cmd/internal/gc: detect bad append(f()) during type check

Today's earlier fix can stay, but it's a band-aid over the real problem,
which is that bad code was slipping through the type checker
into the back end (and luckily causing a type error there).

I discovered this because my new append does not use the same
temporaries and failed the test as written.

Fixes #9521.

Change-Id: I7e33e2ea15743406e15c6f3fdf73e1edecda69bd
Reviewed-on: https://go-review.googlesource.com/9921Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/gc/typecheck.go

@@ -1636,11 +1636,10 @@ OpSwitch:
 		}
 
 		// Unpack multiple-return result before type-checking.
+		var funarg *Type
 		if Istype(t, TSTRUCT) && t.Funarg != 0 {
-			t = t.Type
-			if Istype(t, TFIELD) {
-				t = t.Type
-			}
+			funarg = t
+			t = t.Type.Type
 		}
 
 		n.Type = t
@@ -1678,11 +1677,19 @@ OpSwitch:
 			break OpSwitch
 		}
 
-		for args = args.Next; args != nil; args = args.Next {
-			if args.N.Type == nil {
-				continue
+		if funarg != nil {
+			for t := funarg.Type.Down; t != nil; t = t.Down {
+				if assignop(t.Type, n.Type.Type, nil) == 0 {
+					Yyerror("cannot append %v value to []%v", t.Type, n.Type.Type)
+				}
+			}
+		} else {
+			for args = args.Next; args != nil; args = args.Next {
+				if args.N.Type == nil {
+					continue
+				}
+				args.N = assignconv(args.N, t.Type, "append")
 			}
-			args.N = assignconv(args.N, t.Type, "append")
 		}
 
 		break OpSwitch

test/fixedbugs/issue9521.go

@@ -9,8 +9,10 @@
 
 package main
 
-func f() (_, _ []int) { return }
+func f() (_, _ []int)         { return }
+func g() (x []int, y float64) { return }
 
 func main() {
-	_ = append(f()) // ERROR "cannot use _"
+	_ = append(f()) // ERROR "cannot append \[\]int value to \[\]int"
+	_ = append(g()) // ERROR "cannot append float64 value to \[\]int"
 }