Rudimentary support for passing type parameters into generic functions.

Instead of generating an independent function instance for every
combination of type parameters at compile time we construct generic
function instances at runtime using "generic factory functions". Such a
factory takes type params as arguments and returns a concrete instance
of the function for the given type params (type param values are
captured by the returned function as a closure and can be used as
necessary).

Here is an abbreviated example of how a generic function is compiled and
called:

```
// Go:
func F[T any](t T) {}
f(1)

// JS:
F = function(T){ return function(t) {}; };
F($Int)(1);
```

This approach minimizes the size of the generated JS source, which is
critical for the client-side use case, at the cost of runtime
performance. See gopherjs#1013 (comment)
for the detailed description.

Note that the implementation in this commit is far from complete:

  - Generic function instances are not cached.
  - Generic types are not supported.
  - Declaring types dependent on type parameters doesn't work correctly.
  - Operators (such as `+`) do not work correctly with generic
    arguments.

compiler/expressions.go

@@ -492,10 +492,18 @@ func (fc *funcContext) translateExpr(expr ast.Expr) *expression {
 			)
 		case *types.Basic:
 			return fc.formatExpr("%e.charCodeAt(%f)", e.X, e.Index)
+		case *types.Signature:
+			return fc.translateGenericInstance(e)
 		default:
-			panic(fmt.Sprintf("Unhandled IndexExpr: %T\n", t))
+			panic(fmt.Errorf("unhandled IndexExpr: %T", t))
+		}
+	case *ast.IndexListExpr:
+		switch t := fc.pkgCtx.TypeOf(e.X).Underlying().(type) {
+		case *types.Signature:
+			return fc.translateGenericInstance(e)
+		default:
+			panic(fmt.Errorf("unhandled IndexListExpr: %T", t))
 		}
-
 	case *ast.SliceExpr:
 		if b, isBasic := fc.pkgCtx.TypeOf(e.X).Underlying().(*types.Basic); isBasic && isString(b) {
 			switch {
@@ -749,6 +757,10 @@ func (fc *funcContext) translateExpr(expr ast.Expr) *expression {
 		case *types.Var, *types.Const:
 			return fc.formatExpr("%s", fc.objectName(o))
 		case *types.Func:
+			if _, ok := fc.pkgCtx.Info.Instances[e]; ok {
+				// Generic function call with auto-inferred types.
+				return fc.translateGenericInstance(e)
+			}
 			return fc.formatExpr("%s", fc.objectName(o))
 		case *types.TypeName:
 			return fc.formatExpr("%s", fc.typeName(o.Type()))
@@ -788,6 +800,38 @@ func (fc *funcContext) translateExpr(expr ast.Expr) *expression {
 	}
 }
 
+// translateGenericInstance translates a generic function instantiation.
+//
+// The returned JS expression evaluates into a callable function with type params
+// substituted.
+func (fc *funcContext) translateGenericInstance(e ast.Expr) *expression {
+	var identifier *ast.Ident
+	switch e := e.(type) {
+	case *ast.Ident:
+		identifier = e
+	case *ast.IndexExpr:
+		identifier = e.X.(*ast.Ident)
+	case *ast.IndexListExpr:
+		identifier = e.X.(*ast.Ident)
+	default:
+		err := bailout(fmt.Errorf("unexpected generic instantiation expression type %T at %s", e, fc.pkgCtx.fileSet.Position(e.Pos())))
+		panic(err)
+	}
+
+	instance, ok := fc.pkgCtx.Info.Instances[identifier]
+	if !ok {
+		err := fmt.Errorf("no matching generic instantiation for %q at %s", identifier, fc.pkgCtx.fileSet.Position(identifier.Pos()))
+		bailout(err)
+	}
+	typeParams := []string{}
+	for i := 0; i < instance.TypeArgs.Len(); i++ {
+		t := instance.TypeArgs.At(i)
+		typeParams = append(typeParams, fc.typeName(t))
+	}
+	o := fc.pkgCtx.Uses[identifier]
+	return fc.formatExpr("%s(%s)", fc.objectName(o), strings.Join(typeParams, ", "))
+}
+
 func (fc *funcContext) translateCall(e *ast.CallExpr, sig *types.Signature, fun *expression) *expression {
 	args := fc.translateArgs(sig, e.Args, e.Ellipsis.IsValid())
 	if fc.Blocking[e] {

compiler/package.go

@@ -180,6 +180,7 @@ func Compile(importPath string, files []*ast.File, fileSet *token.FileSet, impor
 		Implicits:  make(map[ast.Node]types.Object),
 		Selections: make(map[*ast.SelectorExpr]*types.Selection),
 		Scopes:     make(map[ast.Node]*types.Scope),
+		Instances:  make(map[*ast.Ident]types.Instance),
 	}
 
 	var errList ErrorList
@@ -294,7 +295,7 @@ func Compile(importPath string, files []*ast.File, fileSet *token.FileSet, impor
 			// but now we do it here to maintain previous behavior.
 			continue
 		}
-		funcCtx.pkgCtx.pkgVars[importedPkg.Path()] = funcCtx.newVariable(importedPkg.Name(), true)
+		funcCtx.pkgCtx.pkgVars[importedPkg.Path()] = funcCtx.newVariable(importedPkg.Name(), varPackage)
 		importedPaths = append(importedPaths, importedPkg.Path())
 	}
 	sort.Strings(importedPaths)
@@ -521,7 +522,7 @@ func Compile(importPath string, files []*ast.File, fileSet *token.FileSet, impor
 			d.DeclCode = funcCtx.CatchOutput(0, func() {
 				typeName := funcCtx.objectName(o)
 				lhs := typeName
-				if isPkgLevel(o) {
+				if typeVarLevel(o) == varPackage {
 					lhs += " = $pkg." + encodeIdent(o.Name())
 				}
 				size := int64(0)
@@ -898,5 +899,22 @@ func translateFunction(typ *ast.FuncType, recv *ast.Ident, body *ast.BlockStmt,
 
 	c.pkgCtx.escapingVars = prevEV
 
-	return params, fmt.Sprintf("function%s(%s) {\n%s%s}", functionName, strings.Join(params, ", "), bodyOutput, c.Indentation(0))
+	if !c.sigTypes.IsGeneric() {
+		return params, fmt.Sprintf("function%s(%s) {\n%s%s}", functionName, strings.Join(params, ", "), bodyOutput, c.Indentation(0))
+	}
+
+	// Generic functions are generated as factories to allow passing type parameters
+	// from the call site.
+	// TODO(nevkontakte): Cache function instances for a given combination of type
+	// parameters.
+	// TODO(nevkontakte): Generate type parameter arguments and derive all dependent
+	// types inside the function.
+	typeParams := []string{}
+	for i := 0; i < c.sigTypes.Sig.TypeParams().Len(); i++ {
+		typeParam := c.sigTypes.Sig.TypeParams().At(i)
+		typeParams = append(typeParams, c.typeName(typeParam))
+	}
+
+	return params, fmt.Sprintf("function%s(%s){ return function(%s) {\n%s%s}; }",
+		functionName, strings.Join(typeParams, ", "), strings.Join(params, ", "), bodyOutput, c.Indentation(0))
 }

compiler/statements.go

@@ -444,7 +444,7 @@ func (fc *funcContext) translateStmt(stmt ast.Stmt, label *types.Label) {
 			for _, spec := range decl.Specs {
 				o := fc.pkgCtx.Defs[spec.(*ast.TypeSpec).Name].(*types.TypeName)
 				fc.pkgCtx.typeNames = append(fc.pkgCtx.typeNames, o)
-				fc.pkgCtx.objectNames[o] = fc.newVariable(o.Name(), true)
+				fc.pkgCtx.objectNames[o] = fc.newVariable(o.Name(), varPackage)
 				fc.pkgCtx.dependencies[o] = true
 			}
 		case token.CONST:

compiler/utils.go

@@ -237,9 +237,23 @@ func (fc *funcContext) newConst(t types.Type, value constant.Value) ast.Expr {
 // local variable name. In this context "local" means "in scope of the current"
 // functionContext.
 func (fc *funcContext) newLocalVariable(name string) string {
-	return fc.newVariable(name, false)
+	return fc.newVariable(name, varFuncLocal)
 }
 
+// varLevel specifies at which level a JavaScript variable should be declared.
+type varLevel int
+
+const (
+	// A variable defined at a function level (e.g. local variables).
+	varFuncLocal = iota
+	// A variable that should be declared in a generic type or function factory.
+	// This is mainly for type parameters and generic-dependent types.
+	varGenericFactory
+	// A variable that should be declared in a package factory. This user is for
+	// top-level functions, types, etc.
+	varPackage
+)
+
 // newVariable assigns a new JavaScript variable name for the given Go variable
 // or type.
 //
@@ -252,7 +266,7 @@ func (fc *funcContext) newLocalVariable(name string) string {
 // to this functionContext, as well as all parents, but not to the list of local
 // variables. If false, it is added to this context only, as well as the list of
 // local vars.
-func (fc *funcContext) newVariable(name string, pkgLevel bool) string {
+func (fc *funcContext) newVariable(name string, level varLevel) string {
 	if name == "" {
 		panic("newVariable: empty name")
 	}
@@ -261,7 +275,7 @@ func (fc *funcContext) newVariable(name string, pkgLevel bool) string {
 		i := 0
 		for {
 			offset := int('a')
-			if pkgLevel {
+			if level == varPackage {
 				offset = int('A')
 			}
 			j := i
@@ -286,9 +300,22 @@ func (fc *funcContext) newVariable(name string, pkgLevel bool) string {
 		varName = fmt.Sprintf("%s$%d", name, n)
 	}
 
-	if pkgLevel {
-		for c2 := fc.parent; c2 != nil; c2 = c2.parent {
-			c2.allVars[name] = n + 1
+	// Package-level variables are registered in all outer scopes.
+	if level == varPackage {
+		for c := fc.parent; c != nil; c = c.parent {
+			c.allVars[name] = n + 1
+		}
+		return varName
+	}
+
+	// Generic-factory level variables are registered in outer scopes up to the
+	// level of the generic function or method.
+	if level == varGenericFactory {
+		for c := fc; c != nil; c = c.parent {
+			c.allVars[name] = n + 1
+			if c.sigTypes.IsGeneric() {
+				break
+			}
 		}
 		return varName
 	}
@@ -331,14 +358,20 @@ func isVarOrConst(o types.Object) bool {
 	return false
 }
 
-func isPkgLevel(o types.Object) bool {
-	return o.Parent() != nil && o.Parent().Parent() == types.Universe
+func typeVarLevel(o types.Object) varLevel {
+	if _, ok := o.Type().(*types.TypeParam); ok {
+		return varGenericFactory
+	}
+	if o.Parent() != nil && o.Parent().Parent() == types.Universe {
+		return varPackage
+	}
+	return varFuncLocal
 }
 
 // objectName returns a JS identifier corresponding to the given types.Object.
 // Repeated calls for the same object will return the same name.
 func (fc *funcContext) objectName(o types.Object) string {
-	if isPkgLevel(o) {
+	if typeVarLevel(o) == varPackage {
 		fc.pkgCtx.dependencies[o] = true
 
 		if o.Pkg() != fc.pkgCtx.Pkg || (isVarOrConst(o) && o.Exported()) {
@@ -348,7 +381,7 @@ func (fc *funcContext) objectName(o types.Object) string {
 
 	name, ok := fc.pkgCtx.objectNames[o]
 	if !ok {
-		name = fc.newVariable(o.Name(), isPkgLevel(o))
+		name = fc.newVariable(o.Name(), typeVarLevel(o))
 		fc.pkgCtx.objectNames[o] = name
 	}
 
@@ -359,13 +392,13 @@ func (fc *funcContext) objectName(o types.Object) string {
 }
 
 func (fc *funcContext) varPtrName(o *types.Var) string {
-	if isPkgLevel(o) && o.Exported() {
+	if typeVarLevel(o) == varPackage && o.Exported() {
 		return fc.pkgVar(o.Pkg()) + "." + o.Name() + "$ptr"
 	}
 
 	name, ok := fc.pkgCtx.varPtrNames[o]
 	if !ok {
-		name = fc.newVariable(o.Name()+"$ptr", isPkgLevel(o))
+		name = fc.newVariable(o.Name()+"$ptr", typeVarLevel(o))
 		fc.pkgCtx.varPtrNames[o] = name
 	}
 	return name
@@ -385,6 +418,8 @@ func (fc *funcContext) typeName(ty types.Type) string {
 			return "$error"
 		}
 		return fc.objectName(t.Obj())
+	case *types.TypeParam:
+		return fc.objectName(t.Obj())
 	case *types.Interface:
 		if t.Empty() {
 			return "$emptyInterface"
@@ -397,8 +432,8 @@ func (fc *funcContext) typeName(ty types.Type) string {
 	// repeatedly.
 	anonType, ok := fc.pkgCtx.anonTypeMap.At(ty).(*types.TypeName)
 	if !ok {
-		fc.initArgs(ty) // cause all embedded types to be registered
-		varName := fc.newVariable(strings.ToLower(typeKind(ty)[5:])+"Type", true)
+		fc.initArgs(ty) // cause all dependency types to be registered
+		varName := fc.newVariable(strings.ToLower(typeKind(ty)[5:])+"Type", varPackage)
 		anonType = types.NewTypeName(token.NoPos, fc.pkgCtx.Pkg, varName, ty) // fake types.TypeName
 		fc.pkgCtx.anonTypes = append(fc.pkgCtx.anonTypes, anonType)
 		fc.pkgCtx.anonTypeMap.Set(ty, anonType)
@@ -815,6 +850,12 @@ func (st signatureTypes) HasNamedResults() bool {
 	return st.HasResults() && st.Sig.Results().At(0).Name() != ""
 }
 
+// IsGeneric returns true if the signature represents a generic function or a
+// method of a generic type.
+func (st signatureTypes) IsGeneric() bool {
+	return st.Sig.TypeParams().Len() > 0 || st.Sig.RecvTypeParams().Len() > 0
+}
+
 // ErrorAt annotates an error with a position in the source code.
 func ErrorAt(err error, fset *token.FileSet, pos token.Pos) error {
 	return fmt.Errorf("%s: %w", fset.Position(pos), err)