go/types: convert untyped switch expressions to default type

R=1.6

Fixes #11667.
Fixes #11687.

Change-Id: I060db212e8e0ee35fdefb4d482398f8f71650b38
Reviewed-on: https://go-review.googlesource.com/12713Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/expr.go

@@ -1462,9 +1462,9 @@ func (check *Checker) expr(x *operand, e ast.Expr) {
 	x.mode = invalid
 }
 
-// exprWithHint typechecks expression e and initializes x with the expression value.
+// exprWithHint typechecks expression e and initializes x with the expression value;
+// hint is the type of a composite literal element.
 // If an error occurred, x.mode is set to invalid.
-// If hint != nil, it is the type of a composite literal element.
 //
 func (check *Checker) exprWithHint(x *operand, e ast.Expr, hint Type) {
 	assert(hint != nil)

src/go/types/stmt.go

@@ -155,25 +155,20 @@ func (check *Checker) suspendedCall(keyword string, call *ast.CallExpr) {
 	check.errorf(x.pos(), "%s %s %s", keyword, msg, &x)
 }
 
-func (check *Checker) caseValues(x operand /* copy argument (not *operand!) */, values []ast.Expr) {
+func (check *Checker) caseValues(x *operand, values []ast.Expr) {
 	// No duplicate checking for now. See issue 4524.
 	for _, e := range values {
-		var y operand
-		check.expr(&y, e)
-		if y.mode == invalid {
-			return
-		}
-		// TODO(gri) The convertUntyped call pair below appears in other places. Factor!
-		// Order matters: By comparing y against x, error positions are at the case values.
-		check.convertUntyped(&y, x.typ)
-		if y.mode == invalid {
-			return
+		var v operand
+		check.expr(&v, e)
+		if x.mode == invalid || v.mode == invalid {
+			continue
 		}
-		check.convertUntyped(&x, y.typ)
-		if x.mode == invalid {
-			return
+		check.convertUntyped(&v, x.typ)
+		if v.mode == invalid {
+			continue
 		}
-		check.comparison(&y, &x, token.EQL)
+		// Order matters: By comparing v against x, error positions are at the case values.
+		check.comparison(&v, x, token.EQL)
 	}
 }
 
@@ -399,6 +394,9 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		var x operand
 		if s.Tag != nil {
 			check.expr(&x, s.Tag)
+			// By checking assignment of x to an invisible temporary
+			// (as a compiler would), we get all the relevant checks.
+			check.assignment(&x, nil)
 		} else {
 			// spec: "A missing switch expression is
 			// equivalent to the boolean value true."
@@ -416,9 +414,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				check.invalidAST(c.Pos(), "incorrect expression switch case")
 				continue
 			}
-			if x.mode != invalid {
-				check.caseValues(x, clause.List)
-			}
+			check.caseValues(&x, clause.List)
 			check.openScope(clause, "case")
 			inner := inner
 			if i+1 < len(s.Body.List) {

src/go/types/testdata/stmt0.src

@@ -512,6 +512,65 @@ func switches1() {
 	}
 }
 
+func switches2() {
+	// untyped nil is not permitted as switch expression
+	switch nil /* ERROR "use of untyped nil" */ {
+	case 1, 2, "foo": // don't report additional errors here
+	}
+
+	// untyped constants are converted to default types
+	switch 1<<63-1 {
+	}
+	switch 1 /* ERROR "overflows int" */ << 63 {
+	}
+	var x int
+	switch 1.0 {
+	case 1.0, 2.0, x /* ERROR "mismatched types int and float64" */ :
+	}
+	switch x {
+	case 1.0:
+	}
+
+	// untyped bools become of type bool
+	type B bool
+	var b B = true
+	switch x == x {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+	switch {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+}
+
+func issue11667() {
+	switch 9223372036854775808 /* ERROR "overflows int" */ {
+	}
+	switch 9223372036854775808 /* ERROR "overflows int" */ {
+	case 9223372036854775808:
+	}
+	var x int
+	switch x {
+	case 9223372036854775808 /* ERROR "overflows int" */ :
+	}
+	var y float64
+	switch y {
+	case 9223372036854775808:
+	}
+}
+
+func issue11687() {
+	f := func() (_, _ int) { return }
+	switch f /* ERROR "2-valued expression" */ () {
+	}
+	var x int
+	switch f /* ERROR "2-valued expression" */ () {
+	case x:
+	}
+	switch x {
+	case f /* ERROR "cannot compare" */ (): // TODO(gri) better error message (issue 11896)
+	}
+}
+
 type I interface {
 	m()
 }