[dev.typeparams] cmd/compile/internal/types2: embedded type cannot be a (pointer to) a type parameter

Change-Id: I5eb03ae349925f0799dd866e207221429bc9fb3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/337353
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -135,7 +135,7 @@ func (check *Checker) structType(styp *Struct, e *syntax.StructType) {
 			embeddedPos := pos
 			check.later(func() {
 				t, isPtr := deref(embeddedTyp)
-				switch t := optype(t).(type) {
+				switch t := under(t).(type) {
 				case *Basic:
 					if t == Typ[Invalid] {
 						// error was reported before
@@ -147,6 +147,8 @@ func (check *Checker) structType(styp *Struct, e *syntax.StructType) {
 					}
 				case *Pointer:
 					check.error(embeddedPos, "embedded field type cannot be a pointer")
+				case *TypeParam:
+					check.error(embeddedPos, "embedded field type cannot be a (pointer to a) type parameter")
 				case *Interface:
 					if isPtr {
 						check.error(embeddedPos, "embedded field type cannot be a pointer to an interface")

src/cmd/compile/internal/types2/testdata/check/typeparams.go2

@@ -79,11 +79,11 @@ var _ *int = new[int]()
 
 func _[T any](map[T /* ERROR invalid map key type T \(missing comparable constraint\) */]int) // w/o constraint we don't know if T is comparable
 
-func f1[T1 any](struct{T1}) int
+func f1[T1 any](struct{T1 /* ERROR cannot be a .* type parameter */ }) int
 var _ = f1[int](struct{T1}{})
 type T1 = int
 
-func f2[t1 any](struct{t1; x float32}) int
+func f2[t1 any](struct{t1 /* ERROR cannot be a .* type parameter */ ; x float32}) int
 var _ = f2[t1](struct{t1; x float32}{})
 type t1 = int
 

src/cmd/compile/internal/types2/testdata/fixedbugs/issue39938.go2

@@ -8,8 +8,8 @@ package p
 
 type E0[P any] P
 type E1[P any] *P
-type E2[P any] struct{ P }
-type E3[P any] struct{ *P }
+type E2[P any] struct{ _ P }
+type E3[P any] struct{ _ *P }
 
 type T0 /* ERROR illegal cycle */ struct {
         _ E0[T0]

test/typeparam/interfacearg.go

@@ -9,14 +9,14 @@ package main
 type I interface{}
 
 type _S[T any] struct {
-	*T
+	x *T
 }
 
 // F is a non-generic function, but has a type _S[I] which is instantiated from a
 // generic type. Test that _S[I] is successfully exported.
 func F() {
 	v := _S[I]{}
-	if v.T != nil {
+	if v.x != nil {
 		panic(v)
 	}
 }
@@ -33,9 +33,9 @@ func _F1[T interface{ M() }](t T) {
 }
 
 func F2() {
-        _F1(&S1{})
-        _F1(S2{})
-        _F1(&S2{})
+	_F1(&S1{})
+	_F1(S2{})
+	_F1(&S2{})
 }
 
 func main() {

test/typeparam/lockable.go

@@ -11,7 +11,7 @@ import "sync"
 // A Lockable is a value that may be safely simultaneously accessed
 // from multiple goroutines via the Get and Set methods.
 type Lockable[T any] struct {
-	T
+	x T
 	mu sync.Mutex
 }
 
@@ -19,18 +19,18 @@ type Lockable[T any] struct {
 func (l *Lockable[T]) get() T {
 	l.mu.Lock()
 	defer l.mu.Unlock()
-	return l.T
+	return l.x
 }
 
 // set sets the value in a Lockable.
 func (l *Lockable[T]) set(v T) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
-	l.T = v
+	l.x = v
 }
 
 func main() {
-	sl := Lockable[string]{T: "a"}
+	sl := Lockable[string]{x: "a"}
 	if got := sl.get(); got != "a" {
 		panic(got)
 	}
@@ -39,7 +39,7 @@ func main() {
 		panic(got)
 	}
 
-	il := Lockable[int]{T: 1}
+	il := Lockable[int]{x: 1}
 	if got := il.get(); got != 1 {
 		panic(got)
 	}