go.tools/ssa/interp: fix bug: recovered panic without NRPs should return zero values.

+ test.

R=gri
CC=golang-dev
https://golang.org/cl/38040043

ssa/interp/interp.go

@@ -532,9 +532,13 @@ func callSSA(i *interpreter, caller *frame, callpos token.Pos, fn *ssa.Function,
 // After a normal return, fr.result contains the result of the call
 // and fr.block is nil.
 //
-// After a recovered panic, fr.result is undefined and fr.block
-// contains the block at which to resume control, which may be
-// nil for a normal return.
+// A recovered panic in a function without named return parameters
+// (NRPs) becomes a normal return of the zero value of the function's
+// result type.
+//
+// After a recovered panic in a function with NRPs, fr.result is
+// undefined and fr.block contains the block at which to resume
+// control.
 //
 func runFrame(fr *frame) {
 	defer func() {
@@ -550,7 +554,10 @@ func runFrame(fr *frame) {
 			fmt.Fprintf(os.Stderr, "Panicking: %T %v.\n", fr.panic, fr.panic)
 		}
 		fr.runDefers()
-		fr.block = fr.fn.Recover // recovered panic
+		fr.block = fr.fn.Recover
+		if fr.block == nil {
+			fr.result = zero(fr.fn.Signature.Results())
+		}
 	}()
 
 	for {

ssa/interp/ops.go

@@ -235,6 +235,15 @@ func zero(t types.Type) value {
 			s[i] = zero(t.Field(i).Type())
 		}
 		return s
+	case *types.Tuple:
+		if t.Len() == 1 {
+			return zero(t.At(0).Type())
+		}
+		s := make(tuple, t.Len())
+		for i := range s {
+			s[i] = zero(t.At(i).Type())
+		}
+		return s
 	case *types.Chan:
 		return chan value(nil)
 	case *types.Map:

ssa/interp/testdata/recover.go

@@ -2,6 +2,8 @@ package main
 
 // Tests of panic/recover.
 
+import "fmt"
+
 func fortyTwo() (r int) {
 	r = 42
 	// The next two statements simulate a 'return' statement.
@@ -9,8 +11,24 @@ func fortyTwo() (r int) {
 	panic(nil)
 }
 
+func zero() int {
+	defer func() { recover() }()
+	panic(1)
+}
+
+func zeroEmpty() (int, string) {
+	defer func() { recover() }()
+	panic(1)
+}
+
 func main() {
 	if r := fortyTwo(); r != 42 {
 		panic(r)
 	}
+	if r := zero(); r != 0 {
+		panic(r)
+	}
+	if r, s := zeroEmpty(); r != 0 || s != "" {
+		panic(fmt.Sprint(r, s))
+	}
 }