cmd/compile/internal/types2: implement types2.Instantiate

Instantiation support for imports. This is experimental
but it also doesn't affect Go 1.17 as this code is not
executed unless we enable generics (in the parser).

Change-Id: If2da09ac3a557ec6a180707a53f75f3ce354f3e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/314773
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/types2/instantiate.go

@@ -0,0 +1,63 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+)
+
+// Instantiate instantiates the type typ with the given type arguments.
+// typ must be a *Named or a *Signature type, it must be generic, and
+// its number of type parameters must match the number of provided type
+// arguments. The result is a new, instantiated (not generic) type of
+// the same kind (either a *Named or a *Signature). The type arguments
+// are not checked against the constraints of the type parameters.
+// Any methods attached to a *Named are simply copied; they are not
+// instantiated.
+func Instantiate(pos syntax.Pos, typ Type, targs []Type) (res Type) {
+	// TODO(gri) This code is basically identical to the prolog
+	//           in Checker.instantiate. Factor.
+	var tparams []*TypeName
+	switch t := typ.(type) {
+	case *Named:
+		tparams = t.tparams
+	case *Signature:
+		tparams = t.tparams
+		defer func() {
+			// If we had an unexpected failure somewhere don't panic below when
+			// asserting res.(*Signature). Check for *Signature in case Typ[Invalid]
+			// is returned.
+			if _, ok := res.(*Signature); !ok {
+				return
+			}
+			// If the signature doesn't use its type parameters, subst
+			// will not make a copy. In that case, make a copy now (so
+			// we can set tparams to nil w/o causing side-effects).
+			if t == res {
+				copy := *t
+				res = &copy
+			}
+			// After instantiating a generic signature, it is not generic
+			// anymore; we need to set tparams to nil.
+			res.(*Signature).tparams = nil
+		}()
+
+	default:
+		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
+	}
+
+	// the number of supplied types must match the number of type parameters
+	if len(targs) != len(tparams) {
+		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, len(targs), len(tparams)))
+	}
+
+	if len(tparams) == 0 {
+		return typ // nothing to do (minor optimization)
+	}
+
+	smap := makeSubstMap(tparams, targs)
+	return (*Checker)(nil).subst(pos, typ, smap)
+}

src/cmd/compile/internal/types2/subst.go

@@ -308,6 +308,9 @@ func (subst *subster) typ(typ Type) Type {
 		embeddeds, ecopied := subst.typeList(t.embeddeds)
 		if mcopied || types != t.types || ecopied {
 			iface := &Interface{methods: methods, types: types, embeddeds: embeddeds}
+			if subst.check == nil {
+				panic("internal error: cannot instantiate interfaces yet")
+			}
 			subst.check.posMap[iface] = subst.check.posMap[t] // satisfy completeInterface requirement
 			subst.check.completeInterface(nopos, iface)
 			return iface
@@ -327,12 +330,14 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 	case *Named:
+		// dump is for debugging
+		dump := func(string, ...interface{}) {}
+		if subst.check != nil && subst.check.conf.Trace {
 			subst.check.indent++
 			defer func() {
 				subst.check.indent--
 			}()
-		dump := func(format string, args ...interface{}) {
+			dump = func(format string, args ...interface{}) {
-			if subst.check.conf.Trace {
 				subst.check.trace(subst.pos, format, args...)
 			}
 		}
@@ -377,17 +382,21 @@ func (subst *subster) typ(typ Type) Type {
 		// before creating a new named type, check if we have this one already
 		h := instantiatedHash(t, new_targs)
 		dump(">>> new type hash: %s", h)
+		if subst.check != nil {
 			if named, found := subst.check.typMap[h]; found {
 				dump(">>> found %s", named)
 				subst.cache[t] = named
 				return named
 			}
+		}
 
 		// create a new named type and populate caches to avoid endless recursion
 		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
 		named := subst.check.newNamed(tname, t, t.underlying, t.tparams, t.methods) // method signatures are updated lazily
 		named.targs = new_targs
+		if subst.check != nil {
 			subst.check.typMap[h] = named
+		}
 		subst.cache[t] = named
 
 		// do the substitution