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

// Copyright 2021 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 instantiation of generic types
// through substitution of type parameters by type arguments.

package types2

import (
	"cmd/compile/internal/syntax"
	"errors"
	"fmt"
)

// An Environment is an opaque type checking environment. It may be used to
// share identical type instances across type checked packages or calls to
// Instantiate.
type Environment struct {
	// For now, Environment just hides a Checker.
	// Eventually, we strive to remove the need for a checker.
	check *Checker
}

// NewEnvironment returns a new Environment, initialized with the given
// Checker, or nil.
func NewEnvironment(check *Checker) *Environment {
	return &Environment{check}
}

// Instantiate instantiates the type typ with the given type arguments targs.
// typ must be a *Named or a *Signature type, and its number of type parameters
// must match the number of provided type arguments. The result is a new,
// instantiated (not parameterized) type of the same kind (either a *Named or a
// *Signature). Any methods attached to a *Named are simply copied; they are
// not instantiated.
//
// If env is non-nil, it may be used to de-dupe the instance against previous
// instances with the same identity. This functionality is implemented for
// environments with non-nil Checkers.
//
// If verify is set and constraint satisfaction fails, the returned error may
// be of dynamic type ArgumentError indicating which type argument did not
// satisfy its corresponding type parameter constraint, and why.
//
// TODO(rfindley): change this function to also return an error if lengths of
// tparams and targs do not match.
func Instantiate(env *Environment, typ Type, targs []Type, validate bool) (Type, error) {
	var check *Checker
	if env != nil {
		check = env.check
	}
	inst := check.instance(nopos, typ, targs)

	var err error
	if validate {
		var tparams []*TypeParam
		switch t := typ.(type) {
		case *Named:
			tparams = t.TParams().list()
		case *Signature:
			tparams = t.TParams().list()
		}
		if i, err := check.verify(nopos, tparams, targs); err != nil {
			return inst, ArgumentError{i, err}
		}
	}

	return inst, err
}

// instantiate creates an instance and defers verification of constraints to
// later in the type checking pass. For Named types the resulting instance will
// be unexpanded.
func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, posList []syntax.Pos) (res Type) {
	assert(check != nil)
	if check.conf.Trace {
		check.trace(pos, "-- instantiating %s with %s", typ, NewTypeList(targs))
		check.indent++
		defer func() {
			check.indent--
			var under Type
			if res != nil {
				// Calling under() here may lead to endless instantiations.
				// Test case: type T[P any] T[P]
				// TODO(gri) investigate if that's a bug or to be expected.
				under = safeUnderlying(res)
			}
			check.trace(pos, "=> %s (under = %s)", res, under)
		}()
	}

	inst := check.instance(pos, typ, targs)

	assert(len(posList) <= len(targs))
	check.later(func() {
		// Collect tparams again because lazily loaded *Named types may not have
		// had tparams set up above.
		var tparams []*TypeParam
		switch t := typ.(type) {
		case *Named:
			tparams = t.TParams().list()
		case *Signature:
			tparams = t.TParams().list()
		}
		// Avoid duplicate errors; instantiate will have complained if tparams
		// and targs do not have the same length.
		if len(tparams) == len(targs) {
			if i, err := check.verify(pos, tparams, targs); err != nil {
				// best position for error reporting
				pos := pos
				if i < len(posList) {
					pos = posList[i]
				}
				check.softErrorf(pos, err.Error())
			}
		}
	})
	return inst
}

// instance creates a type or function instance using the given original type
// typ and arguments targs. For Named types the resulting instance will be
// unexpanded.
func (check *Checker) instance(pos syntax.Pos, typ Type, targs []Type) Type {
	switch t := typ.(type) {
	case *Named:
		h := instantiatedHash(t, targs)
		if check != nil {
			// typ may already have been instantiated with identical type arguments.
			// In that case, re-use the existing instance.
			if named := check.typMap[h]; named != nil {
				return named
			}
		}
		tname := NewTypeName(pos, t.obj.pkg, t.obj.name, nil)
		named := check.newNamed(tname, t, nil, nil, nil) // methods and tparams are set when named is loaded
		named.targs = NewTypeList(targs)
		named.instPos = &pos
		if check != nil {
			check.typMap[h] = named
		}
		return named

	case *Signature:
		tparams := t.TParams()
		if !check.validateTArgLen(pos, tparams.Len(), len(targs)) {
			return Typ[Invalid]
		}
		if tparams.Len() == 0 {
			return typ // nothing to do (minor optimization)
		}
		sig := check.subst(pos, typ, makeSubstMap(tparams.list(), targs), nil).(*Signature)
		// If the signature doesn't use its type parameters, subst
		// will not make a copy. In that case, make a copy now (so
		// we can set tparams to nil w/o causing side-effects).
		if sig == t {
			copy := *sig
			sig = &copy
		}
		// After instantiating a generic signature, it is not generic
		// anymore; we need to set tparams to nil.
		sig.tparams = nil
		return sig
	}

	// only types and functions can be generic
	panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
}

// validateTArgLen verifies that the length of targs and tparams matches,
// reporting an error if not. If validation fails and check is nil,
// validateTArgLen panics.
func (check *Checker) validateTArgLen(pos syntax.Pos, ntparams, ntargs int) bool {
	if ntargs != ntparams {
		// TODO(gri) provide better error message
		if check != nil {
			check.errorf(pos, "got %d arguments but %d type parameters", ntargs, ntparams)
			return false
		}
		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, ntargs, ntparams))
	}
	return true
}

func (check *Checker) verify(pos syntax.Pos, tparams []*TypeParam, targs []Type) (int, error) {
	smap := makeSubstMap(tparams, targs)
	for i, tpar := range tparams {
		// stop checking bounds after the first failure
		if err := check.satisfies(pos, targs[i], tpar, smap); err != nil {
			return i, err
		}
	}
	return -1, nil
}

// satisfies reports whether the type argument targ satisfies the constraint of type parameter
// parameter tpar (after any of its type parameters have been substituted through smap).
// A suitable error is reported if the result is false.
// TODO(gri) This should be a method of interfaces or type sets.
func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap substMap) error {
	iface := tpar.iface()
	if iface.Empty() {
		return nil // no type bound
	}

	// TODO(rfindley): it would be great if users could pass in a qualifier here,
	// rather than falling back to verbose qualification. Maybe this can be part
	// of a the shared environment.
	var qf Qualifier
	if check != nil {
		qf = check.qualifier
	}
	errorf := func(format string, args ...interface{}) error {
		return errors.New(sprintf(qf, format, args...))
	}

	// The type parameter bound is parameterized with the same type parameters
	// as the instantiated type; before we can use it for bounds checking we
	// need to instantiate it with the type arguments with which we instantiate
	// the parameterized type.
	iface = check.subst(pos, iface, smap, nil).(*Interface)

	// 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() {
			return errorf("%s has no constraints", targ)
		}
		return errorf("%s does not satisfy comparable", targ)
	}

	// targ must implement iface (methods)
	// - check only if we have methods
	if iface.NumMethods() > 0 {
		// 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 {
			return errorf("%s has no methods", targ)
		}
		if m, wrong := check.missingMethod(targ, iface, true); m != nil {
			// TODO(gri) needs to print updated name to avoid major confusion in error message!
			//           (print warning for now)
			// Old warning:
			// check.softErrorf(pos, "%s does not satisfy %s (warning: name not updated) = %s (missing method %s)", targ, tpar.bound, iface, m)
			if wrong != nil {
				// TODO(gri) This can still report uninstantiated types which makes the error message
				//           more difficult to read then necessary.
				return errorf("%s does not satisfy %s: wrong method signature\n\tgot  %s\n\twant %s",
					targ, tpar.bound, wrong, m,
				)
			}
			return errorf("%s does not satisfy %s (missing method %s)", targ, tpar.bound, m.name)
		}
	}

	// targ's underlying type must also be one of the interface types listed, if any
	if !iface.typeSet().hasTerms() {
		return nil // nothing to do
	}

	// If targ is itself a type parameter, each of its possible types, but at least one, must be in the
	// list of iface types (i.e., the targ type list must be a non-empty subset of the iface types).
	if targ := asTypeParam(targ); targ != nil {
		targBound := targ.iface()
		if !targBound.typeSet().hasTerms() {
			return errorf("%s does not satisfy %s (%s has no type constraints)", targ, tpar.bound, targ)
		}
		if !targBound.typeSet().subsetOf(iface.typeSet()) {
			// TODO(gri) need better error message
			return errorf("%s does not satisfy %s", targ, tpar.bound)
		}
		return nil
	}

	// Otherwise, targ's type or underlying type must also be one of the interface types listed, if any.
	if !iface.typeSet().includes(targ) {
		// TODO(gri) better error message
		return errorf("%s does not satisfy %s", targ, tpar.bound)
	}

	return nil
}