go/types: avoid infinite recursion in unification

This is an almost clean port of CL 352832 from types2 to go/types:
The nest files and unify.go where copied verbatim; unify.go was
adjusted with correct package name, a slightly different comment
was restored to what it was. The test files got adjustments for
error position. infer.go got a missing _Todo error code.

For #48619.
For #48656.

Change-Id: Ia1a2d09e8bb37a85032b4b7e7c7a0b08e8c793a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353029
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/types/infer.go

@@ -128,7 +128,7 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 		if inferred != tpar {
 			check.errorf(arg, _Todo, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
 		} else {
-			check.errorf(arg, 0, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
+			check.errorf(arg, _Todo, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
 		}
 	}
 

src/go/types/testdata/fixedbugs/issue48619.go2

@@ -0,0 +1,22 @@
+// 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 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 */)
+}
+
+func g[P any](a, b 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})
+}

src/go/types/testdata/fixedbugs/issue48656.go2

@@ -0,0 +1,13 @@
+// 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
+
+// TODO(gri) Still need better error positions and message here.
+//           But this doesn't crash anymore.
+
+func f[P /* ERROR does not match \*Q */ interface{*Q}, Q any](p P, q Q) {
+	_ = f[P]
+        _ = f /* ERROR cannot infer P */ [*P]
+}

src/go/types/unify.go

@@ -9,7 +9,6 @@ package types
 import (
 	"bytes"
 	"fmt"
-	"sort"
 )
 
 // The unifier maintains two separate sets of type parameters x and y
@@ -64,6 +63,10 @@ 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
@@ -104,9 +107,14 @@ 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.
@@ -138,19 +146,25 @@ func (u *unifier) join(i, j int) bool {
 	case ti > 0:
 		// Only the type parameter for x has an inferred type. Use x slot for y.
 		u.y.setIndex(j, ti)
+	// This case is handled like the default case.
+	// case tj > 0:
+	// 	// Only the type parameter for y has an inferred type. Use y slot for x.
+	// 	u.x.setIndex(i, tj)
 	default:
-		// Either the type parameter for y has an inferred type, or neither type
-		// parameter has an inferred type. In either case, use y slot for x.
+		// Neither type parameter has an inferred type. Use y slot for x
+		// (or x slot for y, it doesn't matter).
 		u.x.setIndex(i, tj)
 	}
 	return true
 }
 
-// If typ is a type parameter of d, index returns the type parameter index.
+// If typ is an unmasked 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 {
-		return tparamIndex(d.tparams, tpar)
+		if i := tparamIndex(d.tparams, tpar); i >= 0 && !d.masks[i] {
+			return i
+		}
 	}
 	return -1
 }
@@ -243,7 +257,7 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		}
 	}
 
-	// Cases where at least one of x or y is a type parameter.
+	// Cases where at least one of x or y is an (unmasked) 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
@@ -256,6 +270,12 @@ 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
@@ -265,6 +285,9 @@ 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
@@ -399,8 +422,8 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 					p = p.prev
 				}
 				if debug {
-					assert(sort.IsSorted(byUniqueMethodName(a)))
-					assert(sort.IsSorted(byUniqueMethodName(b)))
+					assertSortedMethods(a)
+					assertSortedMethods(b)
 				}
 				for i, f := range a {
 					g := b[i]