[dev.typeparams] go/types: disallow type list handling

This is a port of CL 324571 to go/types, though type list handling is
guarded by a const rather than a config option.

Change-Id: I91c940fead048980603e0bb56fcc896dbef4f94c
Reviewed-on: https://go-review.googlesource.com/c/go/+/326683
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api.go

@@ -34,6 +34,8 @@ import (
 	"go/token"
 )
 
+const allowTypeLists = false
+
 // An Error describes a type-checking error; it implements the error interface.
 // A "soft" error is an error that still permits a valid interpretation of a
 // package (such as "unused variable"); "hard" errors may lead to unpredictable

src/go/types/api_test.go

@@ -349,7 +349,7 @@ func TestTypesInfo(t *testing.T) {
 		{genericPkg + `g0; type t[P any] int; var x struct{ f t[int] }; var _ = x.f`, `x.f`, `generic_g0.t[int]`},
 
 		// issue 45096
-		{genericPkg + `issue45096; func _[T interface{ type int8, int16, int32  }](x T) { _ = x < 0 }`, `0`, `T₁`},
+		{genericPkg + `issue45096; func _[T interface{ ~int8 | ~int16 | ~int32  }](x T) { _ = x < 0 }`, `0`, `T₁`},
 	}
 
 	for _, test := range tests {

src/go/types/api_typeparams_test.go

@@ -45,38 +45,38 @@ func TestInferredInfo(t *testing.T) {
 			`func(float64, *byte, ...[]byte)`,
 		},
 
-		{genericPkg + `s1; func f[T any, P interface{type *T}](x T); func _(x string) { f(x) }`,
+		{genericPkg + `s1; func f[T any, P interface{~*T}](x T); func _(x string) { f(x) }`,
 			`f`,
 			[]string{`string`, `*string`},
 			`func(x string)`,
 		},
-		{genericPkg + `s2; func f[T any, P interface{type *T}](x []T); func _(x []int) { f(x) }`,
+		{genericPkg + `s2; func f[T any, P interface{~*T}](x []T); func _(x []int) { f(x) }`,
 			`f`,
 			[]string{`int`, `*int`},
 			`func(x []int)`,
 		},
-		{genericPkg + `s3; type C[T any] interface{type chan<- T}; func f[T any, P C[T]](x []T); func _(x []int) { f(x) }`,
+		{genericPkg + `s3; type C[T any] interface{~chan<- T}; func f[T any, P C[T]](x []T); func _(x []int) { f(x) }`,
 			`f`,
 			[]string{`int`, `chan<- int`},
 			`func(x []int)`,
 		},
-		{genericPkg + `s4; type C[T any] interface{type chan<- T}; func f[T any, P C[T], Q C[[]*P]](x []T); func _(x []int) { f(x) }`,
+		{genericPkg + `s4; type C[T any] interface{~chan<- T}; func f[T any, P C[T], Q C[[]*P]](x []T); func _(x []int) { f(x) }`,
 			`f`,
 			[]string{`int`, `chan<- int`, `chan<- []*chan<- int`},
 			`func(x []int)`,
 		},
 
-		{genericPkg + `t1; func f[T any, P interface{type *T}]() T; func _() { _ = f[string] }`,
+		{genericPkg + `t1; func f[T any, P interface{~*T}]() T; func _() { _ = f[string] }`,
 			`f`,
 			[]string{`string`, `*string`},
 			`func() string`,
 		},
-		{genericPkg + `t2; type C[T any] interface{type chan<- T}; func f[T any, P C[T]]() []T; func _() { _ = f[int] }`,
+		{genericPkg + `t2; type C[T any] interface{~chan<- T}; func f[T any, P C[T]]() []T; func _() { _ = f[int] }`,
 			`f`,
 			[]string{`int`, `chan<- int`},
 			`func() []int`,
 		},
-		{genericPkg + `t3; type C[T any] interface{type chan<- T}; func f[T any, P C[T], Q C[[]*P]]() []T; func _() { _ = f[int] }`,
+		{genericPkg + `t3; type C[T any] interface{~chan<- T}; func f[T any, P C[T], Q C[[]*P]]() []T; func _() { _ = f[int] }`,
 			`f`,
 			[]string{`int`, `chan<- int`, `chan<- []*chan<- int`},
 			`func() []int`,

src/go/types/interface.go

@@ -34,7 +34,13 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 			continue // ignore
 		}
 
+		// TODO(rfindley) Remove type list handling once the parser doesn't accept type lists anymore.
 		if name.Name == "type" {
+			// Report an error for the first type list per interface
+			// if we don't allow type lists, but continue.
+			if !allowTypeLists && tlist == nil {
+				check.softErrorf(name, _Todo, "use generalized embedding syntax instead of a type list")
+			}
 			// For now, collect all type list entries as if it
 			// were a single union, where each union element is
 			// of the form ~T.
@@ -43,7 +49,9 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 			op.Op = token.TILDE
 			op.X = f.Type
 			tlist = append(tlist, op)
-			if tname != nil && tname != name {
+			// Report an error if we have multiple type lists in an
+			// interface, but only if they are permitted in the first place.
+			if allowTypeLists && tname != nil && tname != name {
 				check.errorf(name, _Todo, "cannot have multiple type lists in an interface")
 			}
 			tname = name

src/go/types/testdata/examples/constraints.go2

@@ -6,6 +6,18 @@
 
 package p
 
+type (
+	// Type lists are processed as unions but an error is reported.
+	// TODO(gri) remove this once the parser doesn't accept type lists anymore.
+	_ interface{
+		type /* ERROR use generalized embedding syntax instead of a type list */ int
+	}
+	_ interface{
+		type /* ERROR use generalized embedding syntax instead of a type list */ int
+		type float32
+	}
+)
+
 type (
 	// Arbitrary types may be embedded like interfaces.
 	_ interface{int}

src/go/types/typestring_test.go

@@ -95,7 +95,6 @@ var independentTestTypes = []testEntry{
 	dup("interface{}"),
 	dup("interface{m()}"),
 	dup(`interface{String() string; m(int) float32}`),
-	{"interface{type int, float32, complex128}", "interface{~int|~float32|~complex128}"},
 	dup("interface{int|float32|complex128}"),
 	dup("interface{int|~float32|~complex128}"),