go/types, types2: disable field accesses through type parameters

This is a feature that is not understood well enough and may have
subtle repercussions impacting future changes. Disable for Go 1.18.

The actual change is trivial: disable a branch through a flag.
The remaining changes are adjustments to tests.

Fixes #51576.

Change-Id: Ib77b038b846711a808315a8889b3904e72367bce
Reviewed-on: https://go-review.googlesource.com/c/go/+/391135
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -70,7 +70,8 @@ func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 	// see if there is a matching field (but not a method, those need to be declared
 	// explicitly in the constraint). If the constraint is a named pointer type (see
 	// above), we are ok here because only fields are accepted as results.
-	if obj == nil && isTypeParam(T) {
+	const enableTParamFieldLookup = false // see issue #51576
+	if enableTParamFieldLookup && obj == nil && isTypeParam(T) {
 		if t := coreType(T); t != nil {
 			obj, index, indirect = lookupFieldOrMethod(t, addressable, pkg, name, false)
 			if _, ok := obj.(*Var); !ok {

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50417.go2

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
 package p
 
 type Sf struct {
@@ -9,13 +13,13 @@ type Sf struct {
 }
 
 func f0[P Sf](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
 }
 
 func f0t[P ~struct{f int}](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
 }
 
 var _ = f0[Sf]
@@ -25,8 +29,8 @@ var _ = f0[Sm /* ERROR does not implement */ ]
 var _ = f0t[Sm /* ERROR does not implement */ ]
 
 func f1[P interface{ Sf; m() }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m()
 }
 
@@ -44,8 +48,8 @@ type Sfm struct {
 func (Sfm) m() {}
 
 func f2[P interface{ Sfm; m() }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m()
 }
 
@@ -56,8 +60,8 @@ var _ = f2[Sfm]
 type PSfm *Sfm
 
 func f3[P interface{ PSfm }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m /* ERROR type P has no field or method m */ ()
 }
 

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50782.go2

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
 package p
 
 // The first example from the issue.
@@ -18,9 +22,12 @@ type numericAbs[T Numeric] interface {
 // AbsDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T numericAbs[T /* ERROR T does not implement Numeric */]](a, b T) T {
-	// TODO: the error below should probably be positioned on the '-'.
-	d := a /* ERROR "invalid operation: operator - not defined" */ .Value - b.Value
-	return d.Abs()
+	// Field accesses are not permitted for now. Keep an error so
+	// we can find and fix this code once the situation changes.
+	return a.Value // ERROR a\.Value undefined
+	// TODO: The error below should probably be positioned on the '-'.
+	// d := a /* ERROR "invalid operation: operator - not defined" */ .Value - b.Value
+	// return d.Abs()
 }
 
 // The second example from the issue.

src/go/types/lookup.go

@@ -70,7 +70,8 @@ func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 	// see if there is a matching field (but not a method, those need to be declared
 	// explicitly in the constraint). If the constraint is a named pointer type (see
 	// above), we are ok here because only fields are accepted as results.
-	if obj == nil && isTypeParam(T) {
+	const enableTParamFieldLookup = false // see issue #51576
+	if enableTParamFieldLookup && obj == nil && isTypeParam(T) {
 		if t := coreType(T); t != nil {
 			obj, index, indirect = lookupFieldOrMethod(t, addressable, pkg, name, false)
 			if _, ok := obj.(*Var); !ok {

src/go/types/testdata/fixedbugs/issue50417.go2

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
 package p
 
 type Sf struct {
@@ -9,13 +13,13 @@ type Sf struct {
 }
 
 func f0[P Sf](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
 }
 
 func f0t[P ~struct{f int}](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
 }
 
 var _ = f0[Sf]
@@ -25,8 +29,8 @@ var _ = f0[Sm /* ERROR does not implement */ ]
 var _ = f0t[Sm /* ERROR does not implement */ ]
 
 func f1[P interface{ Sf; m() }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m()
 }
 
@@ -44,8 +48,8 @@ type Sfm struct {
 func (Sfm) m() {}
 
 func f2[P interface{ Sfm; m() }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m()
 }
 
@@ -56,8 +60,8 @@ var _ = f2[Sfm]
 type PSfm *Sfm
 
 func f3[P interface{ PSfm }](p P) {
-        _ = p.f
-        p.f = 0
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
         p.m /* ERROR type P has no field or method m */ ()
 }
 

src/go/types/testdata/fixedbugs/issue50782.go2

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
 package p
 
 // The first example from the issue.
@@ -18,9 +22,12 @@ type numericAbs[T Numeric] interface {
 // AbsDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T numericAbs[T /* ERROR T does not implement Numeric */]](a, b T) T {
-	// TODO: the error below should probably be positioned on the '-'.
-	d := a /* ERROR "invalid operation: operator - not defined" */ .Value - b.Value
-	return d.Abs()
+	// Field accesses are not permitted for now. Keep an error so
+	// we can find and fix this code once the situation changes.
+	return a.Value // ERROR a\.Value undefined
+	// TODO: The error below should probably be positioned on the '-'.
+	// d := a /* ERROR "invalid operation: operator - not defined" */ .Value - b.Value
+	// return d.Abs()
 }
 
 // The second example from the issue.

test/typeparam/absdiff2.go

@@ -23,15 +23,16 @@ type Numeric interface {
 
 // numericAbs matches a struct containing a numeric type that has an Abs method.
 type numericAbs[T Numeric] interface {
-	~struct{ Value T }
+	~struct{ Value_ T }
 	Abs() T
+	Value() T
 }
 
 // absDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T Numeric, U numericAbs[T]](a, b U) T {
-	d := a.Value - b.Value
-	dt := U{Value: d}
+	d := a.Value() - b.Value()
+	dt := U{Value_: d}
 	return dt.Abs()
 }
 
@@ -50,20 +51,29 @@ type Complex interface {
 // orderedAbs is a helper type that defines an Abs method for
 // a struct containing an ordered numeric type.
 type orderedAbs[T orderedNumeric] struct {
-	Value T
+	Value_ T
 }
 
 func (a orderedAbs[T]) Abs() T {
-	if a.Value < 0 {
-		return -a.Value
+	if a.Value_ < 0 {
+		return -a.Value_
 	}
-	return a.Value
+	return a.Value_
+}
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+// Use accessor method instead.
+
+func (a orderedAbs[T]) Value() T {
+	return a.Value_
 }
 
 // complexAbs is a helper type that defines an Abs method for
 // a struct containing a complex type.
 type complexAbs[T Complex] struct {
-	Value T
+	Value_ T
 }
 
 func realimag(x any) (re, im float64) {
@@ -82,13 +92,17 @@ func realimag(x any) (re, im float64) {
 
 func (a complexAbs[T]) Abs() T {
 	// TODO use direct conversion instead of realimag once #50937 is fixed
-	r, i := realimag(a.Value)
+	r, i := realimag(a.Value_)
 	// r := float64(real(a.Value))
 	// i := float64(imag(a.Value))
 	d := math.Sqrt(r*r + i*i)
 	return T(complex(d, 0))
 }
 
+func (a complexAbs[T]) Value() T {
+	return a.Value_
+}
+
 // OrderedAbsDifference returns the absolute value of the difference
 // between a and b, where a and b are of an ordered type.
 func OrderedAbsDifference[T orderedNumeric](a, b T) T {

test/typeparam/absdiffimp2.dir/a.go

@@ -17,15 +17,16 @@ type Numeric interface {
 
 // numericAbs matches a struct containing a numeric type that has an Abs method.
 type numericAbs[T Numeric] interface {
-	~struct{ Value T }
+	~struct{ Value_ T }
 	Abs() T
+	Value() T
 }
 
 // absDifference computes the absolute value of the difference of
 // a and b, where the absolute value is determined by the Abs method.
 func absDifference[T Numeric, U numericAbs[T]](a, b U) T {
-	d := a.Value - b.Value
-	dt := U{Value: d}
+	d := a.Value() - b.Value()
+	dt := U{Value_: d}
 	return dt.Abs()
 }
 
@@ -44,20 +45,29 @@ type Complex interface {
 // orderedAbs is a helper type that defines an Abs method for
 // a struct containing an ordered numeric type.
 type orderedAbs[T orderedNumeric] struct {
-	Value T
+	Value_ T
 }
 
 func (a orderedAbs[T]) Abs() T {
-	if a.Value < 0 {
-		return -a.Value
+	if a.Value_ < 0 {
+		return -a.Value_
 	}
-	return a.Value
+	return a.Value_
+}
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+// Use accessor method instead.
+
+func (a orderedAbs[T]) Value() T {
+	return a.Value_
 }
 
 // complexAbs is a helper type that defines an Abs method for
 // a struct containing a complex type.
 type complexAbs[T Complex] struct {
-	Value T
+	Value_ T
 }
 
 func realimag(x any) (re, im float64) {
@@ -76,13 +86,17 @@ func realimag(x any) (re, im float64) {
 
 func (a complexAbs[T]) Abs() T {
 	// TODO use direct conversion instead of realimag once #50937 is fixed
-	r, i := realimag(a.Value)
+	r, i := realimag(a.Value_)
 	// r := float64(real(a.Value))
 	// i := float64(imag(a.Value))
 	d := math.Sqrt(r*r + i*i)
 	return T(complex(d, 0))
 }
 
+func (a complexAbs[T]) Value() T {
+	return a.Value_
+}
+
 // OrderedAbsDifference returns the absolute value of the difference
 // between a and b, where a and b are of an ordered type.
 func OrderedAbsDifference[T orderedNumeric](a, b T) T {

test/typeparam/issue50417.go

@@ -8,6 +8,11 @@ package main
 
 func main() {}
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
+/*
 type Sf struct {
 	f int
 }
@@ -138,3 +143,4 @@ func f8[P Int4](p P) {
 }
 
 var _ = f8[*Sf]
+*/

test/typeparam/issue50417b.go

@@ -6,6 +6,13 @@
 
 package main
 
+func main() {}
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
+/*
 import "fmt"
 
 type MyStruct struct {
@@ -48,3 +55,4 @@ func main() {
 		panic(fmt.Sprintf("got %d, want %d", got, want))
 	}
 }
+*/

test/typeparam/issue50690a.go

@@ -29,34 +29,47 @@ func Sum[T Numeric](args ...T) T {
 
 // Ledger is an identifiable, financial record.
 type Ledger[T ~string, K Numeric] struct {
-
 	// ID identifies the ledger.
-	ID T
+	ID_ T
 
 	// Amounts is a list of monies associated with this ledger.
-	Amounts []K
+	Amounts_ []K
 
 	// SumFn is a function that can be used to sum the amounts
 	// in this ledger.
-	SumFn func(...K) K
+	SumFn_ func(...K) K
 }
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+// Use accessor methods instead.
+
+func (l Ledger[T, _]) ID() T               { return l.ID_ }
+func (l Ledger[_, K]) Amounts() []K        { return l.Amounts_ }
+func (l Ledger[_, K]) SumFn() func(...K) K { return l.SumFn_ }
+
 func PrintLedger[
 	T ~string,
 	K Numeric,
-	L ~struct {
-		ID      T
-		Amounts []K
-		SumFn   func(...K) K
+	L interface {
+		~struct {
+			ID_      T
+			Amounts_ []K
+			SumFn_   func(...K) K
+		}
+		ID() T
+		Amounts() []K
+		SumFn() func(...K) K
 	},
 ](l L) {
-	fmt.Printf("%s has a sum of %v\n", l.ID, l.SumFn(l.Amounts...))
+	fmt.Printf("%s has a sum of %v\n", l.ID(), l.SumFn()(l.Amounts()...))
 }
 
 func main() {
 	PrintLedger(Ledger[string, int]{
-		ID:      "fake",
-		Amounts: []int{1, 2, 3},
-		SumFn:   Sum[int],
+		ID_:      "fake",
+		Amounts_: []int{1, 2, 3},
+		SumFn_:   Sum[int],
 	})
 }

test/typeparam/issue50690b.go

@@ -18,24 +18,34 @@ func Print[T ~string](s T) {
 	fmt.Println(s)
 }
 
-func PrintWithPrinter[T ~string, S ~struct {
-	ID      T
-	PrintFn func(T)
+func PrintWithPrinter[T ~string, S interface {
+	~struct {
+		ID       T
+		PrintFn_ func(T)
+	}
+	PrintFn() func(T)
 }](message T, obj S) {
-	obj.PrintFn(message)
+	obj.PrintFn()(message)
 }
 
 type PrintShop[T ~string] struct {
-	ID      T
-	PrintFn func(T)
+	ID       T
+	PrintFn_ func(T)
 }
 
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+// Use accessor method instead.
+
+func (s PrintShop[T]) PrintFn() func(T) { return s.PrintFn_ }
+
 func main() {
 	PrintWithPrinter(
 		"Hello, world.",
 		PrintShop[string]{
-			ID:      "fake",
-			PrintFn: Print[string],
+			ID:       "fake",
+			PrintFn_: Print[string],
 		},
 	)
 }

test/typeparam/issue50690c.go

@@ -18,19 +18,33 @@ func Print[T ~string](s T) {
 	fmt.Println(s)
 }
 
-func PrintWithPrinter[T ~string, S struct {
-	ID      T
-	PrintFn func(T)
+func PrintWithPrinter[T ~string, S interface {
+	~struct {
+		ID       T
+		PrintFn_ func(T)
+	}
+	PrintFn() func(T)
 }](message T, obj S) {
-	obj.PrintFn(message)
+	obj.PrintFn()(message)
 }
 
 func main() {
 	PrintWithPrinter(
 		"Hello, world.",
-		struct {
-			ID      string
-			PrintFn func(string)
-		}{ID: "fake", PrintFn: Print[string]},
+		StructWithPrinter{ID: "fake", PrintFn_: Print[string]},
 	)
 }
+
+type StructWithPrinter struct {
+	ID       string
+	PrintFn_ func(string)
+}
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+// Use accessor method instead.
+
+func (s StructWithPrinter) PrintFn() func(string) {
+	return s.PrintFn_
+}