go/types: remove asTypeParam and simplify some code

This is a port of CL 363438 from types2 to go/types.

Change-Id: I87c76d31b398b9ce406f96b0030ee458619b3dbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/364235
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

13 files changed, 33 insertions, 54 deletions

diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index c1932232aa..5418d66aeb 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -298,7 +298,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		// the argument types must be of floating-point type
 		// (applyTypeFunc never calls f with a type parameter)
 		f := func(typ Type) Type {
-			assert(asTypeParam(typ) == nil)
+			assert(!isTypeParam(typ))
 			if t, _ := under(typ).(*Basic); t != nil {
 				switch t.kind {
 				case Float32:
@@ -441,7 +441,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		// the argument must be of complex type
 		// (applyTypeFunc never calls f with a type parameter)
 		f := func(typ Type) Type {
-			assert(asTypeParam(typ) == nil)
+			assert(!isTypeParam(typ))
 			if t, _ := under(typ).(*Basic); t != nil {
 				switch t.kind {
 				case Complex64:
@@ -822,7 +822,7 @@ func hasVarSize(t Type) bool {
 // applyTypeFunc returns nil.
 // If x is not a type parameter, the result is f(x).
 func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
-	if tp := asTypeParam(x); tp != nil {
+	if tp, _ := x.(*TypeParam); tp != nil {
 		// Test if t satisfies the requirements for the argument
 		// type and collect possible result types at the same time.
 		var terms []*Term
diff --git a/src/go/types/call.go b/src/go/types/call.go
index dfd7142094..7cb6027f3b 100644
--- a/src/go/types/call.go
+++ b/src/go/types/call.go
@@ -531,7 +531,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 			check.errorf(e.Sel, _InvalidMethodExpr, "cannot call pointer method %s on %s", sel, x.typ)
 		default:
 			var why string
-			if tpar := asTypeParam(x.typ); tpar != nil {
+			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
 				// Type parameter bounds don't specify fields, so don't mention "field".
 				if tname := tpar.iface().obj; tname != nil {
 					why = check.sprintf("interface %s has no method %s", tname.name, sel)
diff --git a/src/go/types/conversions.go b/src/go/types/conversions.go
index 18d24e404c..eadc923f5e 100644
--- a/src/go/types/conversions.go
+++ b/src/go/types/conversions.go
@@ -47,7 +47,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 		// If T's type set is empty, or if it doesn't
 		// have specific types, constant x cannot be
 		// converted.
-		ok = under(T).(*TypeParam).underIs(func(u Type) bool {
+		ok = T.(*TypeParam).underIs(func(u Type) bool {
 			// t is nil if there are no specific type terms
 			if u == nil {
 				cause = check.sprintf("%s does not contain specific types", T)
@@ -186,8 +186,8 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 	}
 
 	// optimization: if we don't have type parameters, we're done
-	Vp, _ := Vu.(*TypeParam)
-	Tp, _ := Tu.(*TypeParam)
+	Vp, _ := V.(*TypeParam)
+	Tp, _ := T.(*TypeParam)
 	if Vp == nil && Tp == nil {
 		return false
 	}
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index e12961416e..2108cf6b05 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -669,10 +669,11 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 	}
 
 	// Disallow a lone type parameter as the RHS of a type declaration (issue #45639).
-	// We can look directly at named.underlying because even if it is still a *Named
-	// type (underlying not fully resolved yet) it cannot become a type parameter due
-	// to this very restriction.
-	if tpar, _ := named.underlying.(*TypeParam); tpar != nil {
+	// We don't need this restriction anymore if we make the underlying type of a type
+	// parameter its constraint interface: if the RHS is a lone type parameter, we will
+	// use its underlying type (like we do for any RHS in a type declaration), and its
+	// underlying type is an interface and the type declaration is well defined.
+	if isTypeParam(rhs) {
 		check.error(tdecl.Type, _MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
 		named.underlying = Typ[Invalid]
 	}
@@ -723,7 +724,7 @@ func (check *Checker) collectTypeParams(dst **TypeParamList, list *ast.FieldList
 
 	check.later(func() {
 		for i, bound := range bounds {
-			if _, ok := under(bound).(*TypeParam); ok {
+			if isTypeParam(bound) {
 				check.error(posns[i], _MisplacedTypeParam, "cannot use a type parameter as constraint")
 			}
 		}
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 6eeb431b73..660c92de3b 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -147,11 +147,10 @@ var op2str2 = [...]string{
 // If typ is a type parameter, underIs returns the result of typ.underIs(f).
 // Otherwise, underIs returns the result of f(under(typ)).
 func underIs(typ Type, f func(Type) bool) bool {
-	u := under(typ)
-	if tpar, _ := u.(*TypeParam); tpar != nil {
+	if tpar, _ := typ.(*TypeParam); tpar != nil {
 		return tpar.underIs(f)
 	}
-	return f(u)
+	return f(under(typ))
 }
 
 // The unary expression e may be nil. It's passed in for better error messages only.
diff --git a/src/go/types/instantiate.go b/src/go/types/instantiate.go
index 13d6e3114d..c9ce6f6ae1 100644
--- a/src/go/types/instantiate.go
+++ b/src/go/types/instantiate.go
@@ -157,7 +157,7 @@ func (check *Checker) satisfies(pos token.Pos, targ Type, tpar *TypeParam, smap
 
 	// A type argument that is a type parameter with an empty type set satisfies any constraint.
 	// (The empty set is a subset of any set.)
-	if targ := asTypeParam(targ); targ != nil && targ.iface().typeSet().IsEmpty() {
+	if targ, _ := targ.(*TypeParam); targ != nil && targ.iface().typeSet().IsEmpty() {
 		return nil
 	}
 
@@ -186,7 +186,7 @@ func (check *Checker) satisfies(pos token.Pos, targ Type, tpar *TypeParam, smap
 	// if iface is comparable, targ must be comparable
 	// TODO(gri) the error messages needs to be better, here
 	if iface.IsComparable() && !Comparable(targ) {
-		if tpar := asTypeParam(targ); tpar != nil && tpar.iface().typeSet().IsAll() {
+		if tpar, _ := targ.(*TypeParam); tpar != nil && tpar.iface().typeSet().IsAll() {
 			return errorf("%s has no constraints", targ)
 		}
 		return errorf("%s does not satisfy comparable", targ)
@@ -198,7 +198,7 @@ func (check *Checker) satisfies(pos token.Pos, targ Type, tpar *TypeParam, smap
 		// If the type argument is a pointer to a type parameter, the type argument's
 		// method set is empty.
 		// TODO(gri) is this what we want? (spec question)
-		if base, isPtr := deref(targ); isPtr && asTypeParam(base) != nil {
+		if base, isPtr := deref(targ); isPtr && isTypeParam(base) {
 			return errorf("%s has no methods", targ)
 		}
 		if m, wrong := check.missingMethod(targ, iface, true); m != nil {
@@ -227,7 +227,7 @@ func (check *Checker) satisfies(pos token.Pos, targ Type, tpar *TypeParam, smap
 	// If targ is itself a type parameter, each of its possible types must be in the set
 	// of iface types (i.e., the targ type set must be a subset of the iface type set).
 	// Type arguments with empty type sets were already excluded above.
-	if targ := asTypeParam(targ); targ != nil {
+	if targ, _ := targ.(*TypeParam); targ != nil {
 		targBound := targ.iface()
 		if !targBound.typeSet().subsetOf(iface.typeSet()) {
 			// TODO(gri) report which type is missing
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index aae6fa206d..98af6bfcd7 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -134,14 +134,8 @@ func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 					continue // we can't have a matching field or interface method
 				}
 
-				// continue with underlying type, but only if it's not a type parameter
-				// TODO(gri) is this what we want to do for type parameters? (spec question)
-				// TODO(#45639) the error message produced as a result of skipping an
-				//              underlying type parameter should be improved.
+				// continue with underlying type
 				typ = named.under()
-				if asTypeParam(typ) != nil {
-					continue
-				}
 			}
 
 			tpar = nil
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 8b76e939b6..6f902e9749 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -166,7 +166,7 @@ func operandString(x *operand, qf Qualifier) string {
 			}
 			buf.WriteString(intro)
 			WriteType(&buf, x.typ, qf)
-			if tpar := asTypeParam(x.typ); tpar != nil {
+			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
 				buf.WriteString(" constrained by ")
 				WriteType(&buf, tpar.bound, qf) // do not compute interface type sets here
 			}
@@ -241,8 +241,8 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 
 	Vu := under(V)
 	Tu := under(T)
-	Vp, _ := Vu.(*TypeParam)
-	Tp, _ := Tu.(*TypeParam)
+	Vp, _ := V.(*TypeParam)
+	Tp, _ := T.(*TypeParam)
 
 	// x is an untyped value representable by a value of type T.
 	if isUntyped(Vu) {
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 2d9b9c4c07..d0697b1ad7 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -92,7 +92,7 @@ func IsInterface(t Type) bool {
 
 // isTypeParam reports whether t is a type parameter.
 func isTypeParam(t Type) bool {
-	_, ok := under(t).(*TypeParam)
+	_, ok := t.(*TypeParam)
 	return ok
 }
 
diff --git a/src/go/types/type.go b/src/go/types/type.go
index e26d8189d1..555eb9e8b9 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -88,9 +88,3 @@ func asNamed(t Type) *Named {
 	}
 	return e
 }
-
-// If t is a type parameter, asTypeParam returns that type, otherwise it returns nil.
-func asTypeParam(t Type) *TypeParam {
-	u, _ := under(t).(*TypeParam)
-	return u
-}
diff --git a/src/go/types/typeset.go b/src/go/types/typeset.go
index 1e6b9dd390..d0464aeaa0 100644
--- a/src/go/types/typeset.go
+++ b/src/go/types/typeset.go
@@ -289,10 +289,6 @@ func computeInterfaceTypeSet(check *Checker, pos token.Pos, ityp *Interface) *_T
 				continue // ignore invalid unions
 			}
 			terms = tset.terms
-		case *TypeParam:
-			// Embedding stand-alone type parameters is not permitted.
-			// Union parsing reports a (delayed) error, so we can ignore this entry.
-			continue
 		default:
 			if u == Typ[Invalid] {
 				continue
@@ -370,10 +366,6 @@ func computeUnionTypeSet(check *Checker, pos token.Pos, utyp *Union) *_TypeSet {
 		switch u := under(t.typ).(type) {
 		case *Interface:
 			terms = computeInterfaceTypeSet(check, pos, u).terms
-		case *TypeParam:
-			// A stand-alone type parameters is not permitted as union term.
-			// Union parsing reports a (delayed) error, so we can ignore this entry.
-			continue
 		default:
 			if t.typ == Typ[Invalid] {
 				continue
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index 5828c2e7c3..89264ee9eb 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -337,7 +337,7 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 		check.later(func() {
 			if !Comparable(typ.key) {
 				var why string
-				if asTypeParam(typ.key) != nil {
+				if isTypeParam(typ.key) {
 					why = " (missing comparable constraint)"
 				}
 				check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s%s", typ.key, why)
diff --git a/src/go/types/union.go b/src/go/types/union.go
index bb08174728..2a65ca4d8e 100644
--- a/src/go/types/union.go
+++ b/src/go/types/union.go
@@ -118,15 +118,14 @@ func parseTilde(check *Checker, x ast.Expr) (tilde bool, typ Type) {
 	}
 	typ = check.typ(x)
 	// Embedding stand-alone type parameters is not permitted (issue #47127).
-	// Do this check later because it requires computation of the underlying type (see also issue #46461).
-	// Note: If an underlying type cannot be a type parameter, the call to
-	//       under() will not be needed and then we don't need to delay this
-	//       check to later and could return Typ[Invalid] instead.
-	check.later(func() {
-		if _, ok := under(typ).(*TypeParam); ok {
-			check.error(x, _MisplacedTypeParam, "cannot embed a type parameter")
-		}
-	})
+	// We don't need this restriction anymore if we make the underlying type of a type
+	// parameter its constraint interface: if we embed a lone type parameter, we will
+	// simply use its underlying type (like we do for other named, embedded interfaces),
+	// and since the underlying type is an interface the embedding is well defined.
+	if isTypeParam(typ) {
+		check.error(x, _MisplacedTypeParam, "cannot embed a type parameter")
+		typ = Typ[Invalid]
+	}
 	return
 }