cmd/compile: better errors for float constants with large exponents

Also: Removed misleading comment.

Fixes #20232.

Change-Id: I0b141b1360ac53267b7ebfcec7a2e2a238f3f46c
Reviewed-on: https://go-review.googlesource.com/42930
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/mpfloat.go

@@ -176,25 +176,14 @@ func (a *Mpflt) Neg() {
 	}
 }
 
-//
-// floating point input
-// required syntax is [+-]d*[.]d*[e[+-]d*] or [+-]0xH*[e[+-]d*]
-//
 func (a *Mpflt) SetString(as string) {
 	for len(as) > 0 && (as[0] == ' ' || as[0] == '\t') {
 		as = as[1:]
 	}
 
-	f, ok := a.Val.SetString(as)
-	if !ok {
-		// At the moment we lose precise error cause;
-		// the old code additionally distinguished between:
-		// - malformed hex constant
-		// - decimal point in hex constant
-		// - constant exponent out of range
-		// - decimal point and binary point in constant
-		// TODO(gri) use different conversion function or check separately
-		yyerror("malformed constant: %s", as)
+	f, _, err := a.Val.Parse(as, 10)
+	if err != nil {
+		yyerror("malformed constant: %s (%v)", as, err)
 		a.Val.SetFloat64(0)
 		return
 	}

src/go/types/stdlib_test.go

@@ -159,6 +159,7 @@ func TestStdFixed(t *testing.T) {
 		"issue16369.go", // go/types handles this correctly - not an issue
 		"issue18459.go", // go/types doesn't check validity of //go:xxx directives
 		"issue18882.go", // go/types doesn't check validity of //go:xxx directives
+		"issue20232.go", // go/types handles larger constants than gc
 	)
 }
 

test/fixedbugs/issue20232.go

@@ -0,0 +1,11 @@
+// errorcheck
+
+// Copyright 2017 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.
+
+package main
+
+const _ = 6e5518446744 // ERROR "malformed constant: 6e5518446744 \(exponent overflow\)"
+const _ = 1e-1000000000
+const _ = 1e+1000000000 // ERROR "constant too large"