cmd/compile: move duplicate type-case checking into typecheck

Part of the general trend of moving yyerror calls out of walk and into
typecheck.

Notably, this requires splitting test/typeswitch2.go into two files,
because now some of the errors are reported during typecheck and
others are still reported during walk; and if there were any errors
during typecheck, then cmd/compile exits without invoking walk.

Passes toolstash-check.

Change-Id: I05ee0c00b99af659ee1eef098d342d0d736cf31e
Reviewed-on: https://go-review.googlesource.com/c/go/+/194659
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 0 insertions, 10 deletions

diff --git a/test/typeswitch2.go b/test/typeswitch2.go
index 5958b7db8e..62c96c8330 100644
--- a/test/typeswitch2.go
+++ b/test/typeswitch2.go
@@ -35,13 +35,3 @@ func whatis(x interface{}) string {
 	}
 	return ""
 }
-
-func notused(x interface{}) {
-	// The first t is in a different scope than the 2nd t; it cannot
-	// be accessed (=> declared and not used error); but it is legal
-	// to declare it.
-	switch t := 0; t := x.(type) { // ERROR "declared and not used"
-	case int:
-		_ = t // this is using the t of "t := x.(type)"
-	}
-}