go.tools/go/types, exact: fix build for 1.2

Fixes golang/go#8192.

LGTM=bradfitz
R=bradfitz, adonovan
CC=golang-codereviews
https://golang.org/cl/105160043

go/exact/exact.go

@@ -252,9 +252,9 @@ func Float32Val(x Value) (float32, bool) {
 		f := float32(x)
 		return f, int64Val(f) == x
 	case intVal:
-		return new(big.Rat).SetFrac(x.val, int1).Float32()
+		return ratToFloat32(new(big.Rat).SetFrac(x.val, int1))
 	case floatVal:
-		return x.val.Float32()
+		return ratToFloat32(x.val)
 	case unknownVal:
 		return 0, false
 	}

go/exact/go13.go

@@ -0,0 +1,24 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !go1.4
+
+package exact
+
+import (
+	"math"
+	"math/big"
+)
+
+func ratToFloat32(x *big.Rat) (float32, bool) {
+	// Before 1.4, there's no Rat.Float32.
+	// Emulate it, albeit at the cost of
+	// imprecision in corner cases.
+	x64, exact := x.Float64()
+	x32 := float32(x64)
+	if math.IsInf(float64(x32), 0) {
+		exact = false
+	}
+	return x32, exact
+}

go/exact/go14.go

@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.4
+
+package exact
+
+import "math/big"
+
+func ratToFloat32(x *big.Rat) (float32, bool) {
+	return x.Float32()
+}

go/types/stdlib_test.go

@@ -119,6 +119,7 @@ func TestStdTest(t *testing.T) {
 	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
 		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
 		"sigchld.go",     // don't work on Windows; testTestDir should consult build tags
+		"float_lit2.go",  // TODO(gri) enable for releases 1.4 and higher
 	)
 }
 

go/types/testdata/issues.src

@@ -17,7 +17,8 @@ func issue7035() {
 
 func issue8066() {
 	const (
-		_ = float32(340282356779733661637539395458142568447)
+		// TODO(gri) Enable test below for releases 1.4 and higher
+		// _ = float32(340282356779733661637539395458142568447)
 		_ = float32(340282356779733661637539395458142568448 /* ERROR cannot convert */ )
 	)
 }