cmd/compile/internal/types2: handle recursive type parameter constraints

Check type constraints after the respective type parameter list
has been associated with a parameterized type so that recursive
type parameter constraints "see" a parameterized type.

Fixes #45550.
Fixes #47796.

Change-Id: Iac74610ca017a78013820624230c857395506aff
Reviewed-on: https://go-review.googlesource.com/c/go/+/348090
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: griesemer

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

@@ -567,7 +567,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 	if tdecl.TParamList != nil {
 		check.openScope(tdecl, "type parameters")
 		defer check.closeScope()
-		named.tparams = check.collectTypeParams(tdecl.TParamList)
+		check.collectTypeParams(&named.tparams, tdecl.TParamList)
 	}
 
 	// determine underlying type of named
@@ -598,7 +598,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 	}
 }
 
-func (check *Checker) collectTypeParams(list []*syntax.Field) *TParamList {
+func (check *Checker) collectTypeParams(dst **TParamList, list []*syntax.Field) {
 	tparams := make([]*TypeParam, len(list))
 
 	// Declare type parameters up-front.
@@ -608,6 +608,11 @@ func (check *Checker) collectTypeParams(list []*syntax.Field) *TParamList {
 		tparams[i] = check.declareTypeParam(f.Name)
 	}
 
+	// Set the type parameters before collecting the type constraints because
+	// the parameterized type may be used by the constraints (issue #47887).
+	// Example: type T[P T[P]] interface{}
+	*dst = bindTParams(tparams)
+
 	var bound Type
 	for i, f := range list {
 		// Optimization: Re-use the previous type bound if it hasn't changed.
@@ -618,13 +623,17 @@ func (check *Checker) collectTypeParams(list []*syntax.Field) *TParamList {
 		}
 		tparams[i].bound = bound
 	}
-
-	return bindTParams(tparams)
 }
 
 func (check *Checker) declareTypeParam(name *syntax.Name) *TypeParam {
+	// Use Typ[Invalid] for the type constraint to ensure that a type
+	// is present even if the actual constraint has not been assigned
+	// yet.
+	// TODO(gri) Need to systematically review all uses of type parameter
+	//           constraints to make sure we don't rely on them if they
+	//           are not properly set yet.
 	tname := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
-	tpar := check.NewTypeParam(tname, nil)                   // assigns type to tname as a side-effect
+	tpar := check.NewTypeParam(tname, Typ[Invalid])          // assigns type to tname as a side-effect
 	check.declare(check.scope, name, tname, check.scope.pos) // TODO(gri) check scope position
 	return tpar
 }

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

@@ -157,7 +157,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 	}
 
 	if tparams != nil {
-		sig.tparams = check.collectTypeParams(tparams)
+		check.collectTypeParams(&sig.tparams, tparams)
 		// Always type-check method type parameters but complain if they are not enabled.
 		// (A separate check is needed when type-checking interface method signatures because
 		// they don't have a receiver specification.)

src/cmd/compile/internal/types2/testdata/fixedbugs/issue45550.go2

@@ -0,0 +1,10 @@
+// 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
+
+type Builder[T interface{ struct{ Builder[T] } }] struct{}
+type myBuilder struct {
+	Builder[myBuilder /* ERROR myBuilder does not satisfy */]
+}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue47796.go2

@@ -0,0 +1,33 @@
+// 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
+
+// parameterized types with self-recursive constraints
+type (
+	T1[P T1[P]]                            interface{}
+	T2[P, Q T2[P, Q]]                      interface{}
+	T3[P T2[P, Q], Q interface{ ~string }] interface{}
+
+	T4a[P T4a[P]]                                                        interface{ ~int }
+	T4b[P T4b[int]]                                                      interface{ ~int }
+	T4c[P T4c[string /* ERROR string does not satisfy T4c\[string\] */]] interface{ ~int }
+
+	// mutually recursive constraints
+	T5[P T6[P]] interface{ int }
+	T6[P T5[P]] interface{ int }
+)
+
+// verify that constraints are checked as expected
+var (
+	_ T1[int]
+	_ T2[int, string]
+	_ T3[int, string]
+)
+
+// test case from issue
+
+type Eq[a Eq[a]] interface {
+	Equal(that a) bool
+}