cmd/compile, go/types: fix checking of bad type switch

Consider the following program:

	package p

	func f() {
		x := 1
		v := 2
		switch v.(type) {
		case int:
			println(x)
			println(x / 0)
		case 1:
		}
	}

Before this CL, the compiler prints:

	x.go:4:2: x declared but not used
	x.go:6:9: v (variable of type int) is not an interface

x is in fact used, and other errors in the switch go undiagnosed.

This commit fixes that problem by processing the switch statement
even when the 'not an interface' error is reported.

Now the compiler drops the spurious 'declared but not used'
and adds two previously undiagnosed problems:

	x.go:6:9: v (variable of type int) is not an interface
	x.go:9:15: invalid operation: division by zero
	x.go:10:7: 1 is not a type

go/types was printing roughly the same thing the compiler did before,
and now still prints roughly the same thing the compiler does after.
(The only differences are in the exact reported columns.)

Fixes #50493.

Change-Id: I317883f29077b1b4bbd0e8793617fd3bb31aa0f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/379117
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -305,7 +305,7 @@ L:
 			}
 		}
 		seen[T] = e
-		if T != nil {
+		if T != nil && xtyp != nil {
 			check.typeAssertion(e, x, xtyp, T, true)
 		}
 	}
@@ -733,15 +733,16 @@ func (check *Checker) typeSwitchStmt(inner stmtContext, s *syntax.SwitchStmt, gu
 	if x.mode == invalid {
 		return
 	}
+
 	// TODO(gri) we may want to permit type switches on type parameter values at some point
+	var xtyp *Interface
 	if isTypeParam(x.typ) {
 		check.errorf(&x, "cannot use type switch on type parameter value %s", &x)
-		return
-	}
-	xtyp, _ := under(x.typ).(*Interface)
-	if xtyp == nil {
-		check.errorf(&x, "%s is not an interface", &x)
-		return
+	} else {
+		xtyp, _ = under(x.typ).(*Interface)
+		if xtyp == nil {
+			check.errorf(&x, "%s is not an interface", &x)
+		}
 	}
 
 	check.multipleSwitchDefaults(s.Body)

src/cmd/compile/internal/types2/testdata/check/stmt0.src

@@ -695,7 +695,7 @@ func typeswitches() {
 		_ = y
 	}
 
-	switch x := i /* ERROR "not an interface" */ .(type) {}
+	switch /* ERROR "x declared but not used" */ x := i /* ERROR "not an interface" */ .(type) {}
 
 	switch t := x.(type) {
 	case nil:
@@ -719,6 +719,18 @@ func typeswitches() {
 	case T2 /* ERROR "wrong type for method m" */ :
 	case I2 /* STRICT "wrong type for method m" */ : // only an error in strict mode (issue 8561)
 	}
+
+
+	{
+		x := 1
+		v := 2
+		switch v /* ERROR "v [(]variable of type int[)] is not an interface" */ .(type) {
+		case int:
+			println(x)
+			println(x / /* ERROR "invalid operation: division by zero" */ 0)
+		case /* ERROR "1 is not a type" */ 1:
+		}
+	}
 }
 
 // Test that each case clause uses the correct type of the variable

src/go/types/stmt.go

@@ -310,7 +310,7 @@ L:
 			}
 		}
 		seen[T] = e
-		if T != nil {
+		if T != nil && xtyp != nil {
 			check.typeAssertion(e, x, xtyp, T)
 		}
 	}
@@ -686,14 +686,14 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			return
 		}
 		// TODO(gri) we may want to permit type switches on type parameter values at some point
+		var xtyp *Interface
 		if isTypeParam(x.typ) {
 			check.errorf(&x, _InvalidTypeSwitch, "cannot use type switch on type parameter value %s", &x)
-			return
-		}
-		xtyp, _ := under(x.typ).(*Interface)
-		if xtyp == nil {
-			check.errorf(&x, _InvalidTypeSwitch, "%s is not an interface", &x)
-			return
+		} else {
+			xtyp, _ = under(x.typ).(*Interface)
+			if xtyp == nil {
+				check.errorf(&x, _InvalidTypeSwitch, "%s is not an interface", &x)
+			}
 		}
 
 		check.multipleDefaults(s.Body.List)

src/go/types/testdata/check/stmt0.src

@@ -695,7 +695,7 @@ func typeswitches() {
 		_ = y
 	}
 
-	switch x := i /* ERROR "not an interface" */ .(type) {}
+	switch x /* ERROR "x declared but not used" */ := i /* ERROR "not an interface" */ .(type) {}
 
 	switch t := x.(type) {
 	case nil:
@@ -719,6 +719,18 @@ func typeswitches() {
 	case T2 /* ERROR "wrong type for method m" */ :
 	case I2 /* STRICT "wrong type for method m" */ : // only an error in strict mode (issue 8561)
 	}
+
+
+	{
+		x := 1
+		v := 2
+		switch v /* ERROR "v [(]variable of type int[)] is not an interface" */ .(type) {
+		case int:
+			println(x)
+			println(x / 0 /* ERROR "invalid operation: division by zero" */)
+		case 1 /* ERROR "expected type, found 1" */:
+		}
+	}
 }
 
 // Test that each case clause uses the correct type of the variable