[dev.typeparams] cmd/compile/internal/types2: accept embedded interface elements

Accept embedded interface elements of the form ~T or A|B and
treat them like type lists: for now the elements of a union
cannot be interfaces. Also, translate existing style "type"-
lists in interfaces into interface elements: "type a, b, c"
becomes a union element "~a|~b|~c" which in turn is handled
internally like a type list.

For now, "~" is still ignored and type lists are mapped to
Sum types as before, thus ensuring that all existing tests
work as before (with some minor adjustments).

Introduced a new Union type to represent union elements.
For now they don't make it past interface completion where
they are represented as a Sum type. Thus, except for printing
(and the respective tests) and substitution for interfaces,
the various type switches ignore Union types. In a next step,
we'll replace Sum types with union types and then consider
the ~ functionality as well.

Because union elements are no different from embedded interfaces
we don't need a separate Interface.types field anymore. Removed.

For #45346.

Change-Id: I98ac3286aea9d706e98aee80241d4712ed99af08
Reviewed-on: https://go-review.googlesource.com/c/go/+/321689
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 105 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/union.go b/src/cmd/compile/internal/types2/union.go
new file mode 100644
index 0000000000..70dc3bc360
--- /dev/null
+++ b/src/cmd/compile/internal/types2/union.go
@@ -0,0 +1,105 @@
+// 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 types2
+
+import "cmd/compile/internal/syntax"
+
+// ----------------------------------------------------------------------------
+// API
+
+// A Union represents a union of terms.
+// A term is a type, possibly with a ~ (tilde) indication.
+type Union struct {
+	terms []Type // terms are unique
+	tilde []bool // if tilde[i] is set, terms[i] is of the form ~T
+}
+
+func NewUnion(terms []Type, tilde []bool) Type { return newUnion(terms, tilde) }
+
+func (u *Union) NumTerms() int           { return len(u.terms) }
+func (u *Union) Term(i int) (Type, bool) { return u.terms[i], u.tilde[i] }
+
+func (u *Union) Underlying() Type { return u }
+func (u *Union) String() string   { return TypeString(u, nil) }
+
+// ----------------------------------------------------------------------------
+// Implementation
+
+func newUnion(terms []Type, tilde []bool) Type {
+	assert(len(terms) == len(tilde))
+	if terms == nil {
+		return nil
+	}
+	t := new(Union)
+	t.terms = terms
+	t.tilde = tilde
+	return t
+}
+
+func parseUnion(check *Checker, tlist []syntax.Expr) Type {
+	var terms []Type
+	var tilde []bool
+	for _, x := range tlist {
+		t, d := parseTilde(check, x)
+		if len(tlist) == 1 && !d {
+			return t // single type
+		}
+		terms = append(terms, t)
+		tilde = append(tilde, d)
+	}
+
+	// Ensure that each type is only present once in the type list.
+	// It's ok to do this check at the end because it's not a requirement
+	// for correctness of the code.
+	// Note: This is a quadratic algorithm, but unions tend to be short.
+	check.later(func() {
+		for i, t := range terms {
+			t := expand(t)
+			if t == Typ[Invalid] {
+				continue
+			}
+
+			x := tlist[i]
+			pos := syntax.StartPos(x)
+			// We may not know the position of x if it was a typechecker-
+			// introduced ~T type of a type list entry T. Use the position
+			// of T instead.
+			// TODO(gri) remove this test once we don't support type lists anymore
+			if !pos.IsKnown() {
+				if op, _ := x.(*syntax.Operation); op != nil {
+					pos = syntax.StartPos(op.X)
+				}
+			}
+
+			u := under(t)
+			if tilde[i] {
+				// TODO(gri) enable this check once we have converted tests
+				// if !Identical(u, t) {
+				// 	check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", t, u)
+				// }
+			}
+			if _, ok := u.(*Interface); ok {
+				check.errorf(pos, "cannot use interface %s with ~ or inside a union (implementation restriction)", t)
+			}
+
+			// Complain about duplicate entries a|a, but also a|~a, and ~a|~a.
+			if includes(terms[:i], t) {
+				// TODO(gri) this currently doesn't print the ~ if present
+				check.softErrorf(pos, "duplicate term %s in union element", t)
+			}
+		}
+	})
+
+	return newUnion(terms, tilde)
+}
+
+func parseTilde(check *Checker, x syntax.Expr) (Type, bool) {
+	tilde := false
+	if op, _ := x.(*syntax.Operation); op != nil && op.Op == syntax.Tilde {
+		x = op.X
+		tilde = true
+	}
+	return check.anyType(x), tilde
+}