[dev.regabi] cmd/compile: cleanup type switch typechecking

Address outstanding TODO, which simplifies subsequent CLs.

Now the compiler always type checks type-switch case clauses (like
gccgo), but it treats clause variables as broken if an appropriate
type cannot be determined for it (like go/types).

Passes toolstash-check.

Change-Id: Iedfe9cdf38c6865211e4b93391f1cf72c1bed136
Reviewed-on: https://go-review.googlesource.com/c/go/+/272648
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -89,22 +89,22 @@ func typecheckTypeSwitch(n *Node) {
 			if len(ls) == 1 {
 				if ls[0].Op == OTYPE {
 					vt = ls[0].Type
-				} else if ls[0].Op != OLITERAL { // TODO(mdempsky): Should be !ls[0].isNil()
+				} else if !ls[0].isNil() {
 					// Invalid single-type case;
 					// mark variable as broken.
 					vt = nil
 				}
 			}
 
-			// TODO(mdempsky): It should be possible to
-			// still typecheck the case body.
-			if vt == nil {
-				continue
-			}
-
 			nvar := ncase.Rlist.First()
 			nvar.Type = vt
-			nvar = typecheck(nvar, ctxExpr|ctxAssign)
+			if vt != nil {
+				nvar = typecheck(nvar, ctxExpr|ctxAssign)
+			} else {
+				// Clause variable is broken; prevent typechecking.
+				nvar.SetTypecheck(1)
+				nvar.SetWalkdef(1)
+			}
 			ncase.Rlist.SetFirst(nvar)
 		}
 

test/fixedbugs/bug340.go

@@ -12,6 +12,7 @@ func main() {
 	var x interface{}
 	switch t := x.(type) {
 	case 0:		// ERROR "type"
-		t.x = 1 // ERROR "type interface \{\}|reference to undefined field or method"
+		t.x = 1
+		x.x = 1 // ERROR "type interface \{\}|reference to undefined field or method"
 	}
 }