cmd/compile/internal/types2: partial revert of incorrect unification "fix"

The "fix" (CL 352832) for #48619 was incorrect and broke
the unification algorithm in some cases (e.g., #48695).

This CL reverts the changes made by CL 352832 to unify.go,
and comments out code in corresponding tests.

As a result, #48695 will be fixed, and we will re-open #48619.

Fixes #48695.
For #48619.
For #48656.

Change-Id: I91bc492062dbcc8dae7626f6b33f6dfabf48bcb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354690
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: griesemer

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48619.go2

@@ -2,21 +2,23 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// This issue has been re-opened.
+
 package p
 
 func f[P any](a, _ P) {
-	var x int
-	f(a, x /* ERROR type int of x does not match P */)
-	f(x, a /* ERROR type P of a does not match inferred type int for P */)
+	// var x int
+	// f(a, x /* ERROR type int of x does not match P */)
+	// f(x, a /* ERROR type P of a does not match inferred type int for P */)
 }
 
 func g[P any](a, b P) {
-	g(a, b)
-	g(&a, &b)
-	g([]P{}, []P{})
+	// g(a, b)
+	// g(&a, &b)
+	// g([]P{}, []P{})
 }
 
 func h[P any](a, b P) {
-	h(&a, &b)
-	h([]P{a}, []P{b})
+	// h(&a, &b)
+	// h([]P{a}, []P{b})
 }

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48656.go2

@@ -2,12 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package p
+// This issue is still open.
 
-// TODO(gri) Still need better error positions and message here.
-//           But this doesn't crash anymore.
+package p
 
-func f[P /* ERROR does not match \*Q */ interface{*Q}, Q any](p P, q Q) {
-	_ = f[P]
-        _ = f[/* ERROR cannot infer P */ *P]
+func f[P *Q, Q any](p P, q Q) {
+	// _ = f[P]
+        // _ = f[/* ERROR cannot infer P */ *P]
 }

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48695.go2

@@ -0,0 +1,14 @@
+// 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
+
+func g[P ~func(T) P, T any](P) {}
+
+func _() {
+	type F func(int) F
+	var f F
+	g(f)
+	_ = g[F]
+}

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

@@ -63,10 +63,6 @@ func (u *unifier) unify(x, y Type) bool {
 type tparamsList struct {
 	unifier *unifier
 	tparams []*TypeParam
-	// For each tparams element, there is a corresponding mask bit in masks.
-	// If set, the corresponding type parameter is masked and doesn't appear
-	// as a type parameter with tparamsList.index.
-	masks []bool
 	// For each tparams element, there is a corresponding type slot index in indices.
 	// index  < 0: unifier.types[-index-1] == nil
 	// index == 0: no type slot allocated yet
@@ -107,14 +103,9 @@ func (d *tparamsList) init(tparams []*TypeParam) {
 		}
 	}
 	d.tparams = tparams
-	d.masks = make([]bool, len(tparams))
 	d.indices = make([]int, len(tparams))
 }
 
-// mask and unmask permit the masking/unmasking of the i'th type parameter of d.
-func (d *tparamsList) mask(i int)   { d.masks[i] = true }
-func (d *tparamsList) unmask(i int) { d.masks[i] = false }
-
 // join unifies the i'th type parameter of x with the j'th type parameter of y.
 // If both type parameters already have a type associated with them and they are
 // not joined, join fails and returns false.
@@ -158,13 +149,11 @@ func (u *unifier) join(i, j int) bool {
 	return true
 }
 
-// If typ is an unmasked type parameter of d, index returns the type parameter index.
+// If typ is a type parameter of d, index returns the type parameter index.
 // Otherwise, the result is < 0.
 func (d *tparamsList) index(typ Type) int {
 	if tpar, ok := typ.(*TypeParam); ok {
-		if i := tparamIndex(d.tparams, tpar); i >= 0 && !d.masks[i] {
-			return i
-		}
+		return tparamIndex(d.tparams, tpar)
 	}
 	return -1
 }
@@ -257,7 +246,7 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		}
 	}
 
-	// Cases where at least one of x or y is an (unmasked) type parameter.
+	// Cases where at least one of x or y is a type parameter.
 	switch i, j := u.x.index(x), u.y.index(y); {
 	case i >= 0 && j >= 0:
 		// both x and y are type parameters
@@ -270,12 +259,6 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 	case i >= 0:
 		// x is a type parameter, y is not
 		if tx := u.x.at(i); tx != nil {
-			// The inferred type tx may be or contain x again but we don't
-			// want to "unpack" it again when unifying tx with y: tx is the
-			// inferred type. Mask type parameter x for this recursion, so
-			// that subsequent encounters treat x like an ordinary type.
-			u.x.mask(i)
-			defer u.x.unmask(i)
 			return u.nifyEq(tx, y, p)
 		}
 		// otherwise, infer type from y
@@ -285,9 +268,6 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 	case j >= 0:
 		// y is a type parameter, x is not
 		if ty := u.y.at(j); ty != nil {
-			// see comment above
-			u.y.mask(j)
-			defer u.y.unmask(j)
 			return u.nifyEq(x, ty, p)
 		}
 		// otherwise, infer type from x