cmd/compile/internal/types2: disallow lone type parameter on RHS of type declaration

We may revisit this decision in a future release. By disallowing this
for Go 1.18 we are ensuring that we don't lock in the generics design
in a place that may need to change later. (Type declarations are the
primary construct where it crucially matters what the underlying type
of a type parameter is.)

Comment out all tests that rely on this feature; add comments referring
to issue so we can find all places easily should we change our minds.

Fixes #45639.

Change-Id: I730510e4da66d3716d455a9071c7778a1e4a1152
Reviewed-on: https://go-review.googlesource.com/c/go/+/359177
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

13 files changed, 143 insertions, 145 deletions

diff --git a/src/cmd/compile/internal/types2/api_test.go b/src/cmd/compile/internal/types2/api_test.go
index ecb5a29830..26652f3e74 100644
--- a/src/cmd/compile/internal/types2/api_test.go
+++ b/src/cmd/compile/internal/types2/api_test.go
@@ -622,13 +622,6 @@ func TestDefsInfo(t *testing.T) {
 		{`package p3; type x int`, `x`, `type p3.x int`},
 		{`package p4; func f()`, `f`, `func p4.f()`},
 		{`package p5; func f() int { x, _ := 1, 2; return x }`, `_`, `var _ int`},
-
-		// generic types must be sanitized
-		// (need to use sufficiently nested types to provoke unexpanded types)
-		{genericPkg + `g0; type t[P any] P; const x = t[int](42)`, `x`, `const generic_g0.x generic_g0.t[int]`},
-		{genericPkg + `g1; type t[P any] P; var x = t[int](42)`, `x`, `var generic_g1.x generic_g1.t[int]`},
-		{genericPkg + `g2; type t[P any] P; type x struct{ f t[int] }`, `x`, `type generic_g2.x struct{f generic_g2.t[int]}`},
-		{genericPkg + `g3; type t[P any] P; func f(x struct{ f t[string] }); var g = f`, `g`, `var generic_g3.g func(x struct{f generic_g3.t[string]})`},
 	}
 
 	for _, test := range tests {
@@ -667,13 +660,6 @@ func TestUsesInfo(t *testing.T) {
 		{`package p2; func _() { _ = x }; var x int`, `x`, `var p2.x int`},
 		{`package p3; func _() { type _ x }; type x int`, `x`, `type p3.x int`},
 		{`package p4; func _() { _ = f }; func f()`, `f`, `func p4.f()`},
-
-		// generic types must be sanitized
-		// (need to use sufficiently nested types to provoke unexpanded types)
-		{genericPkg + `g0; func _() { _ = x }; type t[P any] P; const x = t[int](42)`, `x`, `const generic_g0.x generic_g0.t[int]`},
-		{genericPkg + `g1; func _() { _ = x }; type t[P any] P; var x = t[int](42)`, `x`, `var generic_g1.x generic_g1.t[int]`},
-		{genericPkg + `g2; func _() { type _ x }; type t[P any] P; type x struct{ f t[int] }`, `x`, `type generic_g2.x struct{f generic_g2.t[int]}`},
-		{genericPkg + `g3; func _() { _ = f }; type t[P any] P; func f(x struct{ f t[string] })`, `f`, `func generic_g3.f(x struct{f generic_g3.t[string]})`},
 	}
 
 	for _, test := range tests {
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index 63be4b3223..5d2a6c531b 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -604,9 +604,12 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		named.underlying = Typ[Invalid]
 	}
 
-	// If the RHS is a type parameter, it must be from this type declaration.
-	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.TypeParams().list(), tpar) < 0 {
-		check.errorf(tdecl.Type, "cannot use function type parameter %s as RHS in type declaration", tpar)
+	// Disallow a lone type parameter as the RHS of a type declaration (issue #45639).
+	// We can look directly at named.underlying because even if it is still a *Named
+	// type (underlying not fully resolved yet) it cannot become a type parameter due
+	// to this very restriction.
+	if tpar, _ := named.underlying.(*TypeParam); tpar != nil {
+		check.error(tdecl.Type, "cannot use a type parameter as RHS in type declaration")
 		named.underlying = Typ[Invalid]
 	}
 }
diff --git a/src/cmd/compile/internal/types2/testdata/check/linalg.go2 b/src/cmd/compile/internal/types2/testdata/check/linalg.go2
index efc090a1d1..f02e773dbe 100644
--- a/src/cmd/compile/internal/types2/testdata/check/linalg.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/linalg.go2
@@ -4,8 +4,6 @@
 
 package linalg
 
-import "math"
-
 // Numeric is type bound that matches any numeric type.
 // It would likely be in a constraints package in the standard library.
 type Numeric interface {
@@ -52,32 +50,33 @@ type Complex interface {
 	~complex64 | ~complex128
 }
 
-// OrderedAbs is a helper type that defines an Abs method for
-// ordered numeric types.
-type OrderedAbs[T OrderedNumeric] T
-
-func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
-	if a < 0 {
-		return -a
-	}
-	return a
-}
-
-// ComplexAbs is a helper type that defines an Abs method for
-// complex types.
-type ComplexAbs[T Complex] T
-
-func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
-	r := float64(real(a))
-	i := float64(imag(a))
-	d := math.Sqrt(r * r + i * i)
-	return ComplexAbs[T](complex(d, 0))
-}
-
-func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
-	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
-}
-
-func ComplexAbsDifference[T Complex](a, b T) T {
-	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // OrderedAbs is a helper type that defines an Abs method for
+// // ordered numeric types.
+// type OrderedAbs[T OrderedNumeric] T
+// 
+// func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
+// 	if a < 0 {
+// 		return -a
+// 	}
+// 	return a
+// }
+// 
+// // ComplexAbs is a helper type that defines an Abs method for
+// // complex types.
+// type ComplexAbs[T Complex] T
+// 
+// func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
+// 	r := float64(real(a))
+// 	i := float64(imag(a))
+// 	d := math.Sqrt(r * r + i * i)
+// 	return ComplexAbs[T](complex(d, 0))
+// }
+// 
+// func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+// 	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
+// }
+// 
+// func ComplexAbsDifference[T Complex](a, b T) T {
+// 	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
+// }
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeinst.go2 b/src/cmd/compile/internal/types2/testdata/check/typeinst.go2
index 14f1b07ee2..a3d1b5e28f 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeinst.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeinst.go2
@@ -8,7 +8,8 @@ type myInt int
 
 // Parameterized type declarations
 
-type T1[P any] P
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+type T1[P any] P // ERROR cannot use a type parameter as RHS in type declaration
 
 type T2[P any] struct {
         f P
@@ -19,7 +20,7 @@ type List[P any] []P
 
 // Alias type declarations cannot have type parameters.
 // Issue #46477 proposses to change that.
-type A1[P any] = /* ERROR cannot be alias */ P
+type A1[P any] = /* ERROR cannot be alias */ struct{}
 
 // Pending clarification of #46477 we disallow aliases
 // of generic types.
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeinst2.go2 b/src/cmd/compile/internal/types2/testdata/check/typeinst2.go2
index 783ff346c9..5529bd093d 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeinst2.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeinst2.go2
@@ -87,25 +87,27 @@ type NumericAbs[T any] interface {
 
 func AbsDifference[T NumericAbs[T]](x T) { panic(0) }
 
-type OrderedAbs[T any] T
-
-func (a OrderedAbs[T]) Abs() OrderedAbs[T]
-
-func OrderedAbsDifference[T any](x T) {
-	AbsDifference(OrderedAbs[T](x))
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type OrderedAbs[T any] T
+// 
+// func (a OrderedAbs[T]) Abs() OrderedAbs[T]
+// 
+// func OrderedAbsDifference[T any](x T) {
+// 	AbsDifference(OrderedAbs[T](x))
+// }
 
 // same code, reduced to essence
 
 func g[P interface{ m() P }](x P) { panic(0) }
 
-type T4[P any] P
-
-func (_ T4[P]) m() T4[P]
-
-func _[Q any](x Q) {
-	g(T4[Q](x))
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T4[P any] P
+// 
+// func (_ T4[P]) m() T4[P]
+// 
+// func _[Q any](x Q) {
+// 	g(T4[Q](x))
+// }
 
 // Another test case that caused  problems in the past
 
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2 b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
index 1200a6e874..11adb21d95 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
@@ -353,15 +353,16 @@ func _() {
 
 // the previous example was extracted from
 
-func f12[T interface{m() T}]() {}
-
-type A[T any] T
-
-func (a A[T]) m() A[T]
-
-func _[T any]() {
-	f12[A[T]]()
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// func f12[T interface{m() T}]() {}
+// 
+// type A[T any] T
+// 
+// func (a A[T]) m() A[T]
+// 
+// func _[T any]() {
+// 	f12[A[T]]()
+// }
 
 // method expressions
 
diff --git a/src/cmd/compile/internal/types2/testdata/examples/methods.go2 b/src/cmd/compile/internal/types2/testdata/examples/methods.go2
index 4e87041e54..1d76d553dc 100644
--- a/src/cmd/compile/internal/types2/testdata/examples/methods.go2
+++ b/src/cmd/compile/internal/types2/testdata/examples/methods.go2
@@ -6,8 +6,6 @@
 
 package p
 
-import "unsafe"
-
 // Parameterized types may have methods.
 type T1[A any] struct{ a A }
 
@@ -97,17 +95,18 @@ type T0 struct{}
 func (T0) _() {}
 func (T1[A]) _() {}
 
-// A generic receiver type may constrain its type parameter such
-// that it must be a pointer type. Such receiver types are not
-// permitted.
-type T3a[P interface{ ~int | ~string | ~float64 }] P
-
-func (T3a[_]) m() {} // this is ok
-
-type T3b[P interface{ ~unsafe.Pointer }] P
-
-func (T3b /* ERROR invalid receiver */ [_]) m() {}
-
-type T3c[P interface{ *int | *string }] P
-
-func (T3c /* ERROR invalid receiver */ [_]) m() {}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // A generic receiver type may constrain its type parameter such
+// // that it must be a pointer type. Such receiver types are not
+// // permitted.
+// type T3a[P interface{ ~int | ~string | ~float64 }] P
+// 
+// func (T3a[_]) m() {} // this is ok
+// 
+// type T3b[P interface{ ~unsafe.Pointer }] P
+// 
+// func (T3b /* ERROR invalid receiver */ [_]) m() {}
+// 
+// type T3c[P interface{ *int | *string }] P
+// 
+// func (T3c /* ERROR invalid receiver */ [_]) m() {}
diff --git a/src/cmd/compile/internal/types2/testdata/examples/types.go2 b/src/cmd/compile/internal/types2/testdata/examples/types.go2
index f177c78d06..72b74cee01 100644
--- a/src/cmd/compile/internal/types2/testdata/examples/types.go2
+++ b/src/cmd/compile/internal/types2/testdata/examples/types.go2
@@ -185,12 +185,13 @@ type _ struct {
 // 	_ = y < 0
 // }
 
-// It is not permitted to declare a local type whose underlying
-// type is a type parameter not declared by that type declaration.
-func _[T any]() {
-	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
-	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // It is not permitted to declare a local type whose underlying
+// // type is a type parameter not declared by that type declaration.
+// func _[T any]() {
+// 	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+// 	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
+// }
 
 // As a special case, an explicit type argument may be omitted
 // from a type parameter bound if the type bound expects exactly
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2
index 200484b6d9..9a98f7f955 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2
@@ -74,9 +74,10 @@ func F20[t Z20]() { F20(t /* ERROR invalid composite literal type */ {}) }
 type Z21 /* ERROR illegal cycle */ interface{ Z21 }
 func F21[T Z21]() { ( /* ERROR not used */ F21[Z21]) }
 
-// crash 24
-type T24[P any] P
-func (r T24[P]) m() { T24 /* ERROR without instantiation */ .m() }
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // crash 24
+// type T24[P any] P
+// func (r T24[P]) m() { T24 /* ERROR without instantiation */ .m() }
 
 // crash 25
 type T25[A any] int
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39768.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39768.go2
index fb522733e0..696d9d9bee 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39768.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39768.go2
@@ -4,14 +4,15 @@
 
 package p
 
-type T[P any] P
-type A = T  // ERROR cannot use generic type
-var x A[int]
-var _ A
-
-type B = T[int]
-var y B = x
-var _ B /* ERROR not a generic type */ [int]
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T[P any] P
+// type A = T  // ERROR cannot use generic type
+// var x A[int]
+// var _ A
+//
+// type B = T[int]
+// var y B = x
+// var _ B /* ERROR not a generic type */ [int]
 
 // test case from issue
 
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39938.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39938.go2
index 31bec5fb01..114646786d 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39938.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39938.go2
@@ -8,7 +8,7 @@
 
 package p
 
-type E0[P any] P
+type E0[P any] []P
 type E1[P any] *P
 type E2[P any] struct{ _ P }
 type E3[P any] struct{ _ *P }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2
index 441fb4cb34..80148fe481 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2
@@ -4,9 +4,10 @@
 
 package P
 
-// It is not permitted to declare a local type whose underlying
-// type is a type parameters not declared by that type declaration.
-func _[T any]() {
-	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
-	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // It is not permitted to declare a local type whose underlying
+// // type is a type parameters not declared by that type declaration.
+// func _[T any]() {
+// 	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+// 	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
+// }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
index af52056bef..6a2e787bf9 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
@@ -4,15 +4,16 @@
 
 package p
 
-type T1[P any] P
-
-func (T1[_]) m() {}
-
-func _[P any](x *T1[P]) {
-        // x.m exists because x is of type *T1 where T1 is a defined type
-        // (even though under(T1) is a type parameter)
-        x.m()
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T1[P any] P
+// 
+// func (T1[_]) m() {}
+// 
+// func _[P any](x *T1[P]) {
+//         // x.m exists because x is of type *T1 where T1 is a defined type
+//         // (even though under(T1) is a type parameter)
+//         x.m()
+// }
 
 
 func _[P interface{ m() }](x P) {
@@ -40,29 +41,31 @@ type Barer[t any] interface {
 	Bar(t)
 }
 
-type Foo1[t any] t
-type Bar[t any] t
-
-func (l Foo1[t]) Foo(v Barer[t]) { v.Bar(t(l)) }
-func (b *Bar[t]) Bar(l t)        { *b = Bar[t](l) }
-
-func _[t any](f Fooer1[t]) t {
-	var b Bar[t]
-	f.Foo(&b)
-	return t(b)
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type Foo1[t any] t
+// type Bar[t any] t
+// 
+// func (l Foo1[t]) Foo(v Barer[t]) { v.Bar(t(l)) }
+// func (b *Bar[t]) Bar(l t)        { *b = Bar[t](l) }
+// 
+// func _[t any](f Fooer1[t]) t {
+// 	var b Bar[t]
+// 	f.Foo(&b)
+// 	return t(b)
+// }
 
 // Test case 2 from issue
 
-type Fooer2[t any] interface {
-	Foo()
-}
-
-type Foo2[t any] t
-
-func (f *Foo2[t]) Foo() {}
-
-func _[t any](v t) {
-	var f = Foo2[t](v)
-	_ = Fooer2[t](&f)
-}
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type Fooer2[t any] interface {
+// 	Foo()
+// }
+// 
+// type Foo2[t any] t
+// 
+// func (f *Foo2[t]) Foo() {}
+// 
+// func _[t any](v t) {
+// 	var f = Foo2[t](v)
+// 	_ = Fooer2[t](&f)
+// }