cmd/compile/internal/types2: use an identifier map rather than isubst for recv type params

This is a port of CL 354643 from go/types to types2 with adjustments:
- use of syntax rather than go/ast package as needed
- adjustments due to the different code for type parameter declarations
- rename of Checker.rparamMap to Checker.recvTParamMap, which seems clearer

Change-Id: I5311a0c05a13c6b87ea1422b250b90c3d05c5dce
Reviewed-on: https://go-review.googlesource.com/c/go/+/354693
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -100,9 +100,10 @@ type Checker struct {
 	// information collected during type-checking of a set of package files
 	// (initialized by Files, valid only for the duration of check.Files;
 	// maps and lists are allocated on demand)
-	files        []*syntax.File            // list of package files
-	imports      []*PkgName                // list of imported packages
-	dotImportMap map[dotImportKey]*PkgName // maps dot-imported objects to the package they were dot-imported through
+	files         []*syntax.File              // list of package files
+	imports       []*PkgName                  // list of imported packages
+	dotImportMap  map[dotImportKey]*PkgName   // maps dot-imported objects to the package they were dot-imported through
+	recvTParamMap map[*syntax.Name]*TypeParam // maps blank receiver type parameters to their type
 
 	firstErr error                    // first error encountered
 	methods  map[*TypeName][]*Func    // maps package scope type names to associated non-blank (non-interface) methods
@@ -292,6 +293,7 @@ func (check *Checker) checkFiles(files []*syntax.File) (err error) {
 	check.dotImportMap = nil
 	check.pkgPathMap = nil
 	check.seenPkgMap = nil
+	check.recvTParamMap = nil
 
 	// TODO(gri) There's more memory we should release at this point.
 

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

@@ -4,10 +4,7 @@
 
 package types2
 
-import (
-	"cmd/compile/internal/syntax"
-	"fmt"
-)
+import "cmd/compile/internal/syntax"
 
 // ----------------------------------------------------------------------------
 // API
@@ -105,39 +102,33 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 	sig.scope = check.scope
 	defer check.closeScope()
 
-	var recvTyp syntax.Expr // rewritten receiver type; valid if != nil
 	if recvPar != nil {
 		// collect generic receiver type parameters, if any
 		// - a receiver type parameter is like any other type parameter, except that it is declared implicitly
 		// - the receiver specification acts as local declaration for its type parameters, which may be blank
 		_, rname, rparams := check.unpackRecv(recvPar.Type, true)
 		if len(rparams) > 0 {
-			// Blank identifiers don't get declared and regular type-checking of the instantiated
-			// parameterized receiver type expression fails in Checker.collectParams of receiver.
-			// Identify blank type parameters and substitute each with a unique new identifier named
-			// "n_" (where n is the parameter index) and which cannot conflict with any user-defined
-			// name.
-			var smap map[*syntax.Name]*syntax.Name // substitution map from "_" to "!n" identifiers
+			tparams := make([]*TypeParam, len(rparams))
+			for i, rparam := range rparams {
+				tparams[i] = check.declareTypeParam(rparam)
+			}
+			sig.rparams = bindTParams(tparams)
+			// Blank identifiers don't get declared, so naive type-checking of the
+			// receiver type expression would fail in Checker.collectParams below,
+			// when Checker.ident cannot resolve the _ to a type.
+			//
+			// Checker.recvTParamMap maps these blank identifiers to their type parameter
+			// types, so that they may be resolved in Checker.ident when they fail
+			// lookup in the scope.
 			for i, p := range rparams {
 				if p.Value == "_" {
-					new := *p
-					new.Value = fmt.Sprintf("%d_", i)
-					rparams[i] = &new // use n_ identifier instead of _ so it can be looked up
-					if smap == nil {
-						smap = make(map[*syntax.Name]*syntax.Name)
+					tpar := sig.rparams.At(i)
+					if check.recvTParamMap == nil {
+						check.recvTParamMap = make(map[*syntax.Name]*TypeParam)
 					}
-					smap[p] = &new
+					check.recvTParamMap[p] = tpar
 				}
 			}
-			if smap != nil {
-				// blank identifiers were found => use rewritten receiver type
-				recvTyp = isubst(recvPar.Type, smap)
-			}
-			rlist := make([]*TypeParam, len(rparams))
-			for i, rparam := range rparams {
-				rlist[i] = check.declareTypeParam(rparam)
-			}
-			sig.rparams = bindTParams(rlist)
 			// determine receiver type to get its type parameters
 			// and the respective type parameter bounds
 			var recvTParams []*TypeParam
@@ -186,10 +177,10 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 	scope := NewScope(check.scope, nopos, nopos, "function body (temp. scope)")
 	var recvList []*Var // TODO(gri) remove the need for making a list here
 	if recvPar != nil {
-		recvList, _ = check.collectParams(scope, []*syntax.Field{recvPar}, recvTyp, false) // use rewritten receiver type, if any
+		recvList, _ = check.collectParams(scope, []*syntax.Field{recvPar}, false) // use rewritten receiver type, if any
 	}
-	params, variadic := check.collectParams(scope, ftyp.ParamList, nil, true)
-	results, _ := check.collectParams(scope, ftyp.ResultList, nil, false)
+	params, variadic := check.collectParams(scope, ftyp.ParamList, true)
+	results, _ := check.collectParams(scope, ftyp.ResultList, false)
 	scope.Squash(func(obj, alt Object) {
 		var err error_
 		err.errorf(obj, "%s redeclared in this block", obj.Name())
@@ -281,8 +272,8 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 }
 
 // collectParams declares the parameters of list in scope and returns the corresponding
-// variable list. If type0 != nil, it is used instead of the first type in list.
-func (check *Checker) collectParams(scope *Scope, list []*syntax.Field, type0 syntax.Expr, variadicOk bool) (params []*Var, variadic bool) {
+// variable list.
+func (check *Checker) collectParams(scope *Scope, list []*syntax.Field, variadicOk bool) (params []*Var, variadic bool) {
 	if list == nil {
 		return
 	}
@@ -296,9 +287,6 @@ func (check *Checker) collectParams(scope *Scope, list []*syntax.Field, type0 sy
 		// type-check type of grouped fields only once
 		if ftype != prev {
 			prev = ftype
-			if i == 0 && type0 != nil {
-				ftype = type0
-			}
 			if t, _ := ftype.(*syntax.DotsType); t != nil {
 				ftype = t.Elem
 				if variadicOk && i == len(list)-1 {
@@ -348,61 +336,3 @@ func (check *Checker) collectParams(scope *Scope, list []*syntax.Field, type0 sy
 
 	return
 }
-
-// isubst returns an x with identifiers substituted per the substitution map smap.
-// isubst only handles the case of (valid) method receiver type expressions correctly.
-func isubst(x syntax.Expr, smap map[*syntax.Name]*syntax.Name) syntax.Expr {
-	switch n := x.(type) {
-	case *syntax.Name:
-		if alt := smap[n]; alt != nil {
-			return alt
-		}
-	// case *syntax.StarExpr:
-	// 	X := isubst(n.X, smap)
-	// 	if X != n.X {
-	// 		new := *n
-	// 		new.X = X
-	// 		return &new
-	// 	}
-	case *syntax.Operation:
-		if n.Op == syntax.Mul && n.Y == nil {
-			X := isubst(n.X, smap)
-			if X != n.X {
-				new := *n
-				new.X = X
-				return &new
-			}
-		}
-	case *syntax.IndexExpr:
-		Index := isubst(n.Index, smap)
-		if Index != n.Index {
-			new := *n
-			new.Index = Index
-			return &new
-		}
-	case *syntax.ListExpr:
-		var elems []syntax.Expr
-		for i, elem := range n.ElemList {
-			new := isubst(elem, smap)
-			if new != elem {
-				if elems == nil {
-					elems = make([]syntax.Expr, len(n.ElemList))
-					copy(elems, n.ElemList)
-				}
-				elems[i] = new
-			}
-		}
-		if elems != nil {
-			new := *n
-			new.ElemList = elems
-			return &new
-		}
-	case *syntax.ParenExpr:
-		return isubst(n.X, smap) // no need to keep parentheses
-	default:
-		// Other receiver type expressions are invalid.
-		// It's fine to ignore those here as they will
-		// be checked elsewhere.
-	}
-	return x
-}

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

@@ -28,7 +28,15 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 	switch obj {
 	case nil:
 		if e.Value == "_" {
-			check.error(e, "cannot use _ as value or type")
+			// Blank identifiers are never declared, but the current identifier may
+			// be a placeholder for a receiver type parameter. In this case we can
+			// resolve its type and object from Checker.recvTParamMap.
+			if tpar := check.recvTParamMap[e]; tpar != nil {
+				x.mode = typexpr
+				x.typ = tpar
+			} else {
+				check.error(e, "cannot use _ as value or type")
+			}
 		} else {
 			if check.conf.CompilerErrorMessages {
 				check.errorf(e, "undefined: %s", e.Value)