go/types: better errors for invalid short var decls

This is a port of CL 312170 to go/types, adjusted to use go/ast and to
add error codes. go/parser already emits errors for non-identifiers on
the LHS of a short var decl, so a TODO is added to reconsider this
redundancy.

A new error code is added for repeated identifiers in short var decls.
This is a bit specific, but I considered it to be a unique kind of
error.

The x/tools tests for this port turned up a bug: the new logic failed to
call recordDef for blank identifiers. Patchset #2 contains the fix for
this bug, both in go/types and cmd/compile/internal/types2.

Change-Id: Ibdc40b8b4ad0e0696111d431682e1f1056fd5eeb
Reviewed-on: https://go-review.googlesource.com/c/go/+/314629
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 10 insertions, 10 deletions

diff --git a/src/cmd/compile/internal/types2/assignments.go b/src/cmd/compile/internal/types2/assignments.go
index ec9fdbba62..583118c8b2 100644
--- a/src/cmd/compile/internal/types2/assignments.go
+++ b/src/cmd/compile/internal/types2/assignments.go
@@ -344,16 +344,14 @@ func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
 		}
 
 		name := ident.Value
-		if name == "_" {
-			continue
-		}
-
-		if seen[name] {
-			check.errorf(lhs, "%s repeated on left side of :=", lhs)
-			hasErr = true
-			continue
+		if name != "_" {
+			if seen[name] {
+				check.errorf(lhs, "%s repeated on left side of :=", lhs)
+				hasErr = true
+				continue
+			}
+			seen[name] = true
 		}
-		seen[name] = true
 
 		// Use the correct obj if the ident is redeclared. The
 		// variable's scope starts after the declaration; so we
@@ -374,7 +372,9 @@ func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
 		// declare new variable
 		obj := NewVar(ident.Pos(), check.pkg, name, nil)
 		lhsVars[i] = obj
-		newVars = append(newVars, obj)
+		if name != "_" {
+			newVars = append(newVars, obj)
+		}
 		check.recordDef(ident, obj)
 	}