "typeassert.value.ok")
+			if c.targetData.TypeAllocSize(assertedType) > c.targetData.TypeAllocSize(c.i8ptrType) {
-			case llvm.PointerTypeKind:
+				// Value was stored in an allocated buffer, load it from there.
-				valueOk = c.builder.CreateBitCast(valuePtr, assertedType, "typeassert.value.ok")
+				valuePtrCast := c.builder.CreateBitCast(valuePtr, llvm.PointerType(assertedType, 0), "")
-			case llvm.StructTypeKind:
+				valueOk = c.builder.CreateLoad(valuePtrCast, "typeassert.value.ok")
-				// A bitcast would be useful here, but bitcast doesn't allow
+			} else {
-				// aggregate types. So we'll bitcast it using an alloca.
+				// Value was stored directly in the interface.
-				// Hopefully this will get optimized away.
+				switch assertedType.TypeKind() {
-				mem := c.builder.CreateAlloca(c.i8ptrType, "")
+				case llvm.IntegerTypeKind:
-				c.builder.CreateStore(valuePtr, mem)
+					valueOk = c.builder.CreatePtrToInt(valuePtr, assertedType, "typeassert.value.ok")
-				memStructPtr := c.builder.CreateBitCast(mem, llvm.PointerType(assertedType, 0), "")
+				case llvm.PointerTypeKind:
-				valueOk = c.builder.CreateLoad(memStructPtr, "typeassert.value.ok")
+					valueOk = c.builder.CreateBitCast(valuePtr, assertedType, "typeassert.value.ok")
-			default:
+				case llvm.StructTypeKind:
-				return llvm.Value{}, errors.New("todo: typeassert: bitcast small types")
+					// A bitcast would be useful here, but bitcast doesn't allow
+					// aggregate types. So we'll bitcast it using an alloca.
+					// Hopefully this will get optimized away.
+					mem := c.builder.CreateAlloca(c.i8ptrType, "")
+					c.builder.CreateStore(valuePtr, mem)
+					memStructPtr := c.builder.CreateBitCast(mem, llvm.PointerType(assertedType, 0), "")
+					valueOk = c.builder.CreateLoad(memStructPtr, "typeassert.value.ok")
+				default:
+					return llvm.Value{}, errors.New("todo: typeassert: bitcast small types")
+				}
 			}
 		}
 		c.builder.CreateBr(nextBlock)

ir.go

@@ -25,10 +25,11 @@ type Program struct {
 	NamedTypes           []*NamedType
 	needsScheduler       bool
 	goCalls              []*ssa.Go
-	typesWithMethods     map[string]*InterfaceType // see AnalyseInterfaceConversions
+	typesWithMethods     map[string]*TypeWithMethods // see AnalyseInterfaceConversions
-	typesWithoutMethods  map[string]int            // see AnalyseInterfaceConversions
+	typesWithoutMethods  map[string]int              // see AnalyseInterfaceConversions
-	methodSignatureNames map[string]int
+	methodSignatureNames map[string]int              // see MethodNum
-	fpWithContext        map[string]struct{} // see AnalyseFunctionPointers
+	interfaces           map[string]*Interface       // see AnalyseInterfaceConversions
+	fpWithContext        map[string]struct{}         // see AnalyseFunctionPointers
 }
 
 // Function or method.
@@ -60,12 +61,19 @@ type NamedType struct {
 }
 
 // Type that is at some point put in an interface.
-type InterfaceType struct {
+type TypeWithMethods struct {
 	t       types.Type
 	Num     int
 	Methods map[string]*types.Selection
 }
 
+// Interface type that is at some point used in a type assert (to check whether
+// it implements another interface).
+type Interface struct {
+	Num  int
+	Type *types.Interface
+}
+
 // Create and intialize a new *Program from a *ssa.Program.
 func NewProgram(program *ssa.Program, mainPath string) *Program {
 	return &Program{
@@ -74,6 +82,7 @@ func NewProgram(program *ssa.Program, mainPath string) *Program {
 		functionMap:          make(map[*ssa.Function]*Function),
 		globalMap:            make(map[*ssa.Global]*Global),
 		methodSignatureNames: make(map[string]int),
+		interfaces:           make(map[string]*Interface),
 	}
 }
 
@@ -148,6 +157,18 @@ func (p *Program) GetGlobal(ssaGlobal *ssa.Global) *Global {
 	return p.globalMap[ssaGlobal]
 }
 
+// SortMethods sorts the list of methods by method ID.
+func (p *Program) SortMethods(methods []*types.Selection) {
+	m := &methodList{methods: methods, program: p}
+	sort.Sort(m)
+}
+
+// SortFuncs sorts the list of functions by method ID.
+func (p *Program) SortFuncs(funcs []*types.Func) {
+	m := &funcList{funcs: funcs, program: p}
+	sort.Sort(m)
+}
+
 // Parse compiler directives in the preceding comments.
 func (f *Function) parsePragmas() {
 	if f.fn.Syntax() == nil {
@@ -236,3 +257,43 @@ func (g *Global) LinkName() string {
 func (g *Global) IsExtern() bool {
 	return strings.HasPrefix(g.g.Name(), "_extern_")
 }
+
+// Wrapper type to implement sort.Interface for []*types.Selection.
+type methodList struct {
+	methods []*types.Selection
+	program *Program
+}
+
+func (m *methodList) Len() int {
+	return len(m.methods)
+}
+
+func (m *methodList) Less(i, j int) bool {
+	iid := m.program.MethodNum(m.methods[i].Obj().(*types.Func))
+	jid := m.program.MethodNum(m.methods[j].Obj().(*types.Func))
+	return iid < jid
+}
+
+func (m *methodList) Swap(i, j int) {
+	m.methods[i], m.methods[j] = m.methods[j], m.methods[i]
+}
+
+// Wrapper type to implement sort.Interface for []*types.Func.
+type funcList struct {
+	funcs   []*types.Func
+	program *Program
+}
+
+func (fl *funcList) Len() int {
+	return len(fl.funcs)
+}
+
+func (fl *funcList) Less(i, j int) bool {
+	iid := fl.program.MethodNum(fl.funcs[i])
+	jid := fl.program.MethodNum(fl.funcs[j])
+	return iid < jid
+}
+
+func (fl *funcList) Swap(i, j int) {
+	fl.funcs[i], fl.funcs[j] = fl.funcs[j], fl.funcs[i]
+}

passes.go

@@ -2,6 +2,9 @@ package main
 
 import (
 	"go/types"
+	"sort"
+	"strings"
+
 	"golang.org/x/tools/go/ssa"
 )
 
@@ -56,6 +59,18 @@ func Signature(sig *types.Signature) string {
 	return s
 }
 
+// Convert an interface type to a string of all method strings, separated by
+// "; ". For example: "Read([]byte) (int, error); Close() error"
+func InterfaceKey(itf *types.Interface) string {
+	methodStrings := []string{}
+	for i := 0; i < itf.NumMethods(); i++ {
+		method := itf.Method(i)
+		methodStrings = append(methodStrings, MethodSignature(method))
+	}
+	sort.Strings(methodStrings)
+	return strings.Join(methodStrings, ";")
+}
+
 // Fill in parents of all functions.
 //
 // All packages need to be added before this pass can run, or it will produce
@@ -104,7 +119,7 @@ func (p *Program) AnalyseCallgraph() {
 func (p *Program) AnalyseInterfaceConversions() {
 	// Clear, if AnalyseTypes has been called before.
 	p.typesWithoutMethods = map[string]int{"nil": 0}
-	p.typesWithMethods = map[string]*InterfaceType{}
+	p.typesWithMethods = map[string]*TypeWithMethods{}
 
 	for _, f := range p.Functions {
 		for _, block := range f.fn.Blocks {
@@ -114,7 +129,7 @@ func (p *Program) AnalyseInterfaceConversions() {
 					methods := getAllMethods(f.fn.Prog, instr.X.Type())
 					name := instr.X.Type().String()
 					if _, ok := p.typesWithMethods[name]; !ok && len(methods) > 0 {
-						t := &InterfaceType{
+						t := &TypeWithMethods{
 							t:       instr.X.Type(),
 							Num:     len(p.typesWithMethods),
 							Methods: make(map[string]*types.Selection),
@@ -271,7 +286,13 @@ func (p *Program) SimpleDCE() {
 			for _, instr := range block.Instrs {
 				if instr, ok := instr.(*ssa.MakeInterface); ok {
 					for _, sel := range getAllMethods(p.program, instr.X.Type()) {
-						callee := p.GetFunction(p.program.MethodValue(sel))
+						fn := p.program.MethodValue(sel)
+						callee := p.GetFunction(fn)
+						if callee == nil {
+							// TODO: why is this necessary?
+							p.addFunction(fn)
+							callee = p.GetFunction(fn)
+						}
 						if !callee.flag {
 							callee.flag = true
 							worklist = append(worklist, callee.fn)
@@ -361,6 +382,19 @@ func (p *Program) TypeNum(typ types.Type) (int, bool) {
 	}
 }
 
+// InterfaceNum returns the numeric interface ID of this type, for use in type
+// asserts.
+func (p *Program) InterfaceNum(itfType *types.Interface) int {
+	key := InterfaceKey(itfType)
+	if itf, ok := p.interfaces[key]; !ok {
+		num := len(p.interfaces)
+		p.interfaces[key] = &Interface{Num: num, Type: itfType}
+		return num
+	} else {
+		return itf.Num
+	}
+}
+
 // MethodNum returns the numeric ID of this method, to be used in method lookups
 // on interfaces for example.
 func (p *Program) MethodNum(method *types.Func) int {
@@ -381,14 +415,23 @@ func (p *Program) FirstDynamicType() int {
 }
 
 // Return all types with methods, sorted by type ID.
-func (p *Program) AllDynamicTypes() []*InterfaceType {
+func (p *Program) AllDynamicTypes() []*TypeWithMethods {
-	l := make([]*InterfaceType, len(p.typesWithMethods))
+	l := make([]*TypeWithMethods, len(p.typesWithMethods))
 	for _, m := range p.typesWithMethods {
 		l[m.Num] = m
 	}
 	return l
 }
 
+// Return all interface types, sorted by interface ID.
+func (p *Program) AllInterfaces() []*Interface {
+	l := make([]*Interface, len(p.interfaces))
+	for _, itf := range p.interfaces {
+		l[itf.Num] = itf
+	}
+	return l
+}
+
 func (p *Program) FunctionNeedsContext(f *Function) bool {
 	if !f.addressTaken {
 		return false

src/examples/test/test.go

@@ -16,6 +16,18 @@ type Stringer interface {
 	String() string
 }
 
+type Foo int
+
+type Number int
+
+func (n Number) Double() int {
+	return int(n) * 2
+}
+
+type Doubler interface {
+	Double() int
+}
+
 const SIX = 6
 
 var testmap = map[string]int{"data": 3}
@@ -54,6 +66,7 @@ func main() {
 	printItf(*thing)
 	printItf(thing)
 	printItf(Stringer(thing))
+	printItf(Number(3))
 	s := Stringer(thing)
 	println("Stringer.String():", s.String())
 
@@ -107,6 +120,8 @@ func strlen(s string) int {
 
 func printItf(val interface{}) {
 	switch val := val.(type) {
+	case Doubler:
+		println("is Doubler:", val.Double())
 	case int:
 		println("is int:", val)
 	case byte:
@@ -117,6 +132,8 @@ func printItf(val interface{}) {
 		println("is Thing:", val.String())
 	case *Thing:
 		println("is *Thing:", val.String())
+	case Foo:
+		println("is Foo:", val)
 	default:
 		println("is ?")
 	}

src/runtime/interface.go

@@ -10,10 +10,10 @@ package runtime
 // signatures as interned strings at compile time.
 //
 // The typecode is a small number unique for the Go type. All typecodes <
-// firstInterfaceNum do not have any methods and typecodes >= firstInterfaceNum
+// firstTypeWithMethods do not have any methods and typecodes >=
-// all have at least one method. This means that methodSetRanges does not need
+// firstTypeWithMethods all have at least one method. This means that
-// to contain types without methods and is thus indexed starting at a typecode
+// methodSetRanges does not need to contain types without methods and is thus
-// with number firstInterfaceNum.
+// indexed starting at a typecode with number firstTypeWithMethods.
 //
 // To further conserve some space, the methodSetRange (as the name indicates)
 // doesn't contain a list of methods and function pointers directly, but instead
@@ -36,10 +36,13 @@ type methodSetRange struct {
 // which is a dummy value, but will be bigger after the compiler has filled them
 // in.
 var (
-	firstInterfaceNum   uint16            // the lowest typecode that has at least one method
+	firstTypeWithMethods uint16            // the lowest typecode that has at least one method
-	methodSetRanges     [0]methodSetRange // indexes into methodSetSignatures and methodSetFunctions
+	methodSetRanges      [0]methodSetRange // indexes into methodSetSignatures and methodSetFunctions
-	methodSetSignatures [0]uint16         // uniqued method ID
+	methodSetSignatures  [0]uint16         // uniqued method ID
-	methodSetFunctions  [0]*uint8         // function pointer of method
+	methodSetFunctions   [0]*uint8         // function pointer of method
+	interfaceIndex       [0]uint16         // mapping from interface ID to an index in interfaceMethods
+	interfaceLengths     [0]uint8          // mapping from interface ID to the number of methods it has
+	interfaceMethods     [0]uint16         // the method an interface implements (list of method IDs)
 )
 
 // Get the function pointer for the method on the interface.
@@ -50,7 +53,7 @@ func interfaceMethod(itf _interface, method uint16) *uint8 {
 	// in the list of signatures. The compiler will only emit
 	// runtime.interfaceMethod calls when the method actually exists on this
 	// interface (proven by the typechecker).
-	i := methodSetRanges[itf.typecode-firstInterfaceNum].index
+	i := methodSetRanges[itf.typecode-firstTypeWithMethods].index
 	for {
 		if methodSetSignatures[i] == method {
 			return methodSetFunctions[i]
@@ -72,3 +75,49 @@ func interfaceEqual(x, y _interface) bool {
 	// TODO: depends on reflection.
 	panic("unimplemented: interface equality")
 }
+
+// Return true iff the type implements all methods needed by the interface. This
+// means the type satisfies the interface.
+// This is a compiler intrinsic.
+//go:nobounds
+func interfaceImplements(typecode, interfaceNum uint16) bool {
+	// method set indexes of the concrete type
+	methodSet := methodSetRanges[typecode-firstTypeWithMethods]
+	methodIndex := methodSet.index
+	methodIndexEnd := methodSet.index + methodSet.length
+
+	// method set indexes of the interface
+	itfIndex := interfaceIndex[interfaceNum]
+	itfIndexEnd := itfIndex + uint16(interfaceLengths[interfaceNum])
+
+	// Iterate over all methods of the interface:
+	for itfIndex < itfIndexEnd {
+		methodId := interfaceMethods[itfIndex]
+		if methodIndex >= methodIndexEnd {
+			// Reached the end of the list of methods, so interface doesn't
+			// implement this type.
+			return false
+		}
+		if methodId == methodSetSignatures[methodIndex] {
+			// Found a matching method, continue to the next method.
+			itfIndex++
+			methodIndex++
+			continue
+		} else if methodId > methodSetSignatures[methodIndex] {
+			// The method didn't match, but method ID of the concrete type was
+			// lower than that of the interface, so probably it has a method the
+			// interface doesn't implement.
+			// Move on to the next method of the concrete type.
+			methodIndex++
+			continue
+		} else {
+			// The concrete type is missing a method. This means the type assert
+			// fails.
+			return false
+		}
+	}
+
+	// Found a method for each expected method in the interface. This type
+	// assert is successful.
+	return true
+}