[release-branch.go1.21] go/types, types2: disable interface inference for versions before Go 1.21

Change the internal constant enableInterfaceInference to a unifier
field that can be controlled dynamically and set it for Go 1.21
or later.

This restores Go 1.20 unification behavior for interfaces.

Fixes #61930.

Change-Id: Iefd6c0899811f8208a8be9cef2650a07787ae177
Reviewed-on: https://go-review.googlesource.com/c/go/+/519855
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/520601

6 files changed, 48 insertions, 28 deletions

diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index 44d66eb516..5eb916c528 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -96,7 +96,7 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 	// 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)
+	u := newUnifier(tparams, targs, check.allowVersion(check.pkg, pos, go1_21))
 
 	errorf := func(kind string, tpar, targ Type, arg *operand) {
 		// provide a better error message if we can
diff --git a/src/cmd/compile/internal/types2/unify.go b/src/cmd/compile/internal/types2/unify.go
index eddd808cb7..4f04132353 100644
--- a/src/cmd/compile/internal/types2/unify.go
+++ b/src/cmd/compile/internal/types2/unify.go
@@ -53,11 +53,6 @@ const (
 	// the core types, if any, of non-local (unbound) type parameters.
 	enableCoreTypeUnification = true
 
-	// If enableInterfaceInference is set, type inference uses
-	// shared methods for improved type inference involving
-	// interfaces.
-	enableInterfaceInference = 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
@@ -81,15 +76,16 @@ type unifier struct {
 	// 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
+	handles                  map[*TypeParam]*Type
+	depth                    int  // recursion depth during unification
+	enableInterfaceInference bool // use shared methods for better inference
 }
 
 // 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 {
+func newUnifier(tparams []*TypeParam, targs []Type, enableInterfaceInference bool) *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
@@ -103,7 +99,7 @@ func newUnifier(tparams []*TypeParam, targs []Type) *unifier {
 		}
 		handles[x] = &t
 	}
-	return &unifier{handles, 0}
+	return &unifier{handles, 0, enableInterfaceInference}
 }
 
 // unifyMode controls the behavior of the unifier.
@@ -339,7 +335,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	// 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) && !(enableInterfaceInference && IsInterface(x)) {
+	if ny, _ := y.(*Named); mode&exact == 0 && ny != nil && isTypeLit(x) && !(u.enableInterfaceInference && IsInterface(x)) {
 		if traceInference {
 			u.tracef("%s ≡ under %s", x, ny)
 		}
@@ -437,12 +433,12 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		emode |= exact
 	}
 
-	// If EnableInterfaceInference is set and we don't require exact unification,
+	// If u.EnableInterfaceInference is set and we don't require exact unification,
 	// if 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 enableInterfaceInference && mode&exact == 0 {
+	if u.enableInterfaceInference && 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)
@@ -632,7 +628,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		}
 
 	case *Interface:
-		assert(!enableInterfaceInference || mode&exact != 0) // handled before this switch
+		assert(!u.enableInterfaceInference || mode&exact != 0) // 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.
diff --git a/src/go/types/generate_test.go b/src/go/types/generate_test.go
index 75fda025ee..2578cbbd15 100644
--- a/src/go/types/generate_test.go
+++ b/src/go/types/generate_test.go
@@ -242,6 +242,14 @@ func fixInferSig(f *ast.File) {
 						n.Args[0] = arg
 						return false
 					}
+				case "allowVersion":
+					// rewrite check.allowVersion(..., pos, ...) to check.allowVersion(..., posn, ...)
+					if ident, _ := n.Args[1].(*ast.Ident); ident != nil && ident.Name == "pos" {
+						pos := n.Args[1].Pos()
+						arg := newIdent(pos, "posn")
+						n.Args[1] = arg
+						return false
+					}
 				}
 			}
 		}
diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index 7c7898435b..cb7634415b 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -98,7 +98,7 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 	// 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)
+	u := newUnifier(tparams, targs, check.allowVersion(check.pkg, posn, go1_21))
 
 	errorf := func(kind string, tpar, targ Type, arg *operand) {
 		// provide a better error message if we can
diff --git a/src/go/types/unify.go b/src/go/types/unify.go
index 56b258ae35..646eb3e339 100644
--- a/src/go/types/unify.go
+++ b/src/go/types/unify.go
@@ -55,11 +55,6 @@ const (
 	// the core types, if any, of non-local (unbound) type parameters.
 	enableCoreTypeUnification = true
 
-	// If enableInterfaceInference is set, type inference uses
-	// shared methods for improved type inference involving
-	// interfaces.
-	enableInterfaceInference = 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
@@ -83,15 +78,16 @@ type unifier struct {
 	// 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
+	handles                  map[*TypeParam]*Type
+	depth                    int  // recursion depth during unification
+	enableInterfaceInference bool // use shared methods for better inference
 }
 
 // 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 {
+func newUnifier(tparams []*TypeParam, targs []Type, enableInterfaceInference bool) *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
@@ -105,7 +101,7 @@ func newUnifier(tparams []*TypeParam, targs []Type) *unifier {
 		}
 		handles[x] = &t
 	}
-	return &unifier{handles, 0}
+	return &unifier{handles, 0, enableInterfaceInference}
 }
 
 // unifyMode controls the behavior of the unifier.
@@ -341,7 +337,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	// 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) && !(enableInterfaceInference && IsInterface(x)) {
+	if ny, _ := y.(*Named); mode&exact == 0 && ny != nil && isTypeLit(x) && !(u.enableInterfaceInference && IsInterface(x)) {
 		if traceInference {
 			u.tracef("%s ≡ under %s", x, ny)
 		}
@@ -439,12 +435,12 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		emode |= exact
 	}
 
-	// If EnableInterfaceInference is set and we don't require exact unification,
+	// If u.EnableInterfaceInference is set and we don't require exact unification,
 	// if 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 enableInterfaceInference && mode&exact == 0 {
+	if u.enableInterfaceInference && 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)
@@ -634,7 +630,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		}
 
 	case *Interface:
-		assert(!enableInterfaceInference || mode&exact != 0) // handled before this switch
+		assert(!u.enableInterfaceInference || mode&exact != 0) // 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.
diff --git a/src/internal/types/testdata/fixedbugs/issue61903.go b/src/internal/types/testdata/fixedbugs/issue61903.go
new file mode 100644
index 0000000000..8a6fcd9529
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue61903.go
@@ -0,0 +1,20 @@
+// -lang=go1.20
+
+// Copyright 2023 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.
+
+package p
+
+type T[P any] interface{}
+
+func f1[P any](T[P])    {}
+func f2[P any](T[P], P) {}
+
+func _() {
+	var t T[int]
+	f1(t)
+
+	var s string
+	f2(t, s /* ERROR "type string of s does not match inferred type int for P" */)
+}