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go/types, types2: remove code for infer1 

Fixes #58283.

Change-Id: I4a82083cddfed1b1be7776464f926a4c69a35e10
Reviewed-on: https://go-review.googlesource.com/c/go/+/470995
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
This commit is contained in:
Robert Griesemer 2023-02-23 18:04:54 -08:00 committed by Gopher Robot
parent 969c3ba839
commit d81ae7cfc7
4 changed files with 4 additions and 852 deletions
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398
src/cmd/compile/internal/types2/infer.go
View File

@ -9,210 +9,9 @@ package types2
import (
"cmd/compile/internal/syntax"
"fmt"
. "internal/types/errors"
"strings"
)
// infer1 is an implementation of infer.
// Inference proceeds as follows. Starting with given type arguments:
//
// 1. apply FTI (function type inference) with typed arguments,
// 2. apply CTI (constraint type inference),
// 3. apply FTI with untyped function arguments,
// 4. apply CTI.
//
// The process stops as soon as all type arguments are known or an error occurs.
func (check *Checker) infer1(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand, silent bool) (result []Type) {
if debug {
defer func() {
assert(result == nil || len(result) == len(tparams))
for _, targ := range result {
assert(targ != nil)
}
//check.dump("### inferred targs = %s", result)
}()
}
if traceInference {
check.dump("-- inferA %s%s ➞ %s", tparams, params, targs)
defer func() {
check.dump("=> inferA %s ➞ %s", tparams, result)
}()
}
// There must be at least one type parameter, and no more type arguments than type parameters.
n := len(tparams)
assert(n > 0 && len(targs) <= n)
// Function parameters and arguments must match in number.
assert(params.Len() == len(args))
// If we already have all type arguments, we're done.
if len(targs) == n {
return targs
}
// len(targs) < n
// Rename type parameters to avoid conflicts in recursive instantiation scenarios.
tparams, params = check.renameTParams(pos, tparams, params)
// --- 1 ---
// Continue with the type arguments we have. Avoid matching generic
// parameters that already have type arguments against function arguments:
// It may fail because matching uses type identity while parameter passing
// uses assignment rules. Instantiate the parameter list with the type
// arguments we have, and continue with that parameter list.
// First, make sure we have a "full" list of type arguments, some of which
// may be nil (unknown). Make a copy so as to not clobber the incoming slice.
if len(targs) < n {
targs2 := make([]Type, n)
copy(targs2, targs)
targs = targs2
}
// len(targs) == n
// Substitute type arguments for their respective type parameters in params,
// if any. Note that nil targs entries are ignored by check.subst.
// TODO(gri) Can we avoid this (we're setting known type arguments below,
// but that doesn't impact the isParameterized check for now).
if params.Len() > 0 {
smap := makeSubstMap(tparams, targs)
params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)
}
// Unify parameter and argument types for generic parameters with typed arguments
// and collect the indices of generic parameters with untyped arguments.
// Terminology: generic parameter = function parameter with a type-parameterized type
u := newUnifier(tparams, targs)
errorf := func(kind string, tpar, targ Type, arg *operand) {
if silent {
return
}
// provide a better error message if we can
targs := u.inferred(tparams)
if targs[0] == nil {
// The first type parameter couldn't be inferred.
// If none of them could be inferred, don't try
// to provide the inferred type in the error msg.
allFailed := true
for _, targ := range targs {
if targ != nil {
allFailed = false
break
}
}
if allFailed {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeParamsString(tparams))
return
}
}
smap := makeSubstMap(tparams, targs)
// TODO(gri): pass a poser here, rather than arg.Pos().
inferred := check.subst(arg.Pos(), tpar, smap, nil, check.context())
// CannotInferTypeArgs indicates a failure of inference, though the actual
// error may be better attributed to a user-provided type argument (hence
// InvalidTypeArg). We can't differentiate these cases, so fall back on
// the more general CannotInferTypeArgs.
if inferred != tpar {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
} else {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
}
}
// indices of the generic parameters with untyped arguments - save for later
var indices []int
for i, arg := range args {
par := params.At(i)
// If we permit bidirectional unification, this conditional code needs to be
// executed even if par.typ is not parameterized since the argument may be a
// generic function (for which we want to infer its type arguments).
if isParameterized(tparams, par.typ) {
if arg.mode == invalid {
// An error was reported earlier. Ignore this targ
// and continue, we may still be able to infer all
// targs resulting in fewer follow-on errors.
continue
}
if targ := arg.typ; isTyped(targ) {
// If we permit bidirectional unification, and targ is
// a generic function, we need to initialize u.y with
// the respective type parameters of targ.
if !u.unify(par.typ, targ) {
errorf("type", par.typ, targ, arg)
return nil
}
} else if _, ok := par.typ.(*TypeParam); ok {
// Since default types are all basic (i.e., non-composite) types, an
// untyped argument will never match a composite parameter type; the
// only parameter type it can possibly match against is a *TypeParam.
// Thus, for untyped arguments we only need to look at parameter types
// that are single type parameters.
indices = append(indices, i)
}
}
}
// If we've got all type arguments, we're done.
targs = u.inferred(tparams)
if u.unknowns() == 0 {
return targs
}
// --- 2 ---
// See how far we get with constraint type inference.
// Note that even if we don't have any type arguments, constraint type inference
// may produce results for constraints that explicitly specify a type.
targs, index := check.inferB(tparams, targs)
if targs == nil || index < 0 {
return targs
}
// --- 3 ---
// Use any untyped arguments to infer additional type arguments.
// Some generic parameters with untyped arguments may have been given
// a type by now, we can ignore them.
for _, i := range indices {
tpar := params.At(i).typ.(*TypeParam) // is type parameter by construction of indices
// Only consider untyped arguments for which the corresponding type
// parameter doesn't have an inferred type yet.
if targs[tpar.index] == nil {
arg := args[i]
targ := Default(arg.typ)
// The default type for an untyped nil is untyped nil. We must not
// infer an untyped nil type as type parameter type. Ignore untyped
// nil by making sure all default argument types are typed.
if isTyped(targ) && !u.unify(tpar, targ) {
errorf("default type", tpar, targ, arg)
return nil
}
}
}
// If we've got all type arguments, we're done.
targs = u.inferred(tparams)
if u.unknowns() == 0 {
return targs
}
// --- 4 ---
// Again, follow up with constraint type inference.
targs, index = check.inferB(tparams, targs)
if targs == nil || index < 0 {
return targs
}
// At least one type argument couldn't be inferred.
assert(targs != nil && index >= 0 && targs[index] == nil)
tpar := tparams[index]
if !silent {
check.errorf(pos, CannotInferTypeArgs, "cannot infer %s (%s)", tpar.obj.name, tpar.obj.pos)
}
return nil
}
// renameTParams renames the type parameters in a function signature described by its
// type and ordinary parameters (tparams and params) such that each type parameter is
// given a new identity. renameTParams returns the new type and ordinary parameters.
@ -388,203 +187,6 @@ func (w *tpWalker) isParameterizedTypeList(list []Type) bool {
return false
}
// inferB returns the list of actual type arguments inferred from the type parameters'
// bounds and an initial set of type arguments. If type inference is impossible because
// unification fails, an error is reported if report is set to true, the resulting types
// list is nil, and index is 0.
// Otherwise, types is the list of inferred type arguments, and index is the index of the
// first type argument in that list that couldn't be inferred (and thus is nil). If all
// type arguments were inferred successfully, index is < 0. The number of type arguments
// provided may be less than the number of type parameters, but there must be at least one.
func (check *Checker) inferB(tparams []*TypeParam, targs []Type) (types []Type, index int) {
assert(len(tparams) >= len(targs) && len(targs) > 0)
if traceInference {
check.dump("-- inferB %s ➞ %s", tparams, targs)
defer func() {
check.dump("=> inferB %s ➞ %s", tparams, types)
}()
}
// Unify type parameters with their constraints.
u := newUnifier(tparams, targs)
// Repeatedly apply constraint type inference as long as
// there are still unknown type arguments and progress is
// being made.
//
// This is an O(n^2) algorithm where n is the number of
// type parameters: if there is progress (and iteration
// continues), at least one type argument is inferred
// per iteration and we have a doubly nested loop.
// In practice this is not a problem because the number
// of type parameters tends to be very small (< 5 or so).
// (It should be possible for unification to efficiently
// signal newly inferred type arguments; then the loops
// here could handle the respective type parameters only,
// but that will come at a cost of extra complexity which
// may not be worth it.)
for n := u.unknowns(); n > 0; {
nn := n
for _, tpar := range tparams {
// If there is a core term (i.e., a core type with tilde information)
// unify the type parameter with the core type.
if core, single := coreTerm(tpar); core != nil {
if traceInference {
u.tracef("core(%s) = %s (single = %v)", tpar, core, single)
}
// A type parameter can be unified with its core type in two cases.
tx := u.at(tpar)
switch {
case tx != nil:
// The corresponding type argument tx is known.
// In this case, if the core type has a tilde, the type argument's underlying
// type must match the core type, otherwise the type argument and the core type
// must match.
// If tx is an external type parameter, don't consider its underlying type
// (which is an interface). Core type unification will attempt to unify against
// core.typ.
// Note also that even with inexact unification we cannot leave away the under
// call here because it's possible that both tx and core.typ are named types,
// with under(tx) being a (named) basic type matching core.typ. Such cases do
// not match with inexact unification.
if core.tilde && !isTypeParam(tx) {
tx = under(tx)
}
// Unification may fail because it operates with limited information (core type),
// even if a given type argument satisfies the corresponding type constraint.
// For instance, given [P T1|T2, ...] where the type argument for P is (named
// type) T1, and T1 and T2 have the same built-in (named) type T0 as underlying
// type, the core type will be the named type T0, which doesn't match T1.
// Yet the instantiation of P with T1 is clearly valid (see go.dev/issue/53650).
// Reporting an error if unification fails would be incorrect in this case.
// On the other hand, it is safe to ignore failing unification during constraint
// type inference because if the failure is true, an error will be reported when
// checking instantiation.
u.unify(tx, core.typ)
case single && !core.tilde:
// The corresponding type argument tx is unknown and there's a single
// specific type and no tilde.
// In this case the type argument must be that single type; set it.
u.set(tpar, core.typ)
default:
// Unification is not possible and no progress was made.
continue
}
// The number of known type arguments may have changed.
nn = u.unknowns()
if nn == 0 {
break // all type arguments are known
}
} else {
if traceInference {
u.tracef("core(%s) = nil", tpar)
}
}
}
assert(nn <= n)
if nn == n {
break // no progress
}
n = nn
}
// u.inferred(tparams) now contains the incoming type arguments plus any additional type
// arguments which were inferred from core terms. The newly inferred non-nil
// entries may still contain references to other type parameters.
// For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
// was given, unification produced the type list [int, []C, *A]. We eliminate the
// remaining type parameters by substituting the type parameters in this type list
// until nothing changes anymore.
types = u.inferred(tparams)
if debug {
for i, targ := range targs {
assert(targ == nil || types[i] == targ)
}
}
// The data structure of each (provided or inferred) type represents a graph, where
// each node corresponds to a type and each (directed) vertex points to a component
// type. The substitution process described above repeatedly replaces type parameter
// nodes in these graphs with the graphs of the types the type parameters stand for,
// which creates a new (possibly bigger) graph for each type.
// The substitution process will not stop if the replacement graph for a type parameter
// also contains that type parameter.
// For instance, for [A interface{ *A }], without any type argument provided for A,
// unification produces the type list [*A]. Substituting A in *A with the value for
// A will lead to infinite expansion by producing [**A], [****A], [********A], etc.,
// because the graph A -> *A has a cycle through A.
// Generally, cycles may occur across multiple type parameters and inferred types
// (for instance, consider [P interface{ *Q }, Q interface{ func(P) }]).
// We eliminate cycles by walking the graphs for all type parameters. If a cycle
// through a type parameter is detected, cycleFinder nils out the respective type
// which kills the cycle; this also means that the respective type could not be
// inferred.
//
// TODO(gri) If useful, we could report the respective cycle as an error. We don't
// do this now because type inference will fail anyway, and furthermore,
// constraints with cycles of this kind cannot currently be satisfied by
// any user-supplied type. But should that change, reporting an error
// would be wrong.
w := cycleFinder{tparams, types, make(map[Type]bool)}
for _, t := range tparams {
w.typ(t) // t != nil
}
// dirty tracks the indices of all types that may still contain type parameters.
// We know that nil type entries and entries corresponding to provided (non-nil)
// type arguments are clean, so exclude them from the start.
var dirty []int
for i, typ := range types {
if typ != nil && (i >= len(targs) || targs[i] == nil) {
dirty = append(dirty, i)
}
}
for len(dirty) > 0 {
// TODO(gri) Instead of creating a new substMap for each iteration,
// provide an update operation for substMaps and only change when
// needed. Optimization.
smap := makeSubstMap(tparams, types)
n := 0
for _, index := range dirty {
t0 := types[index]
if t1 := check.subst(nopos, t0, smap, nil, check.context()); t1 != t0 {
types[index] = t1
dirty[n] = index
n++
}
}
dirty = dirty[:n]
}
// Once nothing changes anymore, we may still have type parameters left;
// e.g., a constraint with core type *P may match a type parameter Q but
// we don't have any type arguments to fill in for *P or Q (go.dev/issue/45548).
// Don't let such inferences escape, instead nil them out.
for i, typ := range types {
if typ != nil && isParameterized(tparams, typ) {
types[i] = nil
}
}
// update index
index = -1
for i, typ := range types {
if typ == nil {
index = i
break
}
}
return
}
// If the type parameter has a single specific type S, coreTerm returns (S, true).
// Otherwise, if tpar has a core type T, it returns a term corresponding to that
// core type and false. In that case, if any term of tpar has a tilde, the core

30
src/cmd/compile/internal/types2/infer2.go
View File

@ -11,39 +11,13 @@ import (
. "internal/types/errors"
)
// If compareWithInfer1, infer2 results must match infer1 results.
// Disable before releasing Go 1.21.
const compareWithInfer1 = false
// infer attempts to infer the complete set of type arguments for generic function instantiation/call
// based on the given type parameters tparams, type arguments targs, function parameters params, and
// function arguments args, if any. There must be at least one type parameter, no more type arguments
// than type parameters, and params and args must match in number (incl. zero).
// If successful, infer returns the complete list of given and inferred type arguments, one for each
// type parameter. Otherwise the result is nil and appropriate errors will be reported.
func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) []Type {
r2 := check.infer2(pos, tparams, targs, params, args)
if compareWithInfer1 {
r1 := check.infer1(pos, tparams, targs, params, args, r2 == nil) // be silent on errors if infer2 failed
assert(len(r2) == len(r1))
for i, targ2 := range r2 {
targ1 := r1[i]
var c comparer
c.ignoreInvalids = true
if !c.identical(targ2, targ1, nil) {
tpar := tparams[i]
check.dump("%v: type argument for %s: infer1: %s, infer2: %s", tpar.Obj().Pos(), tpar, targ1, targ2)
panic("inconsistent type inference")
}
}
}
return r2
}
// infer2 is an implementation of infer.
func (check *Checker) infer2(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
if debug {
defer func() {
assert(inferred == nil || len(inferred) == len(tparams))
@ -54,7 +28,7 @@ func (check *Checker) infer2(pos syntax.Pos, tparams []*TypeParam, targs []Type,
}
if traceInference {
check.dump("-- infer2 %s%s ➞ %s", tparams, params, targs)
check.dump("-- infer %s%s ➞ %s", tparams, params, targs)
defer func() {
check.dump("=> %s ➞ %s\n", tparams, inferred)
}()

398
src/go/types/infer.go
View File

@ -11,210 +11,9 @@ package types
import (
"fmt"
"go/token"
. "internal/types/errors"
"strings"
)
// infer1 is an implementation of infer.
// Inference proceeds as follows. Starting with given type arguments:
//
// 1. apply FTI (function type inference) with typed arguments,
// 2. apply CTI (constraint type inference),
// 3. apply FTI with untyped function arguments,
// 4. apply CTI.
//
// The process stops as soon as all type arguments are known or an error occurs.
func (check *Checker) infer1(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand, silent bool) (result []Type) {
if debug {
defer func() {
assert(result == nil || len(result) == len(tparams))
for _, targ := range result {
assert(targ != nil)
}
//check.dump("### inferred targs = %s", result)
}()
}
if traceInference {
check.dump("-- inferA %s%s ➞ %s", tparams, params, targs)
defer func() {
check.dump("=> inferA %s ➞ %s", tparams, result)
}()
}
// There must be at least one type parameter, and no more type arguments than type parameters.
n := len(tparams)
assert(n > 0 && len(targs) <= n)
// Function parameters and arguments must match in number.
assert(params.Len() == len(args))
// If we already have all type arguments, we're done.
if len(targs) == n {
return targs
}
// len(targs) < n
// Rename type parameters to avoid conflicts in recursive instantiation scenarios.
tparams, params = check.renameTParams(posn.Pos(), tparams, params)
// --- 1 ---
// Continue with the type arguments we have. Avoid matching generic
// parameters that already have type arguments against function arguments:
// It may fail because matching uses type identity while parameter passing
// uses assignment rules. Instantiate the parameter list with the type
// arguments we have, and continue with that parameter list.
// First, make sure we have a "full" list of type arguments, some of which
// may be nil (unknown). Make a copy so as to not clobber the incoming slice.
if len(targs) < n {
targs2 := make([]Type, n)
copy(targs2, targs)
targs = targs2
}
// len(targs) == n
// Substitute type arguments for their respective type parameters in params,
// if any. Note that nil targs entries are ignored by check.subst.
// TODO(gri) Can we avoid this (we're setting known type arguments below,
// but that doesn't impact the isParameterized check for now).
if params.Len() > 0 {
smap := makeSubstMap(tparams, targs)
params = check.subst(nopos, params, smap, nil, check.context()).(*Tuple)
}
// Unify parameter and argument types for generic parameters with typed arguments
// and collect the indices of generic parameters with untyped arguments.
// Terminology: generic parameter = function parameter with a type-parameterized type
u := newUnifier(tparams, targs)
errorf := func(kind string, tpar, targ Type, arg *operand) {
if silent {
return
}
// provide a better error message if we can
targs := u.inferred(tparams)
if targs[0] == nil {
// The first type parameter couldn't be inferred.
// If none of them could be inferred, don't try
// to provide the inferred type in the error msg.
allFailed := true
for _, targ := range targs {
if targ != nil {
allFailed = false
break
}
}
if allFailed {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeParamsString(tparams))
return
}
}
smap := makeSubstMap(tparams, targs)
// TODO(gri): pass a poser here, rather than arg.Pos().
inferred := check.subst(arg.Pos(), tpar, smap, nil, check.context())
// CannotInferTypeArgs indicates a failure of inference, though the actual
// error may be better attributed to a user-provided type argument (hence
// InvalidTypeArg). We can't differentiate these cases, so fall back on
// the more general CannotInferTypeArgs.
if inferred != tpar {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
} else {
check.errorf(arg, CannotInferTypeArgs, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
}
}
// indices of the generic parameters with untyped arguments - save for later
var indices []int
for i, arg := range args {
par := params.At(i)
// If we permit bidirectional unification, this conditional code needs to be
// executed even if par.typ is not parameterized since the argument may be a
// generic function (for which we want to infer its type arguments).
if isParameterized(tparams, par.typ) {
if arg.mode == invalid {
// An error was reported earlier. Ignore this targ
// and continue, we may still be able to infer all
// targs resulting in fewer follow-on errors.
continue
}
if targ := arg.typ; isTyped(targ) {
// If we permit bidirectional unification, and targ is
// a generic function, we need to initialize u.y with
// the respective type parameters of targ.
if !u.unify(par.typ, targ) {
errorf("type", par.typ, targ, arg)
return nil
}
} else if _, ok := par.typ.(*TypeParam); ok {
// Since default types are all basic (i.e., non-composite) types, an
// untyped argument will never match a composite parameter type; the
// only parameter type it can possibly match against is a *TypeParam.
// Thus, for untyped arguments we only need to look at parameter types
// that are single type parameters.
indices = append(indices, i)
}
}
}
// If we've got all type arguments, we're done.
targs = u.inferred(tparams)
if u.unknowns() == 0 {
return targs
}
// --- 2 ---
// See how far we get with constraint type inference.
// Note that even if we don't have any type arguments, constraint type inference
// may produce results for constraints that explicitly specify a type.
targs, index := check.inferB(tparams, targs)
if targs == nil || index < 0 {
return targs
}
// --- 3 ---
// Use any untyped arguments to infer additional type arguments.
// Some generic parameters with untyped arguments may have been given
// a type by now, we can ignore them.
for _, i := range indices {
tpar := params.At(i).typ.(*TypeParam) // is type parameter by construction of indices
// Only consider untyped arguments for which the corresponding type
// parameter doesn't have an inferred type yet.
if targs[tpar.index] == nil {
arg := args[i]
targ := Default(arg.typ)
// The default type for an untyped nil is untyped nil. We must not
// infer an untyped nil type as type parameter type. Ignore untyped
// nil by making sure all default argument types are typed.
if isTyped(targ) && !u.unify(tpar, targ) {
errorf("default type", tpar, targ, arg)
return nil
}
}
}
// If we've got all type arguments, we're done.
targs = u.inferred(tparams)
if u.unknowns() == 0 {
return targs
}
// --- 4 ---
// Again, follow up with constraint type inference.
targs, index = check.inferB(tparams, targs)
if targs == nil || index < 0 {
return targs
}
// At least one type argument couldn't be inferred.
assert(targs != nil && index >= 0 && targs[index] == nil)
tpar := tparams[index]
if !silent {
check.errorf(posn, CannotInferTypeArgs, "cannot infer %s (%s)", tpar.obj.name, tpar.obj.pos)
}
return nil
}
// renameTParams renames the type parameters in a function signature described by its
// type and ordinary parameters (tparams and params) such that each type parameter is
// given a new identity. renameTParams returns the new type and ordinary parameters.
@ -390,203 +189,6 @@ func (w *tpWalker) isParameterizedTypeList(list []Type) bool {
return false
}
// inferB returns the list of actual type arguments inferred from the type parameters'
// bounds and an initial set of type arguments. If type inference is impossible because
// unification fails, an error is reported if report is set to true, the resulting types
// list is nil, and index is 0.
// Otherwise, types is the list of inferred type arguments, and index is the index of the
// first type argument in that list that couldn't be inferred (and thus is nil). If all
// type arguments were inferred successfully, index is < 0. The number of type arguments
// provided may be less than the number of type parameters, but there must be at least one.
func (check *Checker) inferB(tparams []*TypeParam, targs []Type) (types []Type, index int) {
assert(len(tparams) >= len(targs) && len(targs) > 0)
if traceInference {
check.dump("-- inferB %s ➞ %s", tparams, targs)
defer func() {
check.dump("=> inferB %s ➞ %s", tparams, types)
}()
}
// Unify type parameters with their constraints.
u := newUnifier(tparams, targs)
// Repeatedly apply constraint type inference as long as
// there are still unknown type arguments and progress is
// being made.
//
// This is an O(n^2) algorithm where n is the number of
// type parameters: if there is progress (and iteration
// continues), at least one type argument is inferred
// per iteration and we have a doubly nested loop.
// In practice this is not a problem because the number
// of type parameters tends to be very small (< 5 or so).
// (It should be possible for unification to efficiently
// signal newly inferred type arguments; then the loops
// here could handle the respective type parameters only,
// but that will come at a cost of extra complexity which
// may not be worth it.)
for n := u.unknowns(); n > 0; {
nn := n
for _, tpar := range tparams {
// If there is a core term (i.e., a core type with tilde information)
// unify the type parameter with the core type.
if core, single := coreTerm(tpar); core != nil {
if traceInference {
u.tracef("core(%s) = %s (single = %v)", tpar, core, single)
}
// A type parameter can be unified with its core type in two cases.
tx := u.at(tpar)
switch {
case tx != nil:
// The corresponding type argument tx is known.
// In this case, if the core type has a tilde, the type argument's underlying
// type must match the core type, otherwise the type argument and the core type
// must match.
// If tx is an external type parameter, don't consider its underlying type
// (which is an interface). Core type unification will attempt to unify against
// core.typ.
// Note also that even with inexact unification we cannot leave away the under
// call here because it's possible that both tx and core.typ are named types,
// with under(tx) being a (named) basic type matching core.typ. Such cases do
// not match with inexact unification.
if core.tilde && !isTypeParam(tx) {
tx = under(tx)
}
// Unification may fail because it operates with limited information (core type),
// even if a given type argument satisfies the corresponding type constraint.
// For instance, given [P T1|T2, ...] where the type argument for P is (named
// type) T1, and T1 and T2 have the same built-in (named) type T0 as underlying
// type, the core type will be the named type T0, which doesn't match T1.
// Yet the instantiation of P with T1 is clearly valid (see go.dev/issue/53650).
// Reporting an error if unification fails would be incorrect in this case.
// On the other hand, it is safe to ignore failing unification during constraint
// type inference because if the failure is true, an error will be reported when
// checking instantiation.
u.unify(tx, core.typ)
case single && !core.tilde:
// The corresponding type argument tx is unknown and there's a single
// specific type and no tilde.
// In this case the type argument must be that single type; set it.
u.set(tpar, core.typ)
default:
// Unification is not possible and no progress was made.
continue
}
// The number of known type arguments may have changed.
nn = u.unknowns()
if nn == 0 {
break // all type arguments are known
}
} else {
if traceInference {
u.tracef("core(%s) = nil", tpar)
}
}
}
assert(nn <= n)
if nn == n {
break // no progress
}
n = nn
}
// u.inferred(tparams) now contains the incoming type arguments plus any additional type
// arguments which were inferred from core terms. The newly inferred non-nil
// entries may still contain references to other type parameters.
// For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
// was given, unification produced the type list [int, []C, *A]. We eliminate the
// remaining type parameters by substituting the type parameters in this type list
// until nothing changes anymore.
types = u.inferred(tparams)
if debug {
for i, targ := range targs {
assert(targ == nil || types[i] == targ)
}
}
// The data structure of each (provided or inferred) type represents a graph, where
// each node corresponds to a type and each (directed) vertex points to a component
// type. The substitution process described above repeatedly replaces type parameter
// nodes in these graphs with the graphs of the types the type parameters stand for,
// which creates a new (possibly bigger) graph for each type.
// The substitution process will not stop if the replacement graph for a type parameter
// also contains that type parameter.
// For instance, for [A interface{ *A }], without any type argument provided for A,
// unification produces the type list [*A]. Substituting A in *A with the value for
// A will lead to infinite expansion by producing [**A], [****A], [********A], etc.,
// because the graph A -> *A has a cycle through A.
// Generally, cycles may occur across multiple type parameters and inferred types
// (for instance, consider [P interface{ *Q }, Q interface{ func(P) }]).
// We eliminate cycles by walking the graphs for all type parameters. If a cycle
// through a type parameter is detected, cycleFinder nils out the respective type
// which kills the cycle; this also means that the respective type could not be
// inferred.
//
// TODO(gri) If useful, we could report the respective cycle as an error. We don't
// do this now because type inference will fail anyway, and furthermore,
// constraints with cycles of this kind cannot currently be satisfied by
// any user-supplied type. But should that change, reporting an error
// would be wrong.
w := cycleFinder{tparams, types, make(map[Type]bool)}
for _, t := range tparams {
w.typ(t) // t != nil
}
// dirty tracks the indices of all types that may still contain type parameters.
// We know that nil type entries and entries corresponding to provided (non-nil)
// type arguments are clean, so exclude them from the start.
var dirty []int
for i, typ := range types {
if typ != nil && (i >= len(targs) || targs[i] == nil) {
dirty = append(dirty, i)
}
}
for len(dirty) > 0 {
// TODO(gri) Instead of creating a new substMap for each iteration,
// provide an update operation for substMaps and only change when
// needed. Optimization.
smap := makeSubstMap(tparams, types)
n := 0
for _, index := range dirty {
t0 := types[index]
if t1 := check.subst(nopos, t0, smap, nil, check.context()); t1 != t0 {
types[index] = t1
dirty[n] = index
n++
}
}
dirty = dirty[:n]
}
// Once nothing changes anymore, we may still have type parameters left;
// e.g., a constraint with core type *P may match a type parameter Q but
// we don't have any type arguments to fill in for *P or Q (go.dev/issue/45548).
// Don't let such inferences escape, instead nil them out.
for i, typ := range types {
if typ != nil && isParameterized(tparams, typ) {
types[i] = nil
}
}
// update index
index = -1
for i, typ := range types {
if typ == nil {
index = i
break
}
}
return
}
// If the type parameter has a single specific type S, coreTerm returns (S, true).
// Otherwise, if tpar has a core type T, it returns a term corresponding to that
// core type and false. In that case, if any term of tpar has a tilde, the core

30
src/go/types/infer2.go
View File

@ -13,39 +13,13 @@ import (
. "internal/types/errors"
)
// If compareWithInfer1, infer2 results must match infer1 results.
// Disable before releasing Go 1.21.
const compareWithInfer1 = false
// infer attempts to infer the complete set of type arguments for generic function instantiation/call
// based on the given type parameters tparams, type arguments targs, function parameters params, and
// function arguments args, if any. There must be at least one type parameter, no more type arguments
// than type parameters, and params and args must match in number (incl. zero).
// If successful, infer returns the complete list of given and inferred type arguments, one for each
// type parameter. Otherwise the result is nil and appropriate errors will be reported.
func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) []Type {
r2 := check.infer2(posn, tparams, targs, params, args)
if compareWithInfer1 {
r1 := check.infer1(posn, tparams, targs, params, args, r2 == nil) // be silent on errors if infer2 failed
assert(len(r2) == len(r1))
for i, targ2 := range r2 {
targ1 := r1[i]
var c comparer
c.ignoreInvalids = true
if !c.identical(targ2, targ1, nil) {
tpar := tparams[i]
check.dump("%v: type argument for %s: infer1: %s, infer2: %s", tpar.Obj().Pos(), tpar, targ1, targ2)
panic("inconsistent type inference")
}
}
}
return r2
}
// infer2 is an implementation of infer.
func (check *Checker) infer2(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type, params *Tuple, args []*operand) (inferred []Type) {
if debug {
defer func() {
assert(inferred == nil || len(inferred) == len(tparams))
@ -56,7 +30,7 @@ func (check *Checker) infer2(posn positioner, tparams []*TypeParam, targs []Type
}
if traceInference {
check.dump("-- infer2 %s%s ➞ %s", tparams, params, targs)
check.dump("-- infer %s%s ➞ %s", tparams, params, targs)
defer func() {
check.dump("=> %s ➞ %s\n", tparams, inferred)
}()