go/analysis/passes/stdmethods: recognize Unwrap() []error

Recognize the new multiple-error-wrapping method signature.

For golang/go#53435

Change-Id: Ifba25746323d036d1e6d3e6d3c34cd6ce904b60a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/432575
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>

go/analysis/passes/stdmethods/stdmethods.go

@@ -134,6 +134,19 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident) {
 		}
 	}
 
+	// Special case: Unwrap has two possible signatures.
+	// Check for Unwrap() []error here.
+	if id.Name == "Unwrap" {
+		if args.Len() == 0 && results.Len() == 1 {
+			t := typeString(results.At(0).Type())
+			if t == "error" || t == "[]error" {
+				return
+			}
+		}
+		pass.ReportRangef(id, "method Unwrap() should have signature Unwrap() error or Unwrap() []error")
+		return
+	}
+
 	// Do the =s (if any) all match?
 	if !matchParams(pass, expect.args, args, "=") || !matchParams(pass, expect.results, results, "=") {
 		return

go/analysis/passes/stdmethods/testdata/src/a/a.go

@@ -49,16 +49,26 @@ func (E) Error() string { return "" } // E implements error.
 
 func (E) As()     {} // want `method As\(\) should have signature As\((any|interface\{\})\) bool`
 func (E) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
-func (E) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`
+func (E) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error or Unwrap\(\) \[\]error`
 
 type F int
 
 func (F) Error() string { return "" } // Both F and *F implement error.
 
 func (*F) As()     {} // want `method As\(\) should have signature As\((any|interface\{\})\) bool`
 func (*F) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
-func (*F) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`
+func (*F) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error or Unwrap\(\) \[\]error`
 
 type G int
 
 func (G) As(interface{}) bool // ok
+
+type W int
+
+func (W) Error() string { return "" }
+func (W) Unwrap() error { return nil } // ok
+
+type M int
+
+func (M) Error() string   { return "" }
+func (M) Unwrap() []error { return nil } // ok

go/analysis/passes/stdmethods/testdata/src/typeparams/typeparams.go

@@ -30,12 +30,12 @@ func (E[_]) Error() string { return "" } // E implements error.
 
 func (E[P]) As()     {} // want `method As\(\) should have signature As\((any|interface\{\})\) bool`
 func (E[_]) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
-func (E[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`
+func (E[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error or Unwrap\(\) \[\]error`
 
 type F[P any] int
 
 func (F[_]) Error() string { return "" } // Both F and *F implement error.
 
 func (*F[_]) As()     {} // want `method As\(\) should have signature As\((any|interface\{\})\) bool`
 func (*F[_]) Is()     {} // want `method Is\(\) should have signature Is\(error\) bool`
-func (*F[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error`
+func (*F[_]) Unwrap() {} // want `method Unwrap\(\) should have signature Unwrap\(\) error or Unwrap\(\) \[\]error`