cmd/compile/internal/types2: remove Config.InferFromConstraints flag

Constraint type inference is part of the proposed language.
Use an internal flag to control the feayure for debugging.

Change-Id: I7a9eaee92b5ffc23c25d9e68a729acc0d705e879
Reviewed-on: https://go-review.googlesource.com/c/go/+/306770
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/irgen.go

@@ -36,7 +36,6 @@ func check2(noders []*noder) {
 	// typechecking
 	conf := types2.Config{
 		GoVersion:             base.Flag.Lang,
-		InferFromConstraints:  true,
 		IgnoreLabels:          true, // parser already checked via syntax.CheckBranches mode
 		CompilerErrorMessages: true, // use error strings matching existing compiler errors
 		Error: func(err error) {

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

@@ -110,10 +110,6 @@ type Config struct {
 	// If AcceptMethodTypeParams is set, methods may have type parameters.
 	AcceptMethodTypeParams bool
 
-	// If InferFromConstraints is set, constraint type inference is used
-	// if some function type arguments are missing.
-	InferFromConstraints bool
-
 	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
 	// declares an empty "C" package and errors are omitted for qualified
 	// identifiers referring to package C (which won't find an object).

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

@@ -66,7 +66,6 @@ func mayTypecheck(t *testing.T, path, source string, info *Info) (string, error)
 	}
 	conf := Config{
 		AcceptMethodTypeParams: true,
-		InferFromConstraints:   true,
 		Error:                  func(err error) {},
 		Importer:               defaultImporter(),
 	}

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

@@ -27,7 +27,7 @@ func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
 	// check number of type arguments (got) vs number of type parameters (want)
 	sig := x.typ.(*Signature)
 	got, want := len(targs), len(sig.tparams)
-	if !check.conf.InferFromConstraints && got != want || got > want {
+	if !useConstraintTypeInference && got != want || got > want {
 		check.errorf(xlist[got-1], "got %d type arguments but want %d", got, want)
 		x.mode = invalid
 		x.expr = inst

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

@@ -129,7 +129,6 @@ func checkFiles(t *testing.T, filenames []string, goVersion string, colDelta uin
 	var conf Config
 	conf.GoVersion = goVersion
 	conf.AcceptMethodTypeParams = true
-	conf.InferFromConstraints = true
 	// special case for importC.src
 	if len(filenames) == 1 && strings.HasSuffix(filenames[0], "importC.src") {
 		conf.FakeImportC = true

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

@@ -11,6 +11,8 @@ import (
 	"cmd/compile/internal/syntax"
 )
 
+const useConstraintTypeInference = true
+
 // infer attempts to infer the complete set of type arguments for generic function instantiation/call
 // based on the given type parameters tparams, type arguments targs, function parameters params, and
 // function arguments args, if any. There must be at least one type parameter, no more type arguments
@@ -56,7 +58,7 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeName, targs []Type, p
 	// and types inferred via constraint type inference take precedence over types
 	// inferred from function arguments.
 	// If we have type arguments, see how far we get with constraint type inference.
-	if len(targs) > 0 && check.conf.InferFromConstraints {
+	if len(targs) > 0 && useConstraintTypeInference {
 		var index int
 		targs, index = check.inferB(tparams, targs, report)
 		if targs == nil || index < 0 {
@@ -171,7 +173,7 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeName, targs []Type, p
 	// See how far we get with constraint type inference.
 	// Note that even if we don't have any type arguments, constraint type inference
 	// may produce results for constraints that explicitly specify a type.
-	if check.conf.InferFromConstraints {
+	if useConstraintTypeInference {
 		targs, index = check.inferB(tparams, targs, report)
 		if targs == nil || index < 0 {
 			return targs
@@ -219,7 +221,7 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeName, targs []Type, p
 	}
 
 	// Again, follow up with constraint type inference.
-	if check.conf.InferFromConstraints {
+	if useConstraintTypeInference {
 		targs, index = check.inferB(tparams, targs, report)
 		if targs == nil || index < 0 {
 			return targs