go/types, types2: reorder object processing to avoid broken aliases

By processing non-alias type declarations before alias type declaration,
and those before everything else we can avoid some of the remaining
errors which are due to alias types not being available.

For #25838.
For #50259.
For #50276.
For #50729.

Change-Id: I233da2899a6d4954c239638624dfa8c08662e6b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/380056
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/resolver.go

@@ -656,25 +656,31 @@ func (check *Checker) packageObjects() {
 		}
 	}
 
-	// We process non-alias declarations first, in order to avoid situations where
-	// the type of an alias declaration is needed before it is available. In general
-	// this is still not enough, as it is possible to create sufficiently convoluted
-	// recursive type definitions that will cause a type alias to be needed before it
-	// is available (see issue #25838 for examples).
-	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	// We process non-alias type declarations first, followed by alias declarations,
+	// and then everything else. This appears to avoid most situations where the type
+	// of an alias is needed before it is available.
+	// There may still be cases where this is not good enough (see also issue #25838).
+	// In those cases Checker.ident will report an error ("invalid use of type alias").
 	var aliasList []*TypeName
-	// phase 1
+	var othersList []Object // everything that's not a type
+	// phase 1: non-alias type declarations
 	for _, obj := range objList {
-		// If we have a type alias, collect it for the 2nd phase.
-		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].tdecl.Alias {
-			aliasList = append(aliasList, tname)
-			continue
+		if tname, _ := obj.(*TypeName); tname != nil {
+			if check.objMap[tname].tdecl.Alias {
+				aliasList = append(aliasList, tname)
+			} else {
+				check.objDecl(obj, nil)
+			}
+		} else {
+			othersList = append(othersList, obj)
 		}
-
+	}
+	// phase 2: alias type declarations
+	for _, obj := range aliasList {
 		check.objDecl(obj, nil)
 	}
-	// phase 2
-	for _, obj := range aliasList {
+	// phase 3: all other declarations
+	for _, obj := range othersList {
 		check.objDecl(obj, nil)
 	}
 

src/cmd/compile/internal/types2/testdata/fixedbugs/issue25838.go

@@ -0,0 +1,26 @@
+// Copyright 2022 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
+
+// examples from the issue
+
+type (
+	e = f
+	f = g
+	g = []h
+	h i
+	i = j
+	j = e
+)
+
+type (
+	e1 = []h1
+	h1 e1
+)
+
+type (
+	P = *T
+	T P
+)

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50259.go2

@@ -7,7 +7,7 @@ package p
 var x T[B]
 
 type T[_ any] struct{}
-type A T[B /* ERROR invalid use of type alias */ ]
+type A T[B]
 type B = T[A]
 
 // test case from issue
@@ -15,4 +15,4 @@ type B = T[A]
 var v Box[Step]
 type Box[T any] struct{}
 type Step = Box[StepBox]
-type StepBox Box[Step /* ERROR invalid use of type alias */ ]
+type StepBox Box[Step]

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50276.go2

@@ -11,7 +11,7 @@ type pair[S any] struct {}
 
 var _ transform[step]
 
-type box transform[step /* ERROR invalid use of type alias */ ]
+type box transform[step]
 type step = pair[box]
 
 // test case from issue
@@ -27,7 +27,7 @@ var first Transform[Step]
 // This line doesn't use the Step alias, and it compiles fine if you uncomment it.
 var second Transform[Pair[Box, interface{}]]
 
-type Box *Transform[Step /* ERROR invalid use of type alias */ ]
+type Box *Transform[Step]
 
 // This line is the same as the `first` line, but it comes after the Box declaration and
 // does not break the compile.

src/go/types/resolver.go

@@ -629,25 +629,31 @@ func (check *Checker) packageObjects() {
 		}
 	}
 
-	// We process non-alias declarations first, in order to avoid situations where
-	// the type of an alias declaration is needed before it is available. In general
-	// this is still not enough, as it is possible to create sufficiently convoluted
-	// recursive type definitions that will cause a type alias to be needed before it
-	// is available (see issue #25838 for examples).
-	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	// We process non-alias type declarations first, followed by alias declarations,
+	// and then everything else. This appears to avoid most situations where the type
+	// of an alias is needed before it is available.
+	// There may still be cases where this is not good enough (see also issue #25838).
+	// In those cases Checker.ident will report an error ("invalid use of type alias").
 	var aliasList []*TypeName
-	// phase 1
+	var othersList []Object // everything that's not a type
+	// phase 1: non-alias type declarations
 	for _, obj := range objList {
-		// If we have a type alias, collect it for the 2nd phase.
-		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].tdecl.Assign.IsValid() {
-			aliasList = append(aliasList, tname)
-			continue
+		if tname, _ := obj.(*TypeName); tname != nil {
+			if check.objMap[tname].tdecl.Assign.IsValid() {
+				aliasList = append(aliasList, tname)
+			} else {
+				check.objDecl(obj, nil)
+			}
+		} else {
+			othersList = append(othersList, obj)
 		}
-
+	}
+	// phase 2: alias type declarations
+	for _, obj := range aliasList {
 		check.objDecl(obj, nil)
 	}
-	// phase 2
-	for _, obj := range aliasList {
+	// phase 3: all other declarations
+	for _, obj := range othersList {
 		check.objDecl(obj, nil)
 	}
 

src/go/types/testdata/fixedbugs/issue25838.go

@@ -0,0 +1,26 @@
+// Copyright 2022 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
+
+// examples from the issue
+
+type (
+	e = f
+	f = g
+	g = []h
+	h i
+	i = j
+	j = e
+)
+
+type (
+	e1 = []h1
+	h1 e1
+)
+
+type (
+	P = *T
+	T P
+)

src/go/types/testdata/fixedbugs/issue50259.go2

@@ -7,7 +7,7 @@ package p
 var x T[B]
 
 type T[_ any] struct{}
-type A T[B /* ERROR invalid use of type alias */ ]
+type A T[B]
 type B = T[A]
 
 // test case from issue
@@ -15,4 +15,4 @@ type B = T[A]
 var v Box[Step]
 type Box[T any] struct{}
 type Step = Box[StepBox]
-type StepBox Box[Step /* ERROR invalid use of type alias */ ]
+type StepBox Box[Step]

src/go/types/testdata/fixedbugs/issue50276.go2

@@ -11,7 +11,7 @@ type pair[S any] struct {}
 
 var _ transform[step]
 
-type box transform[step /* ERROR invalid use of type alias */ ]
+type box transform[step]
 type step = pair[box]
 
 // test case from issue
@@ -27,7 +27,7 @@ var first Transform[Step]
 // This line doesn't use the Step alias, and it compiles fine if you uncomment it.
 var second Transform[Pair[Box, interface{}]]
 
-type Box *Transform[Step /* ERROR invalid use of type alias */ ]
+type Box *Transform[Step]
 
 // This line is the same as the `first` line, but it comes after the Box declaration and
 // does not break the compile.

test/typeparam/issue50259.go

@@ -1,4 +1,4 @@
-// errorcheck -G=3
+// compile -G=3
 
 // Copyright 2022 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -9,5 +9,5 @@ package p
 var x T[B]
 
 type T[_ any] struct{}
-type A T[B] // ERROR "invalid use of type alias B in recursive type"
+type A T[B]
 type B = T[A]