go/types, types2: move emode computation closer to use (cleanup)

[gostls13.git] / src / cmd / compile / internal / types2 / unify.go

// Copyright 2020 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.

// This file implements type unification.
//
// Type unification attempts to make two types x and y structurally
// equivalent by determining the types for a given list of (bound)
// type parameters which may occur within x and y. If x and y are
// structurally different (say []T vs chan T), or conflicting
// types are determined for type parameters, unification fails.
// If unification succeeds, as a side-effect, the types of the
// bound type parameters may be determined.
//
// Unification typically requires multiple calls u.unify(x, y) to
// a given unifier u, with various combinations of types x and y.
// In each call, additional type parameter types may be determined
// as a side effect and recorded in u.
// If a call fails (returns false), unification fails.
//
// In the unification context, structural equivalence of two types
// ignores the difference between a defined type and its underlying
// type if one type is a defined type and the other one is not.
// It also ignores the difference between an (external, unbound)
// type parameter and its core type.
// If two types are not structurally equivalent, they cannot be Go
// identical types. On the other hand, if they are structurally
// equivalent, they may be Go identical or at least assignable, or
// they may be in the type set of a constraint.
// Whether they indeed are identical or assignable is determined
// upon instantiation and function argument passing.

package types2

import (
        "bytes"
        "fmt"
        "sort"
        "strings"
)

const (
        // Upper limit for recursion depth. Used to catch infinite recursions
        // due to implementation issues (e.g., see issues go.dev/issue/48619, go.dev/issue/48656).
        unificationDepthLimit = 50

        // Whether to panic when unificationDepthLimit is reached.
        // If disabled, a recursion depth overflow results in a (quiet)
        // unification failure.
        panicAtUnificationDepthLimit = true

        // If enableCoreTypeUnification is set, unification will consider
        // the core types, if any, of non-local (unbound) type parameters.
        enableCoreTypeUnification = true

        // If traceInference is set, unification will print a trace of its operation.
        // Interpretation of trace:
        //   x ≡ y    attempt to unify types x and y
        //   p ➞ y    type parameter p is set to type y (p is inferred to be y)
        //   p ⇄ q    type parameters p and q match (p is inferred to be q and vice versa)
        //   x ≢ y    types x and y cannot be unified
        //   [p, q, ...] ➞ [x, y, ...]    mapping from type parameters to types
        traceInference = false
)

// A unifier maintains a list of type parameters and
// corresponding types inferred for each type parameter.
// A unifier is created by calling newUnifier.
type unifier struct {
        // handles maps each type parameter to its inferred type through
        // an indirection *Type called (inferred type) "handle".
        // Initially, each type parameter has its own, separate handle,
        // with a nil (i.e., not yet inferred) type.
        // After a type parameter P is unified with a type parameter Q,
        // P and Q share the same handle (and thus type). This ensures
        // that inferring the type for a given type parameter P will
        // automatically infer the same type for all other parameters
        // unified (joined) with P.
        handles map[*TypeParam]*Type
        depth   int // recursion depth during unification
}

// newUnifier returns a new unifier initialized with the given type parameter
// and corresponding type argument lists. The type argument list may be shorter
// than the type parameter list, and it may contain nil types. Matching type
// parameters and arguments must have the same index.
func newUnifier(tparams []*TypeParam, targs []Type) *unifier {
        assert(len(tparams) >= len(targs))
        handles := make(map[*TypeParam]*Type, len(tparams))
        // Allocate all handles up-front: in a correct program, all type parameters
        // must be resolved and thus eventually will get a handle.
        // Also, sharing of handles caused by unified type parameters is rare and
        // so it's ok to not optimize for that case (and delay handle allocation).
        for i, x := range tparams {
                var t Type
                if i < len(targs) {
                        t = targs[i]
                }
                handles[x] = &t
        }
        return &unifier{handles, 0}
}

// unifyMode controls the behavior of the unifier.
type unifyMode uint

const (
        // If assign is set, we are unifying types involved in an assignment:
        // they may match inexactly at the top, but element types must match
        // exactly.
        assign unifyMode = 1 << iota

        // If exact is set, types unify if they are identical (or can be
        // made identical with suitable arguments for type parameters).
        // Otherwise, a named type and a type literal unify if their
        // underlying types unify, channel directions are ignored, and
        // if there is an interface, the other type must implement the
        // interface.
        exact
)

func (m unifyMode) String() string {
        switch m {
        case 0:
                return "inexact"
        case assign:
                return "assign"
        case exact:
                return "exact"
        case assign | exact:
                return "assign, exact"
        }
        return fmt.Sprintf("mode %d", m)
}

// unify attempts to unify x and y and reports whether it succeeded.
// As a side-effect, types may be inferred for type parameters.
// The mode parameter controls how types are compared.
func (u *unifier) unify(x, y Type, mode unifyMode) bool {
        return u.nify(x, y, mode, nil)
}

func (u *unifier) tracef(format string, args ...interface{}) {
        fmt.Println(strings.Repeat(".  ", u.depth) + sprintf(nil, true, format, args...))
}

// String returns a string representation of the current mapping
// from type parameters to types.
func (u *unifier) String() string {
        // sort type parameters for reproducible strings
        tparams := make(typeParamsById, len(u.handles))
        i := 0
        for tpar := range u.handles {
                tparams[i] = tpar
                i++
        }
        sort.Sort(tparams)

        var buf bytes.Buffer
        w := newTypeWriter(&buf, nil)
        w.byte('[')
        for i, x := range tparams {
                if i > 0 {
                        w.string(", ")
                }
                w.typ(x)
                w.string(": ")
                w.typ(u.at(x))
        }
        w.byte(']')
        return buf.String()
}

type typeParamsById []*TypeParam

func (s typeParamsById) Len() int           { return len(s) }
func (s typeParamsById) Less(i, j int) bool { return s[i].id < s[j].id }
func (s typeParamsById) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

// join unifies the given type parameters x and y.
// If both type parameters already have a type associated with them
// and they are not joined, join fails and returns false.
func (u *unifier) join(x, y *TypeParam) bool {
        if traceInference {
                u.tracef("%s ⇄ %s", x, y)
        }
        switch hx, hy := u.handles[x], u.handles[y]; {
        case hx == hy:
                // Both type parameters already share the same handle. Nothing to do.
        case *hx != nil && *hy != nil:
                // Both type parameters have (possibly different) inferred types. Cannot join.
                return false
        case *hx != nil:
                // Only type parameter x has an inferred type. Use handle of x.
                u.setHandle(y, hx)
        // This case is treated like the default case.
        // case *hy != nil:
        //      // Only type parameter y has an inferred type. Use handle of y.
        //      u.setHandle(x, hy)
        default:
                // Neither type parameter has an inferred type. Use handle of y.
                u.setHandle(x, hy)
        }
        return true
}

// asTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u.
// Otherwise, the result is nil.
func (u *unifier) asTypeParam(x Type) *TypeParam {
        if x, _ := x.(*TypeParam); x != nil {
                if _, found := u.handles[x]; found {
                        return x
                }
        }
        return nil
}

// setHandle sets the handle for type parameter x
// (and all its joined type parameters) to h.
func (u *unifier) setHandle(x *TypeParam, h *Type) {
        hx := u.handles[x]
        assert(hx != nil)
        for y, hy := range u.handles {
                if hy == hx {
                        u.handles[y] = h
                }
        }
}

// at returns the (possibly nil) type for type parameter x.
func (u *unifier) at(x *TypeParam) Type {
        return *u.handles[x]
}

// set sets the type t for type parameter x;
// t must not be nil.
func (u *unifier) set(x *TypeParam, t Type) {
        assert(t != nil)
        if traceInference {
                u.tracef("%s ➞ %s", x, t)
        }
        *u.handles[x] = t
}

// unknowns returns the number of type parameters for which no type has been set yet.
func (u *unifier) unknowns() int {
        n := 0
        for _, h := range u.handles {
                if *h == nil {
                        n++
                }
        }
        return n
}

// inferred returns the list of inferred types for the given type parameter list.
// The result is never nil and has the same length as tparams; result types that
// could not be inferred are nil. Corresponding type parameters and result types
// have identical indices.
func (u *unifier) inferred(tparams []*TypeParam) []Type {
        list := make([]Type, len(tparams))
        for i, x := range tparams {
                list[i] = u.at(x)
        }
        return list
}

// asInterface returns the underlying type of x as an interface if
// it is a non-type parameter interface. Otherwise it returns nil.
func asInterface(x Type) (i *Interface) {
        if _, ok := x.(*TypeParam); !ok {
                i, _ = under(x).(*Interface)
        }
        return i
}

// nify implements the core unification algorithm which is an
// adapted version of Checker.identical. For changes to that
// code the corresponding changes should be made here.
// Must not be called directly from outside the unifier.
func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
        u.depth++
        if traceInference {
                u.tracef("%s ≡ %s\t// %s", x, y, mode)
        }
        defer func() {
                if traceInference && !result {
                        u.tracef("%s ≢ %s", x, y)
                }
                u.depth--
        }()

        // nothing to do if x == y
        if x == y {
                return true
        }

        // Stop gap for cases where unification fails.
        if u.depth > unificationDepthLimit {
                if traceInference {
                        u.tracef("depth %d >= %d", u.depth, unificationDepthLimit)
                }
                if panicAtUnificationDepthLimit {
                        panic("unification reached recursion depth limit")
                }
                return false
        }

        // Unification is symmetric, so we can swap the operands.
        // Ensure that if we have at least one
        // - defined type, make sure one is in y
        // - type parameter recorded with u, make sure one is in x
        if _, ok := x.(*Named); ok || u.asTypeParam(y) != nil {
                if traceInference {
                        u.tracef("%s ≡ %s\t// swap", y, x)
                }
                x, y = y, x
        }

        // Unification will fail if we match a defined type against a type literal.
        // If we are matching types in an assignment, at the top-level, types with
        // the same type structure are permitted as long as at least one of them
        // is not a defined type. To accommodate for that possibility, we continue
        // unification with the underlying type of a defined type if the other type
        // is a type literal. This is controlled by the exact unification mode.
        // We also continue if the other type is a basic type because basic types
        // are valid underlying types and may appear as core types of type constraints.
        // If we exclude them, inferred defined types for type parameters may not
        // match against the core types of their constraints (even though they might
        // correctly match against some of the types in the constraint's type set).
        // Finally, if unification (incorrectly) succeeds by matching the underlying
        // type of a defined type against a basic type (because we include basic types
        // as type literals here), and if that leads to an incorrectly inferred type,
        // we will fail at function instantiation or argument assignment time.
        //
        // If we have at least one defined type, there is one in y.
        if ny, _ := y.(*Named); mode&exact == 0 && ny != nil && isTypeLit(x) && !IsInterface(x) {
                if traceInference {
                        u.tracef("%s ≡ under %s", x, ny)
                }
                y = ny.under()
                // Per the spec, a defined type cannot have an underlying type
                // that is a type parameter.
                assert(!isTypeParam(y))
                // x and y may be identical now
                if x == y {
                        return true
                }
        }

        // Cases where at least one of x or y is a type parameter recorded with u.
        // If we have at least one type parameter, there is one in x.
        // If we have exactly one type parameter, because it is in x,
        // isTypeLit(x) is false and y was not changed above. In other
        // words, if y was a defined type, it is still a defined type
        // (relevant for the logic below).
        switch px, py := u.asTypeParam(x), u.asTypeParam(y); {
        case px != nil && py != nil:
                // both x and y are type parameters
                if u.join(px, py) {
                        return true
                }
                // both x and y have an inferred type - they must match
                return u.nify(u.at(px), u.at(py), mode, p)

        case px != nil:
                // x is a type parameter, y is not
                if x := u.at(px); x != nil {
                        // x has an inferred type which must match y
                        if u.nify(x, y, mode, p) {
                                // We have a match, possibly through underlying types.
                                xi := asInterface(x)
                                yi := asInterface(y)
                                _, xn := x.(*Named)
                                _, yn := y.(*Named)
                                // If we have two interfaces, what to do depends on
                                // whether they are named and their method sets.
                                if xi != nil && yi != nil {
                                        // Both types are interfaces.
                                        // If both types are defined types, they must be identical
                                        // because unification doesn't know which type has the "right" name.
                                        if xn && yn {
                                                return Identical(x, y)
                                        }
                                        // In all other cases, the method sets must match.
                                        // The types unified so we know that corresponding methods
                                        // match and we can simply compare the number of methods.
                                        // TODO(gri) We may be able to relax this rule and select
                                        // the more general interface. But if one of them is a defined
                                        // type, it's not clear how to choose and whether we introduce
                                        // an order dependency or not. Requiring the same method set
                                        // is conservative.
                                        if len(xi.typeSet().methods) != len(yi.typeSet().methods) {
                                                return false
                                        }
                                } else if xi != nil || yi != nil {
                                        // One but not both of them are interfaces.
                                        // In this case, either x or y could be viable matches for the corresponding
                                        // type parameter, which means choosing either introduces an order dependence.
                                        // Therefore, we must fail unification (go.dev/issue/60933).
                                        return false
                                }
                                // If y is a defined type, make sure we record that type
                                // for type parameter x, which may have until now only
                                // recorded an underlying type (go.dev/issue/43056).
                                // Either both types are interfaces, or neither type is.
                                // If both are interfaces, they have the same methods.
                                //
                                // Note: Changing the recorded type for a type parameter to
                                // a defined type is only ok when unification is inexact.
                                // But in exact unification, if we have a match, x and y must
                                // be identical, so changing the recorded type for x is a no-op.
                                if yn {
                                        u.set(px, y)
                                }
                                return true
                        }
                        return false
                }
                // otherwise, infer type from y
                u.set(px, y)
                return true
        }

        // x != y if we get here
        assert(x != y)

        // If we don't require exact unification and both types are interfaces,
        // one interface must have a subset of the methods of the other and
        // corresponding method signatures must unify.
        // If only one type is an interface, all its methods must be present in the
        // other type and corresponding method signatures must unify.
        if mode&exact == 0 {
                // One or both interfaces may be defined types.
                // Look under the name, but not under type parameters (go.dev/issue/60564).
                xi := asInterface(x)
                yi := asInterface(y)
                // If we have two interfaces, check the type terms for equivalence,
                // and unify common methods if possible.
                if xi != nil && yi != nil {
                        xset := xi.typeSet()
                        yset := yi.typeSet()
                        if xset.comparable != yset.comparable {
                                return false
                        }
                        // For now we require terms to be equal.
                        // We should be able to relax this as well, eventually.
                        if !xset.terms.equal(yset.terms) {
                                return false
                        }
                        // Interface types are the only types where cycles can occur
                        // that are not "terminated" via named types; and such cycles
                        // can only be created via method parameter types that are
                        // anonymous interfaces (directly or indirectly) embedding
                        // the current interface. Example:
                        //
                        //    type T interface {
                        //        m() interface{T}
                        //    }
                        //
                        // If two such (differently named) interfaces are compared,
                        // endless recursion occurs if the cycle is not detected.
                        //
                        // If x and y were compared before, they must be equal
                        // (if they were not, the recursion would have stopped);
                        // search the ifacePair stack for the same pair.
                        //
                        // This is a quadratic algorithm, but in practice these stacks
                        // are extremely short (bounded by the nesting depth of interface
                        // type declarations that recur via parameter types, an extremely
                        // rare occurrence). An alternative implementation might use a
                        // "visited" map, but that is probably less efficient overall.
                        q := &ifacePair{xi, yi, p}
                        for p != nil {
                                if p.identical(q) {
                                        return true // same pair was compared before
                                }
                                p = p.prev
                        }
                        // The method set of x must be a subset of the method set
                        // of y or vice versa, and the common methods must unify.
                        xmethods := xset.methods
                        ymethods := yset.methods
                        // The smaller method set must be the subset, if it exists.
                        if len(xmethods) > len(ymethods) {
                                xmethods, ymethods = ymethods, xmethods
                        }
                        // len(xmethods) <= len(ymethods)
                        // Collect the ymethods in a map for quick lookup.
                        ymap := make(map[string]*Func, len(ymethods))
                        for _, ym := range ymethods {
                                ymap[ym.Id()] = ym
                        }
                        // All xmethods must exist in ymethods and corresponding signatures must unify.
                        for _, xm := range xmethods {
                                if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
                                        return false
                                }
                        }
                        return true
                }

                // We don't have two interfaces. If we have one, make sure it's in xi.
                if yi != nil {
                        xi = yi
                        y = x
                }

                // If we have one interface, at a minimum each of the interface methods
                // must be implemented and thus unify with a corresponding method from
                // the non-interface type, otherwise unification fails.
                if xi != nil {
                        // All xi methods must exist in y and corresponding signatures must unify.
                        xmethods := xi.typeSet().methods
                        for _, xm := range xmethods {
                                obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name)
                                if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
                                        return false
                                }
                        }
                        return true
                }
        }

        // Unless we have exact unification, neither x nor y are interfaces now.
        // Except for unbound type parameters (see below), x and y must be structurally
        // equivalent to unify.

        // If we get here and x or y is a type parameter, they are unbound
        // (not recorded with the unifier).
        // Ensure that if we have at least one type parameter, it is in x
        // (the earlier swap checks for _recorded_ type parameters only).
        // This ensures that the switch switches on the type parameter.
        //
        // TODO(gri) Factor out type parameter handling from the switch.
        if isTypeParam(y) {
                if traceInference {
                        u.tracef("%s ≡ %s\t// swap", y, x)
                }
                x, y = y, x
        }

        // Type elements (array, slice, etc. elements) use emode for unification.
        // Element types must match exactly if the types are used in an assignment.
        emode := mode
        if mode&assign != 0 {
                emode |= exact
        }

        switch x := x.(type) {
        case *Basic:
                // Basic types are singletons except for the rune and byte
                // aliases, thus we cannot solely rely on the x == y check
                // above. See also comment in TypeName.IsAlias.
                if y, ok := y.(*Basic); ok {
                        return x.kind == y.kind
                }

        case *Array:
                // Two array types unify if they have the same array length
                // and their element types unify.
                if y, ok := y.(*Array); ok {
                        // If one or both array lengths are unknown (< 0) due to some error,
                        // assume they are the same to avoid spurious follow-on errors.
                        return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, emode, p)
                }

        case *Slice:
                // Two slice types unify if their element types unify.
                if y, ok := y.(*Slice); ok {
                        return u.nify(x.elem, y.elem, emode, p)
                }

        case *Struct:
                // Two struct types unify if they have the same sequence of fields,
                // and if corresponding fields have the same names, their (field) types unify,
                // and they have identical tags. Two embedded fields are considered to have the same
                // name. Lower-case field names from different packages are always different.
                if y, ok := y.(*Struct); ok {
                        if x.NumFields() == y.NumFields() {
                                for i, f := range x.fields {
                                        g := y.fields[i]
                                        if f.embedded != g.embedded ||
                                                x.Tag(i) != y.Tag(i) ||
                                                !f.sameId(g.pkg, g.name) ||
                                                !u.nify(f.typ, g.typ, emode, p) {
                                                return false
                                        }
                                }
                                return true
                        }
                }

        case *Pointer:
                // Two pointer types unify if their base types unify.
                if y, ok := y.(*Pointer); ok {
                        return u.nify(x.base, y.base, emode, p)
                }

        case *Tuple:
                // Two tuples types unify if they have the same number of elements
                // and the types of corresponding elements unify.
                if y, ok := y.(*Tuple); ok {
                        if x.Len() == y.Len() {
                                if x != nil {
                                        for i, v := range x.vars {
                                                w := y.vars[i]
                                                if !u.nify(v.typ, w.typ, mode, p) {
                                                        return false
                                                }
                                        }
                                }
                                return true
                        }
                }

        case *Signature:
                // Two function types unify if they have the same number of parameters
                // and result values, corresponding parameter and result types unify,
                // and either both functions are variadic or neither is.
                // Parameter and result names are not required to match.
                // TODO(gri) handle type parameters or document why we can ignore them.
                if y, ok := y.(*Signature); ok {
                        return x.variadic == y.variadic &&
                                u.nify(x.params, y.params, emode, p) &&
                                u.nify(x.results, y.results, emode, p)
                }

        case *Interface:
                assert(mode&exact != 0) // inexact unification is handled before this switch

                // Two interface types unify if they have the same set of methods with
                // the same names, and corresponding function types unify.
                // Lower-case method names from different packages are always different.
                // The order of the methods is irrelevant.
                if y, ok := y.(*Interface); ok {
                        xset := x.typeSet()
                        yset := y.typeSet()
                        if xset.comparable != yset.comparable {
                                return false
                        }
                        if !xset.terms.equal(yset.terms) {
                                return false
                        }
                        a := xset.methods
                        b := yset.methods
                        if len(a) == len(b) {
                                // Interface types are the only types where cycles can occur
                                // that are not "terminated" via named types; and such cycles
                                // can only be created via method parameter types that are
                                // anonymous interfaces (directly or indirectly) embedding
                                // the current interface. Example:
                                //
                                //    type T interface {
                                //        m() interface{T}
                                //    }
                                //
                                // If two such (differently named) interfaces are compared,
                                // endless recursion occurs if the cycle is not detected.
                                //
                                // If x and y were compared before, they must be equal
                                // (if they were not, the recursion would have stopped);
                                // search the ifacePair stack for the same pair.
                                //
                                // This is a quadratic algorithm, but in practice these stacks
                                // are extremely short (bounded by the nesting depth of interface
                                // type declarations that recur via parameter types, an extremely
                                // rare occurrence). An alternative implementation might use a
                                // "visited" map, but that is probably less efficient overall.
                                q := &ifacePair{x, y, p}
                                for p != nil {
                                        if p.identical(q) {
                                                return true // same pair was compared before
                                        }
                                        p = p.prev
                                }
                                if debug {
                                        assertSortedMethods(a)
                                        assertSortedMethods(b)
                                }
                                for i, f := range a {
                                        g := b[i]
                                        if f.Id() != g.Id() || !u.nify(f.typ, g.typ, exact, q) {
                                                return false
                                        }
                                }
                                return true
                        }
                }

        case *Map:
                // Two map types unify if their key and value types unify.
                if y, ok := y.(*Map); ok {
                        return u.nify(x.key, y.key, emode, p) && u.nify(x.elem, y.elem, emode, p)
                }

        case *Chan:
                // Two channel types unify if their value types unify
                // and if they have the same direction.
                // The channel direction is ignored for inexact unification.
                if y, ok := y.(*Chan); ok {
                        return (mode&exact == 0 || x.dir == y.dir) && u.nify(x.elem, y.elem, emode, p)
                }

        case *Named:
                // Two named types unify if their type names originate in the same type declaration.
                // If they are instantiated, their type argument lists must unify.
                if y, ok := y.(*Named); ok {
                        // Check type arguments before origins so they unify
                        // even if the origins don't match; for better error
                        // messages (see go.dev/issue/53692).
                        xargs := x.TypeArgs().list()
                        yargs := y.TypeArgs().list()
                        if len(xargs) != len(yargs) {
                                return false
                        }
                        for i, xarg := range xargs {
                                if !u.nify(xarg, yargs[i], mode, p) {
                                        return false
                                }
                        }
                        return identicalOrigin(x, y)
                }

        case *TypeParam:
                // x must be an unbound type parameter (see comment above).
                if debug {
                        assert(u.asTypeParam(x) == nil)
                }
                // By definition, a valid type argument must be in the type set of
                // the respective type constraint. Therefore, the type argument's
                // underlying type must be in the set of underlying types of that
                // constraint. If there is a single such underlying type, it's the
                // constraint's core type. It must match the type argument's under-
                // lying type, irrespective of whether the actual type argument,
                // which may be a defined type, is actually in the type set (that
                // will be determined at instantiation time).
                // Thus, if we have the core type of an unbound type parameter,
                // we know the structure of the possible types satisfying such
                // parameters. Use that core type for further unification
                // (see go.dev/issue/50755 for a test case).
                if enableCoreTypeUnification {
                        // Because the core type is always an underlying type,
                        // unification will take care of matching against a
                        // defined or literal type automatically.
                        // If y is also an unbound type parameter, we will end
                        // up here again with x and y swapped, so we don't
                        // need to take care of that case separately.
                        if cx := coreType(x); cx != nil {
                                if traceInference {
                                        u.tracef("core %s ≡ %s", x, y)
                                }
                                // If y is a defined type, it may not match against cx which
                                // is an underlying type (incl. int, string, etc.). Use assign
                                // mode here so that the unifier automatically takes under(y)
                                // if necessary.
                                return u.nify(cx, y, assign, p)
                        }
                }
                // x != y and there's nothing to do

        case nil:
                // avoid a crash in case of nil type

        default:
                panic(sprintf(nil, true, "u.nify(%s, %s, %d)", x, y, mode))
        }

        return false
}