go/constant: avoid generating rats for large negative exponents

Fixes #20228

Change-Id: I1893ae3e192da01f9befe5469b2a32e534a691ba
Reviewed-on: https://go-review.googlesource.com/42592Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/constant/value.go

@@ -247,6 +247,13 @@ func makeFloatFromLiteral(lit string) Value {
 	if f, ok := newFloat().SetString(lit); ok {
 		if smallRat(f) {
 			// ok to use rationals
+			if f.Sign() == 0 {
+				// Issue 20228: If the float underflowed to zero, parse just "0".
+				// Otherwise, lit might contain a value with a large negative exponent,
+				// such as -6e-1886451601. As a float, that will underflow to 0,
+				// but it'll take forever to parse as a Rat.
+				lit = "0"
+			}
 			r, _ := newRat().SetString(lit)
 			return ratVal{r}
 		}

src/go/constant/value_test.go

@@ -244,7 +244,8 @@ var stringTests = []struct {
 	{"1e9999", "1e+9999", "0x.f8d4a9da224650a8cb2959e10d985ad92adbd44c62917e608b1f24c0e1b76b6f61edffeb15c135a4b601637315f7662f325f82325422b244286a07663c9415d2p+33216"},
 	{"1e-9999", "1e-9999", "0x.83b01ba6d8c0425eec1b21e96f7742d63c2653ed0a024cf8a2f9686df578d7b07d7a83d84df6a2ec70a921d1f6cd5574893a7eda4d28ee719e13a5dce2700759p-33215"},
 	{"2.71828182845904523536028747135266249775724709369995957496696763", "2.71828", "271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000"},
-	{"0e9999999999", "0", "0"}, // issue #16176
+	{"0e9999999999", "0", "0"},   // issue #16176
+	{"-6e-1886451601", "0", "0"}, // issue #20228
 
 	// Complex
 	{"0i", "(0 + 0i)", "(0 + 0i)"},