go/analysis/passes/printf: update logic now that type parameters are

interfaces

Now that the underlying of type parameters is their constraint
interface, a couple places in the printf analyzer need to be updated:
 - We need to explicitly exclude type parameters when looking at
   interfaces in isFormatter.
 - We need to consider at the element type, not its underlying, when
   looking at pointers to type parameters

Change-Id: I8c6843e001a98d45ff0f30df305e3536335f567e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/364678
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

go/analysis/passes/printf/printf.go

@@ -25,6 +25,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
 	"golang.org/x/tools/go/ast/inspector"
 	"golang.org/x/tools/go/types/typeutil"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func init() {
@@ -520,7 +521,12 @@ func printfNameAndKind(pass *analysis.Pass, call *ast.CallExpr) (fn *types.Func,
 func isFormatter(typ types.Type) bool {
 	// If the type is an interface, the value it holds might satisfy fmt.Formatter.
 	if _, ok := typ.Underlying().(*types.Interface); ok {
-		return true
+		// Don't assume type parameters could be formatters. With the greater
+		// expressiveness of constraint interface syntax we expect more type safety
+		// when using type parameters.
+		if !typeparams.IsTypeParam(typ) {
+			return true
+		}
 	}
 	obj, _, _ := types.LookupFieldOrMethod(typ, false, nil, "Format")
 	fn, ok := obj.(*types.Func)

go/analysis/passes/printf/types.go

@@ -178,10 +178,12 @@ func (m *argMatcher) match(typ types.Type, topLevel bool) bool {
 			return true
 		}
 
+		if typeparams.IsTypeParam(typ.Elem()) {
+			return true // We don't know whether the logic below applies. Give up.
+		}
+
 		under := typ.Elem().Underlying()
 		switch under.(type) {
-		case *typeparams.TypeParam:
-			return true // We don't know whether the logic below applies. Give up.
 		case *types.Struct: // see below
 		case *types.Array: // see below
 		case *types.Slice: // see below

internal/typeparams/common.go

@@ -13,6 +13,7 @@ package typeparams
 import (
 	"go/ast"
 	"go/token"
+	"go/types"
 )
 
 // A IndexExprData holds data from both ast.IndexExpr and the new
@@ -23,3 +24,9 @@ type IndexExprData struct {
 	Indices []ast.Expr // index expressions
 	Rbrack  token.Pos  // position of "]"
 }
+
+// IsTypeParam reports whether t is a type parameter.
+func IsTypeParam(t types.Type) bool {
+	_, ok := t.(*TypeParam)
+	return ok
+}