4 files changed, 48 insertions, 18 deletions
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 19e3b9bc98..c4e8a4696c 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/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() {
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index 4f8441467e..13a3bf8af5 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/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
+}
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 27f3c45ac6..2923d9eb36 100644
--- a/src/go/types/expr.go
+++ b/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) {
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index e09e774f2a..b821b584c1 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -512,3 +512,23 @@ 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
+}