go/analysis: use TypeString when matching types

As Alan rightfully guessed, porting the stdmethods check to use go/types
required the use of types.TypeString not only when printing signatures
in warnings, but also when matching them.

Added a simple test case too.

Fixes golang/go#28792.

Change-Id: Ifbbdd4b1a2f1090d6f9a1674d52b8f0887a67d06
Reviewed-on: https://go-review.googlesource.com/c/149977
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/analysis/passes/stdmethods/stdmethods.go

@@ -131,7 +131,7 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident) {
 			expectFmt += " (" + argjoin(expect.results) + ")"
 		}
 
-		actual := types.TypeString(sign, (*types.Package).Name)
+		actual := typeString(sign)
 		actual = strings.TrimPrefix(actual, "func")
 		actual = id.Name + actual
 
@@ -139,6 +139,10 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident) {
 	}
 }
 
+func typeString(typ types.Type) string {
+	return types.TypeString(typ, (*types.Package).Name)
+}
+
 func argjoin(x []string) string {
 	y := make([]string, len(x))
 	for i, s := range x {
@@ -178,5 +182,5 @@ func matchParamType(fset *token.FileSet, pkg *types.Package, expect string, actu
 	}
 
 	// Overkill but easy.
-	return actual.String() == expect
+	return typeString(actual) == expect
 }

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

@@ -24,6 +24,10 @@ func (U) GobDecode() {} // want `should have signature GobDecode\(\[\]byte\) err
 // Test rendering of type names such as xml.Encoder in diagnostic.
 func (U) MarshalXML(*xml.Encoder) {} // want `method MarshalXML\(\*xml.Encoder\) should...`
 
+func (U) UnmarshalXML(*xml.Decoder, xml.StartElement) error { // no error: signature matches xml.Unmarshaler
+	return nil
+}
+
 type I interface {
 	ReadByte() byte // want `should have signature ReadByte\(\) \(byte, error\)`
 }