[dev.typeparams] cmd/compile: support generic types (with stenciling of method calls)

A type may now have a type param in it, either because it has been
composed from a function type param, or it has been declared as or
derived from a reference to a generic type. No objects or types with
type params can be exported yet. No generic type has a runtime
descriptor (but will likely eventually be associated with a dictionary).

types.Type now has an RParam field, which for a Named type can specify
the type params (in order) that must be supplied to fully instantiate
the type. Also, there is a new flag HasTParam to indicate if there is
a type param (TTYPEPARAM) anywhere in the type.

An instantiated generic type (whether fully instantiated or
re-instantiated to new type params) is a defined type, even though there
was no explicit declaration. This allows us to handle recursive
instantiated types (and improves printing of types).

To avoid the need to transform later in the compiler, an instantiation
of a method of a generic type is immediately represented as a function
with the method as the first argument.

Added 5 tests on generic types to test/typeparams, including list.go,
which tests recursive generic types.

Change-Id: Ib7ff27abd369a06d1c8ea84edc6ca1fd74bbb7c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/292652
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/stencil.go

@@ -13,7 +13,9 @@ import (
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"cmd/internal/src"
 	"fmt"
+	"strings"
 )
 
 // stencil scans functions for instantiated generic function calls and
@@ -61,6 +63,17 @@ func (g *irgen) stencil() {
 			// Replace the OFUNCINST with a direct reference to the
 			// new stenciled function
 			call.X = st.Nname
+			if inst.X.Op() == ir.OCALLPART {
+				// When we create an instantiation of a method
+				// call, we make it a function. So, move the
+				// receiver to be the first arg of the function
+				// call.
+				withRecv := make([]ir.Node, len(call.Args)+1)
+				dot := inst.X.(*ir.SelectorExpr)
+				withRecv[0] = dot.X
+				copy(withRecv[1:], call.Args)
+				call.Args = withRecv
+			}
 			modified = true
 		})
 		if base.Flag.W > 1 && modified {
@@ -74,7 +87,12 @@ func (g *irgen) stencil() {
 // the name of the function and the types of the type params.
 func makeInstName(inst *ir.InstExpr) *types.Sym {
 	b := bytes.NewBufferString("#")
-	b.WriteString(inst.X.(*ir.Name).Name().Sym().Name)
+	if meth, ok := inst.X.(*ir.SelectorExpr); ok {
+		// Write the name of the generic method, including receiver type
+		b.WriteString(meth.Selection.Nname.Sym().Name)
+	} else {
+		b.WriteString(inst.X.(*ir.Name).Name().Sym().Name)
+	}
 	b.WriteString("[")
 	for i, targ := range inst.Targs {
 		if i > 0 {
@@ -90,18 +108,38 @@ func makeInstName(inst *ir.InstExpr) *types.Sym {
 // instantiation of a generic function with specified type arguments.
 type subster struct {
 	newf    *ir.Func // Func node for the new stenciled function
-	tparams *types.Fields
+	tparams []*types.Field
 	targs   []ir.Node
 	// The substitution map from name nodes in the generic function to the
 	// name nodes in the new stenciled function.
 	vars map[*ir.Name]*ir.Name
+	seen map[*types.Type]*types.Type
 }
 
 // genericSubst returns a new function with the specified name. The function is an
-// instantiation of a generic function with type params, as specified by inst.
+// instantiation of a generic function or method with type params, as specified by
+// inst. For a method with a generic receiver, it returns an instantiated function
+// type where the receiver becomes the first parameter. Otherwise the instantiated
+// method would still need to be transformed by later compiler phases.
 func genericSubst(name *types.Sym, inst *ir.InstExpr) *ir.Func {
-	// Similar to noder.go: funcDecl
-	nameNode := inst.X.(*ir.Name)
+	var nameNode *ir.Name
+	var tparams []*types.Field
+	if selExpr, ok := inst.X.(*ir.SelectorExpr); ok {
+		// Get the type params from the method receiver (after skipping
+		// over any pointer)
+		nameNode = ir.AsNode(selExpr.Selection.Nname).(*ir.Name)
+		recvType := selExpr.Type().Recv().Type
+		if recvType.IsPtr() {
+			recvType = recvType.Elem()
+		}
+		tparams = make([]*types.Field, len(recvType.RParams))
+		for i, rparam := range recvType.RParams {
+			tparams[i] = types.NewField(src.NoXPos, nil, rparam)
+		}
+	} else {
+		nameNode = inst.X.(*ir.Name)
+		tparams = nameNode.Type().TParams().Fields().Slice()
+	}
 	gf := nameNode.Func
 	newf := ir.NewFunc(inst.Pos())
 	newf.Nname = ir.NewNameAt(inst.Pos(), name)
@@ -111,9 +149,10 @@ func genericSubst(name *types.Sym, inst *ir.InstExpr) *ir.Func {
 
 	subst := &subster{
 		newf:    newf,
-		tparams: nameNode.Type().TParams().Fields(),
+		tparams: tparams,
 		targs:   inst.Targs,
 		vars:    make(map[*ir.Name]*ir.Name),
+		seen:    make(map[*types.Type]*types.Type),
 	}
 
 	newf.Dcl = make([]*ir.Name, len(gf.Dcl))
@@ -125,9 +164,12 @@ func genericSubst(name *types.Sym, inst *ir.InstExpr) *ir.Func {
 	// Ugly: we have to insert the Name nodes of the parameters/results into
 	// the function type. The current function type has no Nname fields set,
 	// because it came via conversion from the types2 type.
-	oldt := inst.Type()
-	newt := types.NewSignature(oldt.Pkg(), nil, nil, subst.fields(ir.PPARAM, oldt.Params(), newf.Dcl),
-		subst.fields(ir.PPARAMOUT, oldt.Results(), newf.Dcl))
+	oldt := inst.X.Type()
+	// We also transform a generic method type to the corresponding
+	// instantiated function type where the receiver is the first parameter.
+	newt := types.NewSignature(oldt.Pkg(), nil, nil,
+		subst.fields(ir.PPARAM, append(oldt.Recvs().FieldSlice(), oldt.Params().FieldSlice()...), newf.Dcl),
+		subst.fields(ir.PPARAMOUT, oldt.Results().FieldSlice(), newf.Dcl))
 
 	newf.Nname.Ntype = ir.TypeNode(newt)
 	newf.Nname.SetType(newt)
@@ -276,41 +318,81 @@ func (subst *subster) tstruct(t *types.Type) *types.Type {
 
 }
 
-// typ substitutes any type parameter found with the corresponding type argument.
-func (subst *subster) typ(t *types.Type) *types.Type {
-	for i, tp := range subst.tparams.Slice() {
-		if tp.Type == t {
-			return subst.targs[i].Type()
+// instTypeName creates a name for an instantiated type, based on the type args
+func instTypeName(name string, targs []ir.Node) string {
+	b := bytes.NewBufferString(name)
+	b.WriteByte('[')
+	for i, targ := range targs {
+		if i > 0 {
+			b.WriteByte(',')
 		}
+		b.WriteString(targ.Type().String())
+	}
+	b.WriteByte(']')
+	return b.String()
+}
+
+// typ computes the type obtained by substituting any type parameter in t with the
+// corresponding type argument in subst. If t contains no type parameters, the
+// result is t; otherwise the result is a new type.
+// It deals with recursive types by using a map and TFORW types.
+// TODO(danscales) deal with recursion besides ptr/struct cases.
+func (subst *subster) typ(t *types.Type) *types.Type {
+	if !t.HasTParam() {
+		return t
+	}
+	if subst.seen[t] != nil {
+		// We've hit a recursive type
+		return subst.seen[t]
 	}
 
+	var newt *types.Type
 	switch t.Kind() {
+	case types.TTYPEPARAM:
+		for i, tp := range subst.tparams {
+			if tp.Type == t {
+				return subst.targs[i].Type()
+			}
+		}
+		return t
+
 	case types.TARRAY:
 		elem := t.Elem()
 		newelem := subst.typ(elem)
 		if newelem != elem {
-			return types.NewArray(newelem, t.NumElem())
+			newt = types.NewArray(newelem, t.NumElem())
 		}
 
 	case types.TPTR:
 		elem := t.Elem()
+		// In order to deal with recursive generic types, create a TFORW
+		// type initially and store it in the seen map, so it can be
+		// accessed if this type appears recursively within the type.
+		forw := types.New(types.TFORW)
+		subst.seen[t] = forw
 		newelem := subst.typ(elem)
 		if newelem != elem {
-			return types.NewPtr(newelem)
+			forw.SetUnderlying(types.NewPtr(newelem))
+			newt = forw
 		}
+		delete(subst.seen, t)
 
 	case types.TSLICE:
 		elem := t.Elem()
 		newelem := subst.typ(elem)
 		if newelem != elem {
-			return types.NewSlice(newelem)
+			newt = types.NewSlice(newelem)
 		}
 
 	case types.TSTRUCT:
-		newt := subst.tstruct(t)
+		forw := types.New(types.TFORW)
+		subst.seen[t] = forw
+		newt = subst.tstruct(t)
 		if newt != t {
-			return newt
+			forw.SetUnderlying(newt)
+			newt = forw
 		}
+		delete(subst.seen, t)
 
 	case types.TFUNC:
 		newrecvs := subst.tstruct(t.Recvs())
@@ -321,21 +403,39 @@ func (subst *subster) typ(t *types.Type) *types.Type {
 			if newrecvs.NumFields() > 0 {
 				newrecv = newrecvs.Field(0)
 			}
-			return types.NewSignature(t.Pkg(), newrecv, nil, newparams.FieldSlice(), newresults.FieldSlice())
+			newt = types.NewSignature(t.Pkg(), newrecv, nil, newparams.FieldSlice(), newresults.FieldSlice())
 		}
 
 		// TODO: case TCHAN
 		// TODO: case TMAP
 		// TODO: case TINTER
 	}
+	if newt != nil {
+		if t.Sym() != nil {
+			// Since we've substituted types, we also need to change
+			// the defined name of the type, by removing the old types
+			// (in brackets) from the name, and adding the new types.
+			oldname := t.Sym().Name
+			i := strings.Index(oldname, "[")
+			oldname = oldname[:i]
+			sym := t.Sym().Pkg.Lookup(instTypeName(oldname, subst.targs))
+			if sym.Def != nil {
+				// We've already created this instantiated defined type.
+				return sym.Def.Type()
+			}
+			newt.SetSym(sym)
+			sym.Def = ir.TypeNode(newt)
+		}
+		return newt
+	}
+
 	return t
 }
 
 // fields sets the Nname field for the Field nodes inside a type signature, based
 // on the corresponding in/out parameters in dcl. It depends on the in and out
 // parameters being in order in dcl.
-func (subst *subster) fields(class ir.Class, oldt *types.Type, dcl []*ir.Name) []*types.Field {
-	oldfields := oldt.FieldSlice()
+func (subst *subster) fields(class ir.Class, oldfields []*types.Field, dcl []*ir.Name) []*types.Field {
 	newfields := make([]*types.Field, len(oldfields))
 	var i int