go/types, types2: eagerly check that constraints are not type params

As a result of the change to the underlying of a type parameter to be
its constraint interface, we had couple inaccuracies that combined to
cause an infinite recursion when type checking the invalid type
parameter list [A A].
 - We deferred tpar.iface() using check.later twice: once in
   newTypeParam, and then again at the end of collectTypeParams.
 - We deferred the check that type parameter constraints are not type
   parameters, even though this is unnecessary: the constraint type is
   known.

With these inaccuracies, tpar.iface() was executing before our guard
against using type parameters as constraints, causing an infinite
recursion through under().

Fix this by eagerly checking whether the constraint is a type
parameter, and marking it invalid if so. Also remove the unnecessary
calls to tpar.iface() at the end of collectTypeParams, as this will
already have been scheduled by newTypeParam.

Fixes #50321

Change-Id: I4eecbecf21656615867cb94be65b520e9e795bd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/374294
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -657,33 +657,24 @@ func (check *Checker) collectTypeParams(dst **TypeParamList, list []*syntax.Fiel
 	// Keep track of bounds for later validation.
 	var bound Type
 	var bounds []Type
-	var posers []poser
 	for i, f := range list {
 		// Optimization: Re-use the previous type bound if it hasn't changed.
 		// This also preserves the grouped output of type parameter lists
 		// when printing type strings.
 		if i == 0 || f.Type != list[i-1].Type {
 			bound = check.bound(f.Type)
-			bounds = append(bounds, bound)
-			posers = append(posers, f.Type)
-		}
-		tparams[i].bound = bound
-	}
-
-	check.later(func() {
-		for i, bound := range bounds {
 			if isTypeParam(bound) {
 				// We may be able to allow this since it is now well-defined what
 				// the underlying type and thus type set of a type parameter is.
 				// But we may need some additional form of cycle detection within
 				// type parameter lists.
-				check.error(posers[i], "cannot use a type parameter as constraint")
+				check.error(f.Type, "cannot use a type parameter as constraint")
+				bound = Typ[Invalid]
 			}
+			bounds = append(bounds, bound)
 		}
-		for _, tpar := range tparams {
-			tpar.iface() // compute type set
-		}
-	})
+		tparams[i].bound = bound
+	}
 }
 
 func (check *Checker) bound(x syntax.Expr) Type {

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50321.go2

@@ -0,0 +1,8 @@
+// Copyright 2021 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
+
+func Ln[A A /* ERROR cannot use a type parameter as constraint */ ](p A) {
+}

src/go/types/decl.go

@@ -708,34 +708,29 @@ func (check *Checker) collectTypeParams(dst **TypeParamList, list *ast.FieldList
 
 	index := 0
 	var bounds []Type
-	var posns []positioner // bound positions
 	for _, f := range list.List {
-		// TODO(rfindley) we should be able to rely on f.Type != nil at this point
+		var bound Type
+		// NOTE: we may be able to assert that f.Type != nil here, but this is not
+		// an invariant of the AST, so we are cautious.
 		if f.Type != nil {
-			bound := check.bound(f.Type)
-			bounds = append(bounds, bound)
-			posns = append(posns, f.Type)
-			for i := range f.Names {
-				tparams[index+i].bound = bound
-			}
-		}
-		index += len(f.Names)
-	}
-
-	check.later(func() {
-		for i, bound := range bounds {
+			bound = check.bound(f.Type)
 			if isTypeParam(bound) {
 				// We may be able to allow this since it is now well-defined what
 				// the underlying type and thus type set of a type parameter is.
 				// But we may need some additional form of cycle detection within
 				// type parameter lists.
-				check.error(posns[i], _MisplacedTypeParam, "cannot use a type parameter as constraint")
+				check.error(f.Type, _MisplacedTypeParam, "cannot use a type parameter as constraint")
+				bound = Typ[Invalid]
 			}
+		} else {
+			bound = Typ[Invalid]
 		}
-		for _, tpar := range tparams {
-			tpar.iface() // compute type set
+		bounds = append(bounds, bound)
+		for i := range f.Names {
+			tparams[index+i].bound = bound
 		}
-	})
+		index += len(f.Names)
+	}
 }
 
 func (check *Checker) bound(x ast.Expr) Type {

src/go/types/testdata/fixedbugs/issue50321.go2

@@ -0,0 +1,8 @@
+// Copyright 2021 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
+
+func Ln[A A /* ERROR cannot use a type parameter as constraint */ ](p A) {
+}