[dev.typeparams] cmd/compile/internal/types2: provide valid signature in errors involving method expressions

This is an adjusted port of a similar fix in https://golang.org/cl/324733.

Fixes #46583.

Change-Id: Ica1410e4de561e64e58b753e3da04b32156cbaf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/325369
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/call.go

@@ -582,13 +582,27 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 			goto Error
 		}
 
-		// the receiver type becomes the type of the first function
+		// The receiver type becomes the type of the first function
-		// argument of the method expression's function type
+		// argument of the method expression's function type.
 		var params []*Var
 		if sig.params != nil {
 			params = sig.params.vars
 		}
-		params = append([]*Var{NewVar(sig.recv.pos, sig.recv.pkg, sig.recv.name, x.typ)}, params...)
+		// Be consistent about named/unnamed parameters. This is not needed
+		// for type-checking, but the newly constructed signature may appear
+		// in an error message and then have mixed named/unnamed parameters.
+		// (An alternative would be to not print parameter names in errors,
+		// but it's useful to see them; this is cheap and method expressions
+		// are rare.)
+		name := ""
+		if len(params) > 0 && params[0].name != "" {
+			// name needed
+			name = sig.recv.name
+			if name == "" {
+				name = "_"
+			}
+		}
+		params = append([]*Var{NewVar(sig.recv.pos, sig.recv.pkg, name, x.typ)}, params...)
 		x.mode = value
 		x.typ = &Signature{
 			tparams:  sig.tparams,

src/cmd/compile/internal/types2/testdata/fixedbugs/issue46583.src

@@ -0,0 +1,28 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T1 struct{}
+func (t T1) m(int) {}
+var f1 func(T1)
+
+type T2 struct{}
+func (t T2) m(x int) {}
+var f2 func(T2)
+
+type T3 struct{}
+func (T3) m(int) {}
+var f3 func(T3)
+
+type T4 struct{}
+func (T4) m(x int) {}
+var f4 func(T4)
+
+func _() {
+	f1 = T1 /* ERROR func\(T1, int\) */ .m
+	f2 = T2 /* ERROR func\(t T2, x int\) */ .m
+	f3 = T3 /* ERROR func\(T3, int\) */ .m
+	f4 = T4 /* ERROR func\(_ T4, x int\) */ .m
+}