go/types: correctly determine if panic call refers to built-in

R=go1.11

The terminating statement check for functions that declare result
parameters was using the wrong scope to look up calls to `panic`
which in esoteric cases lead to a false positive.

Instead of looking up a panic call again at a time when correct
scope information would have to be recomputed, collect calls to
predeclared panic in a set when type-checking that call.

Fixes #23218.

Change-Id: I35eaf010e5cb8e43696efba7d77cefffb6f3deb2
Reviewed-on: https://go-review.googlesource.com/85198Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/builtins.go

@@ -470,6 +470,19 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 
 	case _Panic:
 		// panic(x)
+		// record panic call if inside a function with result parameters
+		// (for use in Checker.isTerminating)
+		if check.sig.results.Len() > 0 {
+			// function has result parameters
+			p := check.isPanic
+			if p == nil {
+				// allocate lazily
+				p = make(map[*ast.CallExpr]bool)
+				check.isPanic = p
+			}
+			p[call] = true
+		}
+
 		check.assignment(x, &emptyInterface, "argument to panic")
 		if x.mode == invalid {
 			return

src/go/types/check.go

@@ -46,6 +46,7 @@ type context struct {
 	pos           token.Pos              // if valid, identifiers are looked up as if at position pos (used by Eval)
 	iota          constant.Value         // value of iota in a constant declaration; nil otherwise
 	sig           *Signature             // function signature if inside a function; nil otherwise
+	isPanic       map[*ast.CallExpr]bool // set of panic call expressions (used for termination check)
 	hasLabel      bool                   // set if a function makes use of labels (only ~1% of functions); unused outside functions
 	hasCallOrRecv bool                   // set if an expression contains a function call or channel receive operation
 }

src/go/types/return.go

@@ -28,16 +28,10 @@ func (check *Checker) isTerminating(s ast.Stmt, label string) bool {
 		return check.isTerminating(s.Stmt, s.Label.Name)
 
 	case *ast.ExprStmt:
-		// the predeclared (possibly parenthesized) panic() function is terminating
-		if call, _ := unparen(s.X).(*ast.CallExpr); call != nil {
-			if id, _ := call.Fun.(*ast.Ident); id != nil {
-				if _, obj := check.scope.LookupParent(id.Name, token.NoPos); obj != nil {
-					if b, _ := obj.(*Builtin); b != nil && b.id == _Panic {
+		// calling the predeclared (possibly parenthesized) panic() function is terminating
+		if call, ok := unparen(s.X).(*ast.CallExpr); ok && check.isPanic[call] {
 			return true
 		}
-				}
-			}
-		}
 
 	case *ast.ReturnStmt:
 		return true

src/go/types/testdata/stmt1.src

@@ -239,3 +239,21 @@ L:	select {
 	}
 	; ; ;
 } /* ERROR "missing return" */
+
+func parenPanic() int {
+	((((((panic)))(0))))
+}
+
+func issue23218a() int {
+	{
+		panic := func(interface{}){}
+		panic(0)
+	}
+} /* ERROR "missing return" */
+
+func issue23218b() int {
+	{
+		panic := func(interface{}){}
+		((((panic))))(0)
+	}
+} /* ERROR "missing return" */