cmd/vet: avoid internal error for implicitly declared type switch vars

For type switches using a short variable declaration of the form

   switch t := x.(type) {
   case T1:
   ...

go/types doesn't declare the symbolic variable (t in this example)
with the switch; thus such variables are not found in types.Info.Defs.

Instead they are implicitly declared with each type switch case,
and can be found in types.Info.Implicits.

Adjust the shadowing code accordingly.

Added a test case to verify that the issue is fixed, and a test
case verifying that the shadowing code now considers implicitly
declared variables introduces in type switch cases.

While at it, also fixed the (internal) error reporting to provide
more accurate information.

Fixe #26725.

Change-Id: If408ed9e692bf47c640f81de8f46bf5eb43415b0
Reviewed-on: https://go-review.googlesource.com/135117
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/cmd/vet/main.go b/src/cmd/vet/main.go
index c50d4885a0..646adf4d76 100644
--- a/src/cmd/vet/main.go
+++ b/src/cmd/vet/main.go
@@ -467,6 +467,7 @@ type Package struct {
 	path      string
 	defs      map[*ast.Ident]types.Object
 	uses      map[*ast.Ident]types.Object
+	implicits map[ast.Node]types.Object
 	selectors map[*ast.SelectorExpr]*types.Selection
 	types     map[ast.Expr]types.TypeAndValue
 	spans     map[types.Object]Span