go/types: make sure constants valid in integer operations are in integer form

The operation where this manifested in a crash was % (only defined on integers).
However, the existing code was sloppy in that it didn't retain the integer form
after a value (e.g., 3.0) was accepted as representable in integer form (3 for
the example). We would have seen a crash in such cases for / as well except
that there was code to fix it for just that case.

Remove the special code for / and fix more generally by retaining the integer
form for all operations if applicable.

Fixes #14229.

Change-Id: I8bef769e6299839fade27c6e8b5ff29ad6521d0d
Reviewed-on: https://go-review.googlesource.com/19300Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/expr.go

@@ -184,7 +184,8 @@ func roundFloat64(x constant.Value) constant.Value {
 // provided (only needed for int/uint sizes).
 //
 // If rounded != nil, *rounded is set to the rounded value of x for
-// representable floating-point values; it is left alone otherwise.
+// representable floating-point and complex values, and to an Int
+// value for integer values; it is left alone otherwise.
 // It is ok to provide the addressof the first argument for rounded.
 func representableConst(x constant.Value, conf *Config, typ *Basic, rounded *constant.Value) bool {
 	if x.Kind() == constant.Unknown {
@@ -197,6 +198,9 @@ func representableConst(x constant.Value, conf *Config, typ *Basic, rounded *con
 		if x.Kind() != constant.Int {
 			return false
 		}
+		if rounded != nil {
+			*rounded = x
+		}
 		if x, ok := constant.Int64Val(x); ok {
 			switch typ.kind {
 			case Int:
@@ -808,8 +812,6 @@ func (check *Checker) binary(x *operand, e *ast.BinaryExpr, lhs, rhs ast.Expr, o
 		typ := x.typ.Underlying().(*Basic)
 		// force integer division of integer operands
 		if op == token.QUO && isInteger(typ) {
-			xval = constant.ToInt(xval)
-			yval = constant.ToInt(yval)
 			op = token.QUO_ASSIGN
 		}
 		x.val = constant.BinaryOp(xval, op, yval)

src/go/types/testdata/issues.src

@@ -153,3 +153,20 @@ func issue10260() {
 	make(chan I1) <- i0 /* ERROR cannot use .* in send: missing method foo */
 	make(chan I1) <- i2 /* ERROR cannot use .* in send: wrong type for method foo */
 }
+
+// Check that constants representable as integers are in integer form
+// before being used in operations that are only defined on integers.
+func issue14229() {
+	// from the issue
+	const _ = int64(-1<<63) % 1e6
+
+	// related
+	const (
+		a int = 3
+		b = 4.0
+		_ = a / b
+		_ = a % b
+		_ = b / a
+		_ = b % a
+	)
+}