[dev.typeparams] cmd/compile/internal/types2: use Checker-provided type parameter IDs when possible

This is a port of https://golang.org/cl/317472.

For #46003.

Change-Id: Ie7b8880d43d459527b981ed4f60ee4d80a3cd17a
Reviewed-on: https://go-review.googlesource.com/c/go/+/320149
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/api_test.go

@@ -349,7 +349,6 @@ func TestTypesInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		ResetId() // avoid renumbering of type parameter ids when adding tests
 		info := Info{Types: make(map[syntax.Expr]TypeAndValue)}
 		var name string
 		if strings.HasPrefix(test.src, brokenPkg) {

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

@@ -83,6 +83,7 @@ type Checker struct {
 	pkg  *Package
 	*Info
 	version version                     // accepted language version
+	nextID  uint64                      // unique Id for type parameters (first valid Id is 1)
 	objMap  map[Object]*declInfo        // maps package-level objects and (non-interface) methods to declaration info
 	impMap  map[importKey]*Package      // maps (import path, source directory) to (complete or fake) package
 	posMap  map[*Interface][]syntax.Pos // maps interface types to lists of embedded interface positions

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

@@ -732,11 +732,11 @@ func (t *Named) AddMethod(m *Func) {
 // Note: This is a uint32 rather than a uint64 because the
 // respective 64 bit atomic instructions are not available
 // on all platforms.
-var lastId uint32
+var lastID uint32
 
-// nextId returns a value increasing monotonically by 1 with
+// nextID returns a value increasing monotonically by 1 with
 // each call, starting with 1. It may be called concurrently.
-func nextId() uint64 { return uint64(atomic.AddUint32(&lastId, 1)) }
+func nextID() uint64 { return uint64(atomic.AddUint32(&lastID, 1)) }
 
 // A TypeParam represents a type parameter type.
 type TypeParam struct {
@@ -753,7 +753,13 @@ func (t *TypeParam) Obj() *TypeName { return t.obj }
 // NewTypeParam returns a new TypeParam.
 func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
 	assert(bound != nil)
-	typ := &TypeParam{check: check, id: nextId(), obj: obj, index: index, bound: bound}
+	// Always increment lastID, even if it is not used.
+	id := nextID()
+	if check != nil {
+		check.nextID++
+		id = check.nextID
+	}
+	typ := &TypeParam{check: check, id: id, obj: obj, index: index, bound: bound}
 	if obj.typ == nil {
 		obj.typ = typ
 	}

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

@@ -4,14 +4,6 @@
 
 package types2
 
-import "sync/atomic"
-
 func init() {
 	acceptMethodTypeParams = true
 }
-
-// Upon calling ResetId, nextId starts with 1 again.
-// It may be called concurrently. This is only needed
-// for tests where we may want to have a consistent
-// numbering for each individual test case.
-func ResetId() { atomic.StoreUint32(&lastId, 0) }