cmd/compile: better handling of incorrect type switches

Don't report errors if we don't have a correct type switch
guard; instead ignore it and leave it to the type-checker
to report the error. This leads to better error messages
concentrating on the type switch guard rather than errors
around (confusing) syntactic details.

Also clean up some code setting up AssertExpr (they never
have a nil Type field) and remove some incorrect TODOs.

Fixes #24470.

Change-Id: I69512f36e0417e3b5ea9c8856768e04b19d654a8
Reviewed-on: https://go-review.googlesource.com/103615
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 9 insertions, 9 deletions

diff --git a/src/cmd/compile/internal/syntax/nodes.go b/src/cmd/compile/internal/syntax/nodes.go
index 8df9fd5c4e..c1da4adf52 100644
--- a/src/cmd/compile/internal/syntax/nodes.go
+++ b/src/cmd/compile/internal/syntax/nodes.go
@@ -198,12 +198,19 @@ type (
 
 	// X.(Type)
 	AssertExpr struct {
-		X Expr
-		// TODO(gri) consider using Name{"..."} instead of nil (permits attaching of comments)
+		X    Expr
 		Type Expr
 		expr
 	}
 
+	// X.(type)
+	// Lhs := X.(type)
+	TypeSwitchGuard struct {
+		Lhs *Name // nil means no Lhs :=
+		X   Expr  // X.(type)
+		expr
+	}
+
 	Operation struct {
 		Op   Operator
 		X, Y Expr // Y == nil means unary expression
@@ -413,13 +420,6 @@ type (
 		simpleStmt
 	}
 
-	TypeSwitchGuard struct {
-		// TODO(gri) consider using Name{"..."} instead of nil (permits attaching of comments)
-		Lhs *Name // nil means no Lhs :=
-		X   Expr  // X.(type)
-		expr
-	}
-
 	CaseClause struct {
 		Cases Expr // nil means default clause
 		Body  []Stmt