text/template: don't evaluate '.' as a float64

When using a '.' constant literal as a reflect.Value variadic argument,
idealConstant would incorrectly result in a float64. This is because
rune literals can be represented as a float64, and contain a period,
which tricked the logic into thinking the literal must have been a
floating point number.

This also happened with other characters that can be part of a floating
point number, such as 'e' or 'P'.

To fix these edge cases, exit the case sooner if the literal was a rune,
since that should always go to the int case instead.

Finally, add test cases that verify that they behave properly. These
would error before, since eq would receive a mix of int and float64,
which aren't comparable.

Fixes #34483.

Change-Id: Icfcb7803bfa0cf317a1d1adacacad3d69a57eb42
Reviewed-on: https://go-review.googlesource.com/c/go/+/196808
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Payne <tom@airmap.com>
Reviewed-by: Rob Pike <r@golang.org>

src/text/template/exec.go

@@ -496,20 +496,29 @@ func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
 	switch {
 	case constant.IsComplex:
 		return reflect.ValueOf(constant.Complex128) // incontrovertible.
-	case constant.IsFloat && !isHexInt(constant.Text) && strings.ContainsAny(constant.Text, ".eEpP"):
+
+	case constant.IsFloat &&
+		!isHexInt(constant.Text) && !isRuneInt(constant.Text) &&
+		strings.ContainsAny(constant.Text, ".eEpP"):
 		return reflect.ValueOf(constant.Float64)
+
 	case constant.IsInt:
 		n := int(constant.Int64)
 		if int64(n) != constant.Int64 {
 			s.errorf("%s overflows int", constant.Text)
 		}
 		return reflect.ValueOf(n)
+
 	case constant.IsUint:
 		s.errorf("%s overflows int", constant.Text)
 	}
 	return zero
 }
 
+func isRuneInt(s string) bool {
+	return len(s) > 0 && s[0] == '\''
+}
+
 func isHexInt(s string) bool {
 	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
 }

src/text/template/exec_test.go

@@ -663,6 +663,12 @@ var execTests = []execTest{
 	{"bug17c", "{{len .NonEmptyInterfacePtS}}", "2", tVal, true},
 	{"bug17d", "{{index .NonEmptyInterfacePtS 0}}", "a", tVal, true},
 	{"bug17e", "{{range .NonEmptyInterfacePtS}}-{{.}}-{{end}}", "-a--b-", tVal, true},
+
+	// More variadic function corner cases. Some runes would get evaluated
+	// as constant floats instead of ints. Issue 34483.
+	{"bug18a", "{{eq . '.'}}", "true", '.', true},
+	{"bug18b", "{{eq . 'e'}}", "true", 'e', true},
+	{"bug18c", "{{eq . 'P'}}", "true", 'P', true},
 }
 
 func zeroArgs() string {