diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 6c129cd..75cebc9 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -1243,11 +1243,9 @@
 		{newDefined(new(Struct)), new(Struct), true},
 		{newDefined(Typ[Int]), new(Struct), false},
 		{Typ[UntypedInt], Typ[Int], true},
-		// TODO (rFindley): the below behavior is undefined as non-constant untyped
-		// string values are not permitted by the spec. But we should consider
-		// changing this case to return 'true', to have more reasonable behavior in
-		// cases where the API is used for constant expressions.
-		{Typ[UntypedString], Typ[String], false},
+		// Untyped string values are not permitted by the spec, so the below
+		// behavior is undefined.
+		{Typ[UntypedString], Typ[String], true},
 	} {
 		if got := ConvertibleTo(test.v, test.t); got != test.want {
 			t.Errorf("ConvertibleTo(%v, %v) = %t, want %t", test.v, test.t, got, test.want)
@@ -1266,13 +1264,11 @@
 		{newDefined(new(Struct)), new(Struct), true},
 		{Typ[UntypedBool], Typ[Bool], true},
 		{Typ[UntypedString], Typ[Bool], false},
-		// TODO (rFindley): the below behavior is undefined as AssignableTo is
-		// intended for non-constant values (and neither UntypedString or
-		// UntypedInt assignments arise during normal type checking).  But as
-		// described in TestConvertibleTo above, we should consider changing this
-		// behavior.
-		{Typ[UntypedString], Typ[String], false},
-		{Typ[UntypedInt], Typ[Int], false},
+		// Neither untyped string nor untyped numeric assignments arise during
+		// normal type checking, so the below behavior is technically undefined by
+		// the spec.
+		{Typ[UntypedString], Typ[String], true},
+		{Typ[UntypedInt], Typ[Int], true},
 	} {
 		if got := AssignableTo(test.v, test.t); got != test.want {
 			t.Errorf("AssignableTo(%v, %v) = %t, want %t", test.v, test.t, got, test.want)
diff --git a/src/go/types/assignments.go b/src/go/types/assignments.go
index 9697e50..4e8ec27 100644
--- a/src/go/types/assignments.go
+++ b/src/go/types/assignments.go
@@ -34,8 +34,8 @@
 		// spec: "If an untyped constant is assigned to a variable of interface
 		// type or the blank identifier, the constant is first converted to type
 		// bool, rune, int, float64, complex128 or string respectively, depending
-		// on whether the value is a boolean, rune, integer, floating-point, complex,
-		// or string constant."
+		// on whether the value is a boolean, rune, integer, floating-point,
+		// complex, or string constant."
 		if T == nil || IsInterface(T) {
 			if T == nil && x.typ == Typ[UntypedNil] {
 				check.errorf(x.pos(), "use of untyped nil in %s", context)
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 8503a52..94d98f0 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -506,8 +506,6 @@
 	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
 		return nil
 	}
-	// TODO(gri) Sloppy code - clean up. This function is central
-	//           to assignment and expression checking.
 
 	if isUntyped(target) {
 		// both x and target are untyped
@@ -519,80 +517,91 @@
 				check.updateExprType(x.expr, target, false)
 			}
 		} else if xkind != tkind {
-			goto Error
+			return check.newErrorf(x.pos(), "cannot convert %s to %s", x, target)
 		}
 		return nil
 	}
 
-	// typed target
+	if t, ok := target.Underlying().(*Basic); ok && x.mode == constant_ {
+		if err := check.isRepresentable(x, t); err != nil {
+			return err
+		}
+		// Expression value may have been rounded - update if needed.
+		check.updateExprVal(x.expr, x.val)
+	} else {
+		newTarget := check.implicitType(x, target)
+		if newTarget == nil {
+			return check.newErrorf(x.pos(), "cannot convert %s to %s", x, target)
+		}
+		target = newTarget
+	}
+	x.typ = target
+	// Even though implicitType can return UntypedNil, this value is final: the
+	// predeclared identifier nil has no type.
+	check.updateExprType(x.expr, target, true)
+	return nil
+}
+
+// implicitType returns the implicit type of x when used in a context where the
+// target type is expected. If no such implicit conversion is possible, it
+// returns nil.
+func (check *Checker) implicitType(x *operand, target Type) Type {
+	assert(isUntyped(x.typ))
 	switch t := target.Underlying().(type) {
 	case *Basic:
-		if x.mode == constant_ {
-			if err := check.isRepresentable(x, t); err != nil {
-				return err
+		assert(x.mode != constant_)
+		// Non-constant untyped values may appear as the
+		// result of comparisons (untyped bool), intermediate
+		// (delayed-checked) rhs operands of shifts, and as
+		// the value nil.
+		switch x.typ.(*Basic).kind {
+		case UntypedBool:
+			if !isBoolean(target) {
+				return nil
 			}
-			// expression value may have been rounded - update if needed
-			check.updateExprVal(x.expr, x.val)
-		} else {
-			// Non-constant untyped values may appear as the
-			// result of comparisons (untyped bool), intermediate
-			// (delayed-checked) rhs operands of shifts, and as
-			// the value nil.
-			switch x.typ.(*Basic).kind {
-			case UntypedBool:
-				if !isBoolean(target) {
-					goto Error
-				}
-			case UntypedInt, UntypedRune, UntypedFloat, UntypedComplex:
-				if !isNumeric(target) {
-					goto Error
-				}
-			case UntypedString:
-				// Non-constant untyped string values are not
-				// permitted by the spec and should not occur.
-				unreachable()
-			case UntypedNil:
-				// Unsafe.Pointer is a basic type that includes nil.
-				if !hasNil(target) {
-					goto Error
-				}
-			default:
-				goto Error
+		case UntypedInt, UntypedRune, UntypedFloat, UntypedComplex:
+			if !isNumeric(target) {
+				return nil
 			}
+		case UntypedString:
+			// Non-constant untyped string values are not permitted by the spec and
+			// should not occur during normal typechecking passes, but this path is
+			// reachable via the AssignableTo API.
+			if !isString(target) {
+				return nil
+			}
+		case UntypedNil:
+			// Unsafe.Pointer is a basic type that includes nil.
+			if !hasNil(target) {
+				return nil
+			}
+		default:
+			return nil
 		}
 	case *Interface:
-		// Update operand types to the default type rather then
-		// the target (interface) type: values must have concrete
-		// dynamic types. If the value is nil, keep it untyped
-		// (this is important for tools such as go vet which need
-		// the dynamic type for argument checking of say, print
+		// Values must have concrete dynamic types. If the value is nil,
+		// keep it untyped (this is important for tools such as go vet which
+		// need the dynamic type for argument checking of say, print
 		// functions)
 		if x.isNil() {
-			target = Typ[UntypedNil]
-		} else {
-			// cannot assign untyped values to non-empty interfaces
-			check.completeInterface(t)
-			if !t.Empty() {
-				goto Error
-			}
-			target = Default(x.typ)
+			return Typ[UntypedNil]
 		}
+		// cannot assign untyped values to non-empty interfaces
+		check.completeInterface(t)
+		if !t.Empty() {
+			return nil
+		}
+		return Default(x.typ)
 	case *Pointer, *Signature, *Slice, *Map, *Chan:
 		if !x.isNil() {
-			goto Error
+			return nil
 		}
-		// keep nil untyped - see comment for interfaces, above
-		target = Typ[UntypedNil]
+		// Keep nil untyped - see comment for interfaces, above.
+		return Typ[UntypedNil]
 	default:
-		goto Error
+		return nil
 	}
-
-	x.typ = target
-	check.updateExprType(x.expr, target, true) // UntypedNils are final
-	return nil
-
-Error:
-	return check.newErrorf(x.pos(), "cannot convert %s to %s", x, target)
+	return target
 }
 
 func (check *Checker) comparison(x, y *operand, op token.Token) {
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 80d11e2..6fbfe09 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -205,15 +205,11 @@
 	return x.mode == value && x.typ == Typ[UntypedNil]
 }
 
-// TODO(gri) The functions operand.assignableTo, checker.convertUntyped,
-//           checker.representable, and checker.assignment are
-//           overlapping in functionality. Need to simplify and clean up.
-
-// assignableTo reports whether x is assignable to a variable of type T.
-// If the result is false and a non-nil reason is provided, it may be set
-// to a more detailed explanation of the failure (result != "").
-// The check parameter may be nil if assignableTo is invoked through
-// an exported API call, i.e., when all methods have been type-checked.
+// assignableTo reports whether x is assignable to a variable of type T. If the
+// result is false and a non-nil reason is provided, it may be set to a more
+// detailed explanation of the failure (result != ""). The check parameter may
+// be nil if assignableTo is invoked through an exported API call, i.e., when
+// all methods have been type-checked.
 func (x *operand) assignableTo(check *Checker, T Type, reason *string) bool {
 	if x.mode == invalid || T == Typ[Invalid] {
 		return true // avoid spurious errors
@@ -229,34 +225,17 @@
 	Vu := V.Underlying()
 	Tu := T.Underlying()
 
-	// x is an untyped value representable by a value of type T
-	// TODO(gri) This is borrowing from checker.convertUntyped and
-	//           checker.representable. Need to clean up.
+	// x is an untyped value representable by a value of type T.
 	if isUntyped(Vu) {
-		switch t := Tu.(type) {
-		case *Basic:
-			if x.isNil() && t.kind == UnsafePointer {
-				return true
-			}
-			if x.mode == constant_ {
-				return representableConst(x.val, check, t, nil)
-			}
-			// The result of a comparison is an untyped boolean,
-			// but may not be a constant.
-			if Vb, _ := Vu.(*Basic); Vb != nil {
-				return Vb.kind == UntypedBool && isBoolean(Tu)
-			}
-		case *Interface:
-			check.completeInterface(t)
-			return x.isNil() || t.Empty()
-		case *Pointer, *Signature, *Slice, *Map, *Chan:
-			return x.isNil()
+		if t, ok := Tu.(*Basic); ok && x.mode == constant_ {
+			return representableConst(x.val, check, t, nil)
 		}
+		return check.implicitType(x, Tu) != nil
 	}
 	// Vu is typed
 
-	// x's type V and T have identical underlying types
-	// and at least one of V or T is not a named type
+	// x's type V and T have identical underlying types and at least one of V or
+	// T is not a named type.
 	if check.identical(Vu, Tu) && (!isNamed(V) || !isNamed(T)) {
 		return true
 	}