go/types: clarify is/underIs semantics and implementation

This is a port of CL 358594 to go/types. Some code in conversions.go had
to be trivially reorganized to align with types2 -- I'm not sure how
go/types diverged from the base.

Change-Id: I40ce247bbb3b9d0e87ce88c50e440c12774c0745
Reviewed-on: https://go-review.googlesource.com/c/go/+/360475
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

7 files changed, 57 insertions, 30 deletions

diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index de7d7e6b5f..87c26775a6 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -843,7 +843,10 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		// Test if t satisfies the requirements for the argument
 		// type and collect possible result types at the same time.
 		var terms []*Term
-		if !tp.iface().typeSet().is(func(t *term) bool {
+		if !tp.is(func(t *term) bool {
+			if t == nil {
+				return false
+			}
 			if r := f(t.typ); r != nil {
 				terms = append(terms, NewTerm(t.tilde, r))
 				return true
diff --git a/src/go/types/conversions.go b/src/go/types/conversions.go
index fe62adbf10..8c8b63e23a 100644
--- a/src/go/types/conversions.go
+++ b/src/go/types/conversions.go
@@ -20,7 +20,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 	var cause string
 	switch {
 	case constArg && isConstType(T):
-		// constant conversion
+		// constant conversion (T cannot be a type parameter)
 		switch t := asBasic(T); {
 		case representableConst(x.val, check, t, &x.val):
 			ok = true
@@ -92,6 +92,16 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 		return true
 	}
 
+	// determine type parameter operands with specific type terms
+	Vp, _ := under(x.typ).(*TypeParam)
+	Tp, _ := under(T).(*TypeParam)
+	if Vp != nil && !Vp.hasTerms() {
+		Vp = nil
+	}
+	if Tp != nil && !Tp.hasTerms() {
+		Tp = nil
+	}
+
 	errorf := func(format string, args ...interface{}) {
 		if check != nil && cause != nil {
 			msg := check.sprintf(format, args...)
@@ -102,11 +112,7 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 		}
 	}
 
-	// TODO(gri) consider passing under(x.typ), under(T) into convertibleToImpl (optimization)
-	Vp, _ := under(x.typ).(*TypeParam)
-	Tp, _ := under(T).(*TypeParam)
-
-	// generic cases
+	// generic cases with specific type terms
 	// (generic operands cannot be constants, so we can ignore x.val)
 	switch {
 	case Vp != nil && Tp != nil:
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 8b26e64971..ef5958ba3f 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -142,6 +142,8 @@ var op2str2 = [...]string{
 	token.SHL: "shift",
 }
 
+// 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 {
diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index 43b9af348e..9302bd7f57 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -315,7 +315,7 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 			}
 		}
 		return tset.is(func(t *term) bool {
-			return w.isParameterized(t.typ)
+			return t != nil && w.isParameterized(t.typ)
 		})
 
 	case *Map:
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 855dac66aa..ef7d764201 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -258,6 +258,9 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 		if t, _ := under(T).(*TypeParam); t != nil {
 			return t.is(func(t *term) bool {
 				// TODO(gri) this could probably be more efficient
+				if t == nil {
+					return false
+				}
 				if t.tilde {
 					// TODO(gri) We need to check assignability
 					//           for the underlying type of x.
diff --git a/src/go/types/typeparam.go b/src/go/types/typeparam.go
index af36266f11..791e9db8f8 100644
--- a/src/go/types/typeparam.go
+++ b/src/go/types/typeparam.go
@@ -123,10 +123,21 @@ func (t *TypeParam) structuralType() Type {
 	return t.iface().typeSet().structuralType()
 }
 
+// hasTerms reports whether the type parameter constraint has specific type terms.
+func (t *TypeParam) hasTerms() bool {
+	return t.iface().typeSet().hasTerms()
+}
+
+// is calls f with the specific type terms of t's constraint and reports whether
+// all calls to f returned true. If there are no specific terms, is
+// returns the result of f(nil).
 func (t *TypeParam) is(f func(*term) bool) bool {
 	return t.iface().typeSet().is(f)
 }
 
+// underIs calls f with the underlying types of the specific type terms
+// of t's constraint and reports whether all calls to f returned true.
+// If there are no specific terms, underIs returns the result of f(nil).
 func (t *TypeParam) underIs(f func(Type) bool) bool {
 	return t.iface().typeSet().underIs(f)
 }
diff --git a/src/go/types/typeset.go b/src/go/types/typeset.go
index d6c4e5cd8c..215b48488f 100644
--- a/src/go/types/typeset.go
+++ b/src/go/types/typeset.go
@@ -37,7 +37,7 @@ func (s *_TypeSet) IsComparable() bool {
 		return s.comparable
 	}
 	return s.is(func(t *term) bool {
-		return Comparable(t.typ)
+		return t != nil && Comparable(t.typ)
 	})
 }
 
@@ -99,27 +99,29 @@ func (s *_TypeSet) String() string {
 // ----------------------------------------------------------------------------
 // Implementation
 
-func (s *_TypeSet) hasTerms() bool              { return !s.terms.isEmpty() && !s.terms.isAll() }
-func (s *_TypeSet) structuralType() Type        { return s.terms.structuralType() }
-func (s *_TypeSet) includes(t Type) bool        { return s.terms.includes(t) }
+// hasTerms reports whether the type set has specific type terms.
+func (s *_TypeSet) hasTerms() bool { return !s.terms.isEmpty() && !s.terms.isAll() }
+
+// structuralType returns the single type in s if there is exactly one; otherwise the result is nil.
+func (s *_TypeSet) structuralType() Type { return s.terms.structuralType() }
+
+// includes reports whether t ∈ s.
+func (s *_TypeSet) includes(t Type) bool { return s.terms.includes(t) }
+
+// subsetOf reports whether s1 ⊆ s2.
 func (s1 *_TypeSet) subsetOf(s2 *_TypeSet) bool { return s1.terms.subsetOf(s2.terms) }
 
 // TODO(gri) TypeSet.is and TypeSet.underIs should probably also go into termlist.go
 
-var topTerm = term{false, theTop}
-
+// is calls f with the specific type terms of s and reports whether
+// all calls to f returned true. If there are no specific terms, is
+// returns the result of f(nil).
 func (s *_TypeSet) is(f func(*term) bool) bool {
-	if len(s.terms) == 0 {
-		return false
+	if !s.hasTerms() {
+		return f(nil)
 	}
 	for _, t := range s.terms {
-		// Terms represent the top term with a nil type.
-		// The rest of the type checker uses the top type
-		// instead. Convert.
-		// TODO(gri) investigate if we can do without this
-		if t.typ == nil {
-			t = &topTerm
-		}
+		assert(t.typ != nil)
 		if !f(t) {
 			return false
 		}
@@ -127,17 +129,17 @@ func (s *_TypeSet) is(f func(*term) bool) bool {
 	return true
 }
 
+// underIs calls f with the underlying types of the specific type terms
+// of s and reports whether all calls to f returned true. If there are
+// no specific terms, is returns the result of f(nil).
 func (s *_TypeSet) underIs(f func(Type) bool) bool {
-	if len(s.terms) == 0 {
-		return false
+	if !s.hasTerms() {
+		return f(nil)
 	}
 	for _, t := range s.terms {
-		// see corresponding comment in TypeSet.is
+		assert(t.typ != nil)
+		// x == under(x) for ~x terms
 		u := t.typ
-		if u == nil {
-			u = theTop
-		}
-		// t == under(t) for ~t terms
 		if !t.tilde {
 			u = under(u)
 		}