[dev.regabi] cmd/compile: make ir.OuterValue safer

For OINDEX expressions, ir.OuterValue depends on knowing the indexee's
type. Rather than silently acting as though it's not an array, make it
loudly fail.

The only code that needs to be fixed to support this is checkassign
during typechecking, which needs to avoid calling ir.OuterValue now if
typechecking the assigned operand already failed.

Passes toolstash -cmp.

Change-Id: I935cae0dacc837202bc6b63164dc2f0a6fde005c
Reviewed-on: https://go-review.googlesource.com/c/go/+/281539
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/typecheck/typecheck.go b/src/cmd/compile/internal/typecheck/typecheck.go
index 981f4ef1d6..c3a5a3c40f 100644
--- a/src/cmd/compile/internal/typecheck/typecheck.go
+++ b/src/cmd/compile/internal/typecheck/typecheck.go
@@ -1612,6 +1612,14 @@ func checklvalue(n ir.Node, verb string) {
 }
 
 func checkassign(stmt ir.Node, n ir.Node) {
+	// have already complained about n being invalid
+	if n.Type() == nil {
+		if base.Errors() == 0 {
+			base.Fatalf("expected an error about %v", n)
+		}
+		return
+	}
+
 	// Variables declared in ORANGE are assigned on every iteration.
 	if !ir.DeclaredBy(n, stmt) || stmt.Op() == ir.ORANGE {
 		r := ir.OuterValue(n)
@@ -1633,11 +1641,6 @@ func checkassign(stmt ir.Node, n ir.Node) {
 		return
 	}
 
-	// have already complained about n being invalid
-	if n.Type() == nil {
-		return
-	}
-
 	switch {
 	case n.Op() == ir.ODOT && n.(*ir.SelectorExpr).X.Op() == ir.OINDEXMAP:
 		base.Errorf("cannot assign to struct field %v in map", n)