go/types: fix a type instantiation issue, added slices.go2 example

Change-Id: If020572762039adf10973bccd072005084e17e4e

src/go/types/testdata/slices.go2

@@ -0,0 +1,63 @@
+// Package slices implements various slice algorithms.
+package slices
+
+// Map turns a []T1 to a []T2 using a mapping function.
+func Map(type T1, T2)(s []T1, f func(T1) T2) []T2 {
+	r := make([]T2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// Reduce reduces a []T1 to a single value using a reduction function.
+func Reduce(type T1, T2)(s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// Filter filters values from a slice using a filter function.
+func Filter(type T)(s []T, f func(T) bool) []T {
+	var r []T
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
+
+// Example uses
+
+func limiter(x int) byte {
+	switch {
+	case x < 0:
+		return 0
+	default:
+		return byte(x)
+	case x > 255:
+		return 255
+	}
+}
+
+var input = []int{-4, 68954, 7, 44, 0, -555, 6945}
+var limited1 = Map(int, byte)(input, limiter)
+var limited2 = Map(input, limiter) // using type inference
+
+func reducer(x float64, y int) float64 {
+	return x + float64(y)
+}
+
+var reduced1 = Reduce(int, float64)(input, 0, reducer)
+var reduced2 = Reduce(input, 1.0, reducer) // using type inference
+
+func filter(x int) bool {
+	return x&1 != 0
+}
+
+var filtered1 = Filter(int)(input, filter)
+var filtered2 = Filter(input, filter) // using type inference
+

src/go/types/testdata/tmp.go2

@@ -1,8 +1,18 @@
 package p
 
-func Ranger(type T)() (*Receiver(T)) {
-	//return &Receiver(T){}  // TODO(gri) make this work
-	return nil
+func Reduce(type T1, T2)(s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
 }
 
-type Receiver(type T) struct {}
+func reducer(float64, int) float64 {
+	return 0
+}
+
+var _ = Reduce([]int{}, 1.0, reducer)
+
+// TODO(gri) investigate - seems like type inference should accept 1 here
+// var _ = Reduce([]int{}, 1, reducer)

src/go/types/typexpr.go

@@ -143,10 +143,14 @@ func (check *Checker) definedType(e ast.Expr, def *Named) (T Type) {
 }
 
 // instantiatedType is like typ but it ensures that a Parametrized type is
-// fully instantiated.
+// fully instantiated if all type parameters are known.
+// (When we type-check a parameterized function body, parameterized types
+// whose type parameters are incoming parameters cannot be instantiated.)
 func (check *Checker) instantiatedType(e ast.Expr) Type {
 	typ := check.typ(e)
-	if ptyp, _ := typ.(*Parameterized); ptyp != nil {
+	// A parameterized type where all type arguments are known
+	// (i.e., not type parameters themselves) can be instantiated.
+	if ptyp, _ := typ.(*Parameterized); ptyp != nil && !isParameterized(ptyp) {
 		typ = check.inst(ptyp.tname, ptyp.targs)
 		// TODO(gri) can this ever be nil? comment.
 		if typ == nil {