go/types, types2: record (top-level) union types

Fixes #50093.

Change-Id: Ibebeda542d2a81c979670f9098c4a6d2c3e73abb
Reviewed-on: https://go-review.googlesource.com/c/go/+/371514
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

4 files changed, 35 insertions, 15 deletions

diff --git a/src/go/types/api.go b/src/go/types/api.go
index 51d58c49aa..c4d81c1491 100644
--- a/src/go/types/api.go
+++ b/src/go/types/api.go
@@ -197,6 +197,12 @@ type Info struct {
 	// identifier z in a variable declaration 'var z int' is found
 	// only in the Defs map, and identifiers denoting packages in
 	// qualified identifiers are collected in the Uses map.
+	//
+	// For binary expressions representing unions in constraint
+	// position or type elements in interfaces, a union type is
+	// recorded for the top-level expression only. For instance,
+	// given the constraint a|b|c, the union type for (a|b)|c
+	// is recorded, but not the union type for a|b.
 	Types map[ast.Expr]TypeAndValue
 
 	// Instances maps identifiers denoting parameterized types or functions to
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 7b7baa7604..1ee9806fd0 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -373,6 +373,16 @@ func TestTypesInfo(t *testing.T) {
 
 		// issue 47895
 		{`package p; import "unsafe"; type S struct { f int }; var s S; var _ = unsafe.Offsetof(s.f)`, `s.f`, `int`},
+
+		// issue 50093
+		{genericPkg + `u0a; func _[_ interface{int}]() {}`, `int`, `int`},
+		{genericPkg + `u1a; func _[_ interface{~int}]() {}`, `~int`, `~int`},
+		{genericPkg + `u2a; func _[_ interface{int|string}]() {}`, `int | string`, `int|string`},
+		{genericPkg + `u3a; func _[_ interface{int|string|~bool}]() {}`, `int | string | ~bool`, `int|string|~bool`},
+		{genericPkg + `u0b; func _[_ int]() {}`, `int`, `int`},
+		{genericPkg + `u1b; func _[_ ~int]() {}`, `~int`, `~int`},
+		{genericPkg + `u2b; func _[_ int|string]() {}`, `int | string`, `int|string`},
+		{genericPkg + `u3b; func _[_ int|string|~bool]() {}`, `int | string | ~bool`, `int|string|~bool`},
 	}
 
 	for _, test := range tests {
diff --git a/src/go/types/interface.go b/src/go/types/interface.go
index ef65bc6b2b..1ff9015780 100644
--- a/src/go/types/interface.go
+++ b/src/go/types/interface.go
@@ -152,7 +152,7 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 
 	for _, f := range iface.Methods.List {
 		if len(f.Names) == 0 {
-			addEmbedded(f.Type.Pos(), parseUnion(check, flattenUnion(nil, f.Type)))
+			addEmbedded(f.Type.Pos(), parseUnion(check, f.Type))
 			continue
 		}
 		// f.Name != nil
@@ -223,11 +223,3 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 		ityp.check = nil
 	}).describef(iface, "compute type set for %s", ityp)
 }
-
-func flattenUnion(list []ast.Expr, x ast.Expr) []ast.Expr {
-	if o, _ := x.(*ast.BinaryExpr); o != nil && o.Op == token.OR {
-		list = flattenUnion(list, o.X)
-		x = o.Y
-	}
-	return append(list, x)
-}
diff --git a/src/go/types/union.go b/src/go/types/union.go
index 2a65ca4d8e..1437bd4624 100644
--- a/src/go/types/union.go
+++ b/src/go/types/union.go
@@ -49,10 +49,11 @@ func (t *Term) String() string { return (*term)(t).String() }
 // Avoid excessive type-checking times due to quadratic termlist operations.
 const maxTermCount = 100
 
-// parseUnion parses the given list of type expressions tlist as a union of
-// those expressions. The result is a Union type, or Typ[Invalid] for some
-// errors.
-func parseUnion(check *Checker, tlist []ast.Expr) Type {
+// parseUnion parses uexpr as a union of expressions.
+// The result is a Union type, or Typ[Invalid] for some errors.
+func parseUnion(check *Checker, uexpr ast.Expr) Type {
+	tlist := flattenUnion(nil, uexpr)
+
 	var terms []*Term
 	for _, x := range tlist {
 		tilde, typ := parseTilde(check, x)
@@ -60,10 +61,11 @@ func parseUnion(check *Checker, tlist []ast.Expr) Type {
 			// Single type. Ok to return early because all relevant
 			// checks have been performed in parseTilde (no need to
 			// run through term validity check below).
-			return typ
+			return typ // typ already recorded through check.typ in parseTilde
 		}
 		if len(terms) >= maxTermCount {
 			check.errorf(x, _InvalidUnion, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
+			check.recordTypeAndValue(uexpr, typexpr, Typ[Invalid], nil)
 			return Typ[Invalid]
 		}
 		terms = append(terms, NewTerm(tilde, typ))
@@ -108,7 +110,9 @@ func parseUnion(check *Checker, tlist []ast.Expr) Type {
 		}
 	})
 
-	return &Union{terms, nil}
+	u := &Union{terms, nil}
+	check.recordTypeAndValue(uexpr, typexpr, u, nil)
+	return u
 }
 
 func parseTilde(check *Checker, x ast.Expr) (tilde bool, typ Type) {
@@ -146,3 +150,11 @@ func overlappingTerm(terms []*Term, y *Term) int {
 	}
 	return -1
 }
+
+func flattenUnion(list []ast.Expr, x ast.Expr) []ast.Expr {
+	if o, _ := x.(*ast.BinaryExpr); o != nil && o.Op == token.OR {
+		list = flattenUnion(list, o.X)
+		x = o.Y
+	}
+	return append(list, x)
+}