[dev.typeparams] cmd/compile/internal/types2: expand is only required for *Named types

Now that the pointer identity for a *Named type doesn't change
anymore when going from lazy instantiated to actually instantiated
(= expanded) state, expand() only needs to be called when we deal
with *Named types and only if we care about a *Named type's internals.

Remove the expand function and respective calls for all types and
replace with specific t.expand() method calls where t is a *Named.

Change-Id: If82299360d60108b00adc4013b29399aec90b940
Reviewed-on: https://go-review.googlesource.com/c/go/+/340749
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -46,7 +46,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 	default:
 		// make argument getter
 		xlist, _ := check.exprList(call.ArgList, false)
-		arg = func(x *operand, i int) { *x = *xlist[i]; x.typ = expand(x.typ) }
+		arg = func(x *operand, i int) { *x = *xlist[i] }
 		nargs = len(xlist)
 		// evaluate first argument, if present
 		if nargs > 0 {

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

@@ -682,7 +682,6 @@ func (check *Checker) convertUntyped(x *operand, target Type) {
 // If x is a constant operand, the returned constant.Value will be the
 // representation of x in this context.
 func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, constant.Value, errorCode) {
-	target = expand(target)
 	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
 		return x.typ, nil, 0
 	}

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

@@ -279,12 +279,3 @@ func (n *Named) expand() {
 		n.instance = nil
 	}
 }
-
-// expand expands uninstantiated named types and leaves all other types alone.
-// expand does not recurse.
-func expand(typ Type) Type {
-	if t, _ := typ.(*Named); t != nil {
-		t.expand()
-	}
-	return typ
-}

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

@@ -140,10 +140,6 @@ func (p *ifacePair) identical(q *ifacePair) bool {
 
 // For changes to this code the corresponding changes should be made to unifier.nify.
 func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
-	// types must be expanded for comparison
-	x = expand(x)
-	y = expand(y)
-
 	if x == y {
 		return true
 	}
@@ -306,6 +302,8 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 		// Two named types are identical if their type names originate
 		// in the same type declaration.
 		if y, ok := y.(*Named); ok {
+			x.expand()
+			y.expand()
 			// TODO(gri) Why is x == y not sufficient? And if it is,
 			//           we can just return false here because x == y
 			//           is caught in the very beginning of this function.

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

@@ -208,7 +208,6 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 		// TODO(gri) We should delay rtyp expansion to when we actually need the
 		//           receiver; thus all checks here should be delayed to later.
 		rtyp, _ := deref(recv.typ)
-		rtyp = expand(rtyp)
 
 		// spec: "The receiver type must be of the form T or *T where T is a type name."
 		// (ignore invalid types - error was reported before)
@@ -216,6 +215,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 			var err string
 			switch T := rtyp.(type) {
 			case *Named:
+				T.expand()
 				// spec: "The type denoted by T is called the receiver base type; it must not
 				// be a pointer or interface type and it must be declared in the same package
 				// as the method."

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

@@ -114,7 +114,10 @@ func asInterface(t Type) *Interface {
 }
 
 func asNamed(t Type) *Named {
-	e, _ := expand(t).(*Named)
+	e, _ := t.(*Named)
+	if e != nil {
+		e.expand()
+	}
 	return e
 }
 

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

@@ -446,8 +446,7 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 	// make sure we check instantiation works at least once
 	// and that the resulting type is valid
 	check.later(func() {
-		t := expand(typ)
-		check.validType(t, nil)
+		check.validType(typ, nil)
 	})
 
 	return typ

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

@@ -229,10 +229,6 @@ func (u *unifier) nifyEq(x, y Type, p *ifacePair) bool {
 // code the corresponding changes should be made here.
 // Must not be called directly from outside the unifier.
 func (u *unifier) nify(x, y Type, p *ifacePair) bool {
-	// types must be expanded for comparison
-	x = expand(x)
-	y = expand(y)
-
 	if !u.exact {
 		// If exact unification is known to fail because we attempt to
 		// match a type name against an unnamed type literal, consider
@@ -436,6 +432,8 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		// 	return x.obj == y.obj
 		// }
 		if y, ok := y.(*Named); ok {
+			x.expand()
+			y.expand()
 			// TODO(gri) This is not always correct: two types may have the same names
 			//           in the same package if one of them is nested in a function.
 			//           Extremely unlikely but we need an always correct solution.

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

@@ -68,8 +68,7 @@ func parseUnion(check *Checker, tlist []syntax.Expr) Type {
 	// Note: This is a quadratic algorithm, but unions tend to be short.
 	check.later(func() {
 		for i, t := range terms {
-			typ := expand(t.typ)
-			if typ == Typ[Invalid] {
+			if t.typ == Typ[Invalid] {
 				continue
 			}
 
@@ -85,16 +84,16 @@ func parseUnion(check *Checker, tlist []syntax.Expr) Type {
 				}
 			}
 
-			u := under(typ)
+			u := under(t.typ)
 			f, _ := u.(*Interface)
 			if t.tilde {
 				if f != nil {
-					check.errorf(x, "invalid use of ~ (%s is an interface)", typ)
+					check.errorf(x, "invalid use of ~ (%s is an interface)", t.typ)
 					continue // don't report another error for t
 				}
 
-				if !Identical(u, typ) {
-					check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", typ, u)
+				if !Identical(u, t.typ) {
+					check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", t.typ, u)
 					continue // don't report another error for t
 				}
 			}