go/types, types2: factor out maximum type computation

For untyped constant binary operations we need to determine the
"maximum" (untyped) type which includes both constant types.
Factor out this functionality.

Change-Id: If42bd793d38423322885a3063a4321bd56443b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/492619
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: griesemer

src/cmd/compile/internal/types2/expr.go

@@ -688,19 +688,14 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
 		return x.typ, nil, 0
 	}
+	// x is untyped
 
 	if isUntyped(target) {
 		// both x and target are untyped
-		xkind := x.typ.(*Basic).kind
-		tkind := target.(*Basic).kind
-		if isNumeric(x.typ) && isNumeric(target) {
-			if xkind < tkind {
-				return target, nil, 0
-			}
-		} else if xkind != tkind {
-			return nil, nil, InvalidUntypedConversion
+		if m := maxType(x.typ, target); m != nil {
+			return m, nil, 0
 		}
-		return x.typ, nil, 0
+		return nil, nil, InvalidUntypedConversion
 	}
 
 	if x.isNil() {

src/cmd/compile/internal/types2/predicates.go

@@ -510,3 +510,23 @@ func Default(t Type) Type {
 	}
 	return t
 }
+
+// maxType returns the "largest" type that encompasses both x and y.
+// If x and y are different untyped numeric types, the result is the type of x or y
+// that appears later in this list: integer, rune, floating-point, complex.
+// Otherwise, if x != y, the result is nil.
+func maxType(x, y Type) Type {
+	// We only care about untyped types (for now), so == is good enough.
+	// TODO(gri) investigate generalizing this function to simplify code elsewhere
+	if x == y {
+		return x
+	}
+	if isUntyped(x) && isUntyped(y) && isNumeric(x) && isNumeric(y) {
+		// untyped types are basic types
+		if x.(*Basic).kind > y.(*Basic).kind {
+			return x
+		}
+		return y
+	}
+	return nil
+}

src/go/types/expr.go

@@ -652,19 +652,14 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
 		return x.typ, nil, 0
 	}
+	// x is untyped
 
 	if isUntyped(target) {
 		// both x and target are untyped
-		xkind := x.typ.(*Basic).kind
-		tkind := target.(*Basic).kind
-		if isNumeric(x.typ) && isNumeric(target) {
-			if xkind < tkind {
-				return target, nil, 0
-			}
-		} else if xkind != tkind {
-			return nil, nil, InvalidUntypedConversion
+		if m := maxType(x.typ, target); m != nil {
+			return m, nil, 0
 		}
-		return x.typ, nil, 0
+		return nil, nil, InvalidUntypedConversion
 	}
 
 	switch u := under(target).(type) {