[dev.typeparams] cmd/compile/internal/types2: use comparable bit rather than ==() method

This removes the special "==" methods from comparable interfaces in
favor of a "comparable" flag in TypeSets indicating that the interface
is or embeds comparable. Fixes various related implementation
inaccuracies.

While at it, fix setup of the predeclared error and comparable
interface types by associating their respective type name objects
with them.

For #47411.

Change-Id: I409f880c8c8f2fe345621401267e4aaabd17124d
Reviewed-on: https://go-review.googlesource.com/c/go/+/337354
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

9 files changed, 71 insertions, 47 deletions

diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index cc96375027..db398c6563 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -146,6 +146,17 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 	// the parameterized type.
 	iface = check.subst(pos, iface, smap).(*Interface)
 
+	// if iface is comparable, targ must be comparable
+	// TODO(gri) the error messages needs to be better, here
+	if iface.IsComparable() && !Comparable(targ) {
+		if tpar := asTypeParam(targ); tpar != nil && tpar.Bound().typeSet().IsTop() {
+			check.softErrorf(pos, "%s has no constraints", targ)
+			return false
+		}
+		check.softErrorf(pos, "%s does not satisfy comparable", targ)
+		return false
+	}
+
 	// targ must implement iface (methods)
 	// - check only if we have methods
 	if iface.NumMethods() > 0 {
@@ -161,10 +172,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 			//           (print warning for now)
 			// Old warning:
 			// check.softErrorf(pos, "%s does not satisfy %s (warning: name not updated) = %s (missing method %s)", targ, tpar.bound, iface, m)
-			if m.name == "==" {
-				// We don't want to report "missing method ==".
-				check.softErrorf(pos, "%s does not satisfy comparable", targ)
-			} else if wrong != nil {
+			if wrong != nil {
 				// TODO(gri) This can still report uninstantiated types which makes the error message
 				//           more difficult to read then necessary.
 				check.softErrorf(pos,
diff --git a/src/cmd/compile/internal/types2/interface.go b/src/cmd/compile/internal/types2/interface.go
index c344f8ed01..cf8ec1a5e2 100644
--- a/src/cmd/compile/internal/types2/interface.go
+++ b/src/cmd/compile/internal/types2/interface.go
@@ -107,7 +107,7 @@ func (t *Interface) Method(i int) *Func { return t.typeSet().Method(i) }
 // Empty reports whether t is the empty interface.
 func (t *Interface) Empty() bool { return t.typeSet().IsTop() }
 
-// IsComparable reports whether interface t is or embeds the predeclared interface "comparable".
+// IsComparable reports whether each type in interface t's type set is comparable.
 func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 
 // IsConstraint reports whether interface t is not just a method set.
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 91be14bde3..ecf6926c0a 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -308,11 +308,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		for _, m := range T.typeSet().methods {
 			_, f := ityp.typeSet().LookupMethod(m.pkg, m.name)
 
-			if f == nil {
-				// if m is the magic method == we're ok (interfaces are comparable)
-				if m.name == "==" || !static {
-					continue
-				}
+			if f == nil && static {
 				return m, f
 			}
 
@@ -360,10 +356,6 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// we must have a method (not a field of matching function type)
 		f, _ := obj.(*Func)
 		if f == nil {
-			// if m is the magic method == and V is comparable, we're ok
-			if m.name == "==" && Comparable(V) {
-				continue
-			}
 			return m, nil
 		}
 
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index e862c0fca8..f2215b36cb 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -96,19 +96,6 @@ func comparable(T Type, seen map[Type]bool) bool {
 	}
 	seen[T] = true
 
-	// If T is a type parameter not constrained by any type
-	// (i.e., it's operational type is the top type),
-	// T is comparable if it has the == method. Otherwise,
-	// the operational type "wins". For instance
-	//
-	//     interface{ comparable; type []byte }
-	//
-	// is not comparable because []byte is not comparable.
-	// TODO(gri) this code is not 100% correct (see comment for TypeSet.IsComparable)
-	if t := asTypeParam(T); t != nil && optype(t) == theTop {
-		return t.Bound().IsComparable()
-	}
-
 	switch t := under(T).(type) {
 	case *Basic:
 		// assume invalid types to be comparable
@@ -126,9 +113,7 @@ func comparable(T Type, seen map[Type]bool) bool {
 	case *Array:
 		return comparable(t.elem, seen)
 	case *TypeParam:
-		return t.underIs(func(t Type) bool {
-			return comparable(t, seen)
-		})
+		return t.Bound().IsComparable()
 	}
 	return false
 }
diff --git a/src/cmd/compile/internal/types2/sizeof_test.go b/src/cmd/compile/internal/types2/sizeof_test.go
index f7f191f629..22ef369683 100644
--- a/src/cmd/compile/internal/types2/sizeof_test.go
+++ b/src/cmd/compile/internal/types2/sizeof_test.go
@@ -49,7 +49,7 @@ func TestSizeof(t *testing.T) {
 		// Misc
 		{Scope{}, 60, 104},
 		{Package{}, 40, 80},
-		{TypeSet{}, 20, 40},
+		{TypeSet{}, 24, 48},
 	}
 
 	for _, test := range tests {
diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.go2 b/src/cmd/compile/internal/types2/testdata/check/issues.go2
index 32c4320d27..1ede383ebe 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.go2
@@ -58,7 +58,7 @@ func _() {
 type T1[P interface{~uint}] struct{}
 
 func _[P any]() {
-    _ = T1[P /* ERROR P has no type constraints */ ]{}
+    _ = T1[P /* ERROR P has no constraints */ ]{}
 }
 
 // This is the original (simplified) program causing the same issue.
@@ -74,8 +74,8 @@ func (u T2[U]) Add1() U {
     return u.s + 1
 }
 
-func NewT2[U any]() T2[U /* ERROR U has no type constraints */ ] {
-    return T2[U /* ERROR U has no type constraints */ ]{}
+func NewT2[U any]() T2[U /* ERROR U has no constraints */ ] {
+    return T2[U /* ERROR U has no constraints */ ]{}
 }
 
 func _() {
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2
new file mode 100644
index 0000000000..72968f9d43
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2
@@ -0,0 +1,26 @@
+// 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[_ comparable]()
+func g[_ interface{interface{comparable; ~int|~string}}]()
+
+func _[P comparable,
+        Q interface{ comparable; ~int|~string },
+        R any,                               // not comparable
+        S interface{ comparable; ~func() },  // not comparable
+]() {
+        _ = f[int]
+        _ = f[P]
+        _ = f[Q]
+        _ = f[func( /* ERROR does not satisfy comparable */ )]
+        _ = f[R /* ERROR R has no constraints */ ]
+
+        _ = g[int]
+        _ = g[P /* ERROR P has no type constraints */ ]
+        _ = g[Q]
+        _ = g[func( /* ERROR does not satisfy comparable */ )]
+        _ = g[R /* ERROR R has no constraints */ ]
+}
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 8e6af8e65c..cc28625070 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -16,22 +16,30 @@ import (
 
 // A TypeSet represents the type set of an interface.
 type TypeSet struct {
+	comparable bool // if set, the interface is or embeds comparable
 	// TODO(gri) consider using a set for the methods for faster lookup
 	methods []*Func // all methods of the interface; sorted by unique ID
 	types   Type    // typically a *Union; nil means no type restrictions
 }
 
 // IsTop reports whether type set s is the top type set (corresponding to the empty interface).
-func (s *TypeSet) IsTop() bool { return len(s.methods) == 0 && s.types == nil }
+func (s *TypeSet) IsTop() bool { return !s.comparable && len(s.methods) == 0 && s.types == nil }
 
 // IsMethodSet reports whether the type set s is described by a single set of methods.
-func (s *TypeSet) IsMethodSet() bool { return s.types == nil && !s.IsComparable() }
+func (s *TypeSet) IsMethodSet() bool { return !s.comparable && s.types == nil }
 
 // IsComparable reports whether each type in the set is comparable.
-// TODO(gri) this is not correct - there may be s.types values containing non-comparable types
 func (s *TypeSet) IsComparable() bool {
-	_, m := s.LookupMethod(nil, "==")
-	return m != nil
+	if s.types == nil {
+		return s.comparable
+	}
+	tcomparable := s.underIs(func(u Type) bool {
+		return Comparable(u)
+	})
+	if !s.comparable {
+		return tcomparable
+	}
+	return s.comparable && tcomparable
 }
 
 // NumMethods returns the number of methods available.
@@ -54,6 +62,12 @@ func (s *TypeSet) String() string {
 
 	var buf bytes.Buffer
 	buf.WriteByte('{')
+	if s.comparable {
+		buf.WriteString(" comparable")
+		if len(s.methods) > 0 || s.types != nil {
+			buf.WriteByte(';')
+		}
+	}
 	for i, m := range s.methods {
 		if i > 0 {
 			buf.WriteByte(';')
@@ -205,6 +219,9 @@ func computeTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *TypeSet {
 		switch t := under(typ).(type) {
 		case *Interface:
 			tset := computeTypeSet(check, pos, t)
+			if tset.comparable {
+				ityp.tset.comparable = true
+			}
 			for _, m := range tset.methods {
 				addMethod(pos, m, false) // use embedding position pos rather than m.pos
 			}
diff --git a/src/cmd/compile/internal/types2/universe.go b/src/cmd/compile/internal/types2/universe.go
index 0f711a6b68..a3dd4bd0d6 100644
--- a/src/cmd/compile/internal/types2/universe.go
+++ b/src/cmd/compile/internal/types2/universe.go
@@ -88,23 +88,19 @@ func defPredeclaredTypes() {
 		res := NewVar(nopos, nil, "", Typ[String])
 		sig := NewSignature(nil, nil, NewTuple(res), false)
 		err := NewFunc(nopos, nil, "Error", sig)
-		ityp := NewInterfaceType([]*Func{err}, nil)
+		ityp := &Interface{obj, []*Func{err}, nil, nil, true, nil}
 		computeTypeSet(nil, nopos, ityp) // prevent races due to lazy computation of tset
 		typ := NewNamed(obj, ityp, nil)
 		sig.recv = NewVar(nopos, nil, "", typ)
 		def(obj)
 	}
 
-	// type comparable interface{ ==() }
+	// type comparable interface{ /* type set marked comparable */ }
 	{
 		obj := NewTypeName(nopos, nil, "comparable", nil)
 		obj.setColor(black)
-		sig := NewSignature(nil, nil, nil, false)
-		eql := NewFunc(nopos, nil, "==", sig)
-		ityp := NewInterfaceType([]*Func{eql}, nil)
-		computeTypeSet(nil, nopos, ityp) // prevent races due to lazy computation of tset
-		typ := NewNamed(obj, ityp, nil)
-		sig.recv = NewVar(nopos, nil, "", typ)
+		ityp := &Interface{obj, nil, nil, nil, true, &TypeSet{true, nil, nil}}
+		NewNamed(obj, ityp, nil)
 		def(obj)
 	}
 }