go.tools/go/types: fix float32 conversions

Pending CL 93550043.
For submission after the 1.3 release.

Fixes golang/go#8066.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/95580045

go/exact/exact.go

@@ -245,6 +245,22 @@ func Uint64Val(x Value) (uint64, bool) {
 	panic(fmt.Sprintf("%v not an Int", x))
 }
 
+// Float32Val is like Float64Val but for float32 instead of float64.
+func Float32Val(x Value) (float32, bool) {
+	switch x := x.(type) {
+	case int64Val:
+		f := float32(x)
+		return f, int64Val(f) == x
+	case intVal:
+		return new(big.Rat).SetFrac(x.val, int1).Float32()
+	case floatVal:
+		return x.val.Float32()
+	case unknownVal:
+		return 0, false
+	}
+	panic(fmt.Sprintf("%v not a Float", x))
+}
+
 // Float64Val returns the nearest Go float64 value of x and whether the result is exact;
 // x must be numeric but not Complex, or Unknown. For values too small (too close to 0)
 // to represent as float64, Float64Val silently underflows to 0. The result sign always

go/types/expr.go

@@ -149,21 +149,14 @@ func isComparison(op token.Token) bool {
 }
 
 func fitsFloat32(x exact.Value) bool {
-	f, _ := exact.Float64Val(x)
-	// spec: "In all non-constant conversions involving floating-point
-	// or complex values, if the result type cannot represent the value
-	// the conversion succeeds but the result value is implementation-
-	// dependent."
-	//
-	// We assume that float32(f) returns an Inf if f is too large for
-	// a float32, or if f is an Inf; and that it returns 0 for values
-	// with too small a magnitude.
-	return !math.IsInf(float64(float32(f)), 0)
+	f32, _ := exact.Float32Val(x)
+	f := float64(f32)
+	return !math.IsInf(f, 0)
 }
 
 func roundFloat32(x exact.Value) exact.Value {
-	f, _ := exact.Float64Val(x)
-	f = float64(float32(f))
+	f32, _ := exact.Float32Val(x)
+	f := float64(f32)
 	if !math.IsInf(f, 0) {
 		return exact.MakeFloat64(f)
 	}

go/types/stdlib_test.go

@@ -119,7 +119,6 @@ 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) float32 constant conversion requires (missing) Rat.Float32
 	)
 }
 

go/types/testdata/issues.src

@@ -14,3 +14,10 @@ func issue7035() {
 	fmt := new(T)
 	_ = fmt.X
 }
+
+func issue8066() {
+	const (
+		_ = float32(340282356779733661637539395458142568447)
+		_ = float32(340282356779733661637539395458142568448 /* ERROR cannot convert */ )
+	)
+}