go/types: implement singleType and structure (type)

This is a clean port of CL 359015 to go/types.

Change-Id: Iea4e3bfe0a4ae0e5a9052cb6e66c01405bd57c3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/360756
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/builtins.go

@@ -83,7 +83,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		// of S and the respective parameter passing rules apply."
 		S := x.typ
 		var T Type
-		if s, _ := singleUnder(S).(*Slice); s != nil {
+		if s, _ := structure(S).(*Slice); s != nil {
 			T = s.elem
 		} else {
 			check.invalidArg(x, _InvalidAppend, "%s is not a slice", x)
@@ -332,14 +332,14 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 
 	case _Copy:
 		// copy(x, y []T) int
-		dst, _ := singleUnder(x.typ).(*Slice)
+		dst, _ := structure(x.typ).(*Slice)
 
 		var y operand
 		arg(&y, 1)
 		if y.mode == invalid {
 			return
 		}
-		src, _ := singleUnderString(y.typ).(*Slice)
+		src, _ := structureString(y.typ).(*Slice)
 
 		if dst == nil || src == nil {
 			check.invalidArg(x, _InvalidCopy, "copy expects slice arguments; found %s and %s", x, &y)
@@ -473,7 +473,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		}
 
 		var min int // minimum number of arguments
-		switch singleUnder(T).(type) {
+		switch structure(T).(type) {
 		case *Slice:
 			min = 2
 		case *Map, *Chan:
@@ -776,11 +776,11 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 	return true
 }
 
-// If typ is a type parameter, single under returns the single underlying
-// type of all types in the corresponding type constraint if it exists, or
-// nil if it doesn't exist. If typ is not a type parameter, singleUnder
-// just returns the underlying type.
-func singleUnder(typ Type) Type {
+// If typ is a type parameter, structure returns the single underlying
+// type of all types in the corresponding type constraint if it exists,
+// or nil otherwise. If typ is not a type parameter, structure returns
+// the underlying type.
+func structure(typ Type) Type {
 	var su Type
 	if underIs(typ, func(u Type) bool {
 		if su != nil && !Identical(su, u) {
@@ -795,10 +795,10 @@ func singleUnder(typ Type) Type {
 	return nil
 }
 
-// singleUnderString is like singleUnder but also considers []byte and
-// string as "identical". In this case, if successful, the result is always
-// []byte.
-func singleUnderString(typ Type) Type {
+// structureString is like structure but also considers []byte and
+// string as "identical". In this case, if successful, the result
+// is always []byte.
+func structureString(typ Type) Type {
 	var su Type
 	if underIs(typ, func(u Type) bool {
 		if isString(u) {

src/go/types/call.go

@@ -175,7 +175,7 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 	cgocall := x.mode == cgofunc
 
 	// a type parameter may be "called" if all types have the same signature
-	sig, _ := singleUnder(x.typ).(*Signature)
+	sig, _ := structure(x.typ).(*Signature)
 	if sig == nil {
 		check.invalidOp(x, _InvalidCall, "cannot call non-function %s", x)
 		x.mode = invalid

src/go/types/expr.go

@@ -1227,7 +1227,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 			goto Error
 		}
 
-		switch utyp := singleUnder(base).(type) {
+		switch utyp := structure(base).(type) {
 		case *Struct:
 			if len(e.Elts) == 0 {
 				break

src/go/types/index.go

@@ -207,7 +207,7 @@ func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 
 	valid := false
 	length := int64(-1) // valid if >= 0
-	switch u := singleUnder(x.typ).(type) {
+	switch u := structure(x.typ).(type) {
 	case nil:
 		check.errorf(x, _NonSliceableOperand, "cannot slice %s: type set has no single underlying type", x)
 		x.mode = invalid

src/go/types/infer.go

@@ -358,7 +358,7 @@ func (w *tpWalker) isParameterizedTypeList(list []Type) bool {
 func (check *Checker) inferB(tparams []*TypeParam, targs []Type) (types []Type, index int) {
 	assert(len(tparams) >= len(targs) && len(targs) > 0)
 
-	// Setup bidirectional unification between those structural bounds
+	// Setup bidirectional unification between constraints
 	// and the corresponding type arguments (which may be nil!).
 	u := newUnifier(false)
 	u.x.init(tparams)
@@ -371,11 +371,16 @@ func (check *Checker) inferB(tparams []*TypeParam, targs []Type) (types []Type,
 		}
 	}
 
-	// Unify type parameters with their structural constraints, if any.
+	// If a constraint has a structural type, unify the corresponding type parameter with it.
 	for _, tpar := range tparams {
 		typ := tpar
-		sbound := typ.structuralType()
+		sbound := structure(tpar)
 		if sbound != nil {
+			// If the structural type is the underlying type of a single
+			// defined type in the constraint, use that defined type instead.
+			if named, _ := tpar.singleType().(*Named); named != nil {
+				sbound = named
+			}
 			if !u.unify(typ, sbound) {
 				check.errorf(tpar.obj, _Todo, "%s does not match %s", tpar.obj, sbound)
 				return nil, 0
@@ -384,7 +389,7 @@ func (check *Checker) inferB(tparams []*TypeParam, targs []Type) (types []Type,
 	}
 
 	// u.x.types() now contains the incoming type arguments plus any additional type
-	// arguments for which there were structural constraints. The newly inferred non-
+	// arguments which were inferred from structural types. The newly inferred non-
 	// nil entries may still contain references to other type parameters.
 	// For instance, for [A any, B interface{ []C }, C interface{ *A }], if A == int
 	// was given, unification produced the type list [int, []C, *A]. We eliminate the

src/go/types/stmt.go

@@ -834,7 +834,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		if x.mode != invalid {
 			// Ranging over a type parameter is permitted if it has a single underlying type.
 			var cause string
-			u := singleUnder(x.typ)
+			u := structure(x.typ)
 			switch t := u.(type) {
 			case nil:
 				cause = "type set has no single underlying type"

src/go/types/termlist.go

@@ -93,8 +93,8 @@ func (xl termlist) norm() termlist {
 }
 
 // If the type set represented by xl is specified by a single (non-𝓤) term,
-// structuralType returns that type. Otherwise it returns nil.
-func (xl termlist) structuralType() Type {
+// singleType returns that type. Otherwise it returns nil.
+func (xl termlist) singleType() Type {
 	if nl := xl.norm(); len(nl) == 1 {
 		return nl[0].typ // if nl.isAll() then typ is nil, which is ok
 	}

src/go/types/termlist_test.go

@@ -106,7 +106,7 @@ func TestTermlistNorm(t *testing.T) {
 	}
 }
 
-func TestTermlistStructuralType(t *testing.T) {
+func TestTermlistSingleType(t *testing.T) {
 	// helper to deal with nil types
 	tstring := func(typ Type) string {
 		if typ == nil {
@@ -128,9 +128,9 @@ func TestTermlistStructuralType(t *testing.T) {
 		"∅ ∪ ~int ∪ string": "nil",
 	} {
 		xl := maketl(test)
-		got := tstring(xl.structuralType())
+		got := tstring(xl.singleType())
 		if got != want {
-			t.Errorf("(%v).structuralType() == %v; want %v", test, got, want)
+			t.Errorf("(%v).singleType() == %v; want %v", test, got, want)
 		}
 	}
 }

src/go/types/testdata/examples/inference.go2

@@ -99,3 +99,26 @@ func _() {
 	related2(1.0, []int{})
 	related2 /* ERROR does not satisfy */ (float64(1.0), []int{})
 }
+
+type List[P any] []P
+
+func related3[Elem any, Slice []Elem | List[Elem]]() Slice { return nil }
+
+func _() {
+	// related3 can be instantiated explicitly
+	related3[int, []int]()
+	related3[byte, List[byte]]()
+
+	// Alternatively, the 2nd type argument can be inferred
+	// from the first one through constraint type inference.
+	related3[int]()
+
+	// The inferred type is the structural type of the Slice
+	// type parameter.
+	var _ []int = related3[int]()
+
+	// It is not the defined parameterized type List.
+	type anotherList []float32
+	var _ anotherList = related3[float32]() // valid
+	var _ anotherList = related3 /* ERROR cannot use .* \(value of type List\[float32\]\) as anotherList */ [float32, List[float32]]()
+}

src/go/types/typeparam.go

@@ -118,9 +118,9 @@ func (t *TypeParam) iface() *Interface {
 	return ityp
 }
 
-// structuralType returns the structural type of the type parameter's constraint; or nil.
-func (t *TypeParam) structuralType() Type {
-	return t.iface().typeSet().structuralType()
+// singleType returns the single type of the type parameter constraint; or nil.
+func (t *TypeParam) singleType() Type {
+	return t.iface().typeSet().singleType()
 }
 
 // hasTerms reports whether the type parameter constraint has specific type terms.

src/go/types/typeset.go

@@ -102,8 +102,8 @@ func (s *_TypeSet) String() string {
 // hasTerms reports whether the type set has specific type terms.
 func (s *_TypeSet) hasTerms() bool { return !s.terms.isEmpty() && !s.terms.isAll() }
 
-// structuralType returns the single type in s if there is exactly one; otherwise the result is nil.
-func (s *_TypeSet) structuralType() Type { return s.terms.structuralType() }
+// singleType returns the single type in s if there is exactly one; otherwise the result is nil.
+func (s *_TypeSet) singleType() Type { return s.terms.singleType() }
 
 // includes reports whether t ∈ s.
 func (s *_TypeSet) includes(t Type) bool { return s.terms.includes(t) }