[dev.typeparams] cmd/compile/internal/types2: port of https://golang.org/cl/270957

This ports the latest updates to the dev.go2go version of types2
to the dev.typeparams version.

Change-Id: Ic1b09a8aaeefc701a5c194a587be26e0878e64da
Reviewed-on: https://go-review.googlesource.com/c/go/+/270958
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>

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

@@ -1467,18 +1467,18 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 			var key operand
 			check.expr(&key, e.Index)
 			check.assignment(&key, typ.key, "map index")
-			if x.mode == invalid {
-				goto Error
-			}
+			// ok to continue even if indexing failed - map element type is known
 			x.mode = mapindex
 			x.typ = typ.elem
 			x.expr = e
 			return expression
 
 		case *Sum:
-			// A sum type can be indexed if all the sum's types
-			// support indexing and have the same element type.
-			var elem Type
+			// A sum type can be indexed if all of the sum's types
+			// support indexing and have the same index and element
+			// type. Special rules apply for maps in the sum type.
+			var tkey, telem Type // key is for map types only
+			nmaps := 0           // number of map types in sum type
 			if typ.is(func(t Type) bool {
 				var e Type
 				switch t := t.Under().(type) {
@@ -1495,21 +1495,58 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 				case *Slice:
 					e = t.elem
 				case *Map:
+					// If there are multiple maps in the sum type,
+					// they must have identical key types.
+					// TODO(gri) We may be able to relax this rule
+					// but it becomes complicated very quickly.
+					if tkey != nil && !Identical(t.key, tkey) {
+						return false
+					}
+					tkey = t.key
 					e = t.elem
+					nmaps++
 				case *TypeParam:
 					check.errorf(x, "type of %s contains a type parameter - cannot index (implementation restriction)", x)
 				case *instance:
-					unimplemented()
+					panic("unimplemented")
 				}
-				if e != nil && (e == elem || elem == nil) {
-					elem = e
-					return true
+				if e == nil || telem != nil && !Identical(e, telem) {
+					return false
 				}
-				return false
+				telem = e
+				return true
 			}) {
-				valid = true
-				x.mode = variable
-				x.typ = elem
+				// If there are maps, the index expression must be assignable
+				// to the map key type (as for simple map index expressions).
+				if nmaps > 0 {
+					var key operand
+					check.expr(&key, e.Index)
+					check.assignment(&key, tkey, "map index")
+					// ok to continue even if indexing failed - map element type is known
+
+					// If there are only maps, we are done.
+					if nmaps == len(typ.types) {
+						x.mode = mapindex
+						x.typ = telem
+						x.expr = e
+						return expression
+					}
+
+					// Otherwise we have mix of maps and other types. For
+					// now we require that the map key be an integer type.
+					// TODO(gri) This is probably not good enough.
+					valid = isInteger(tkey)
+					// avoid 2nd indexing error if indexing failed above
+					if !valid && key.mode == invalid {
+						goto Error
+					}
+					x.mode = value // map index expressions are not addressable
+				} else {
+					// no maps
+					valid = true
+					x.mode = variable
+				}
+				x.typ = telem
 			}
 		}
 

src/cmd/compile/internal/types2/testdata/expr3.src

@@ -103,6 +103,7 @@ func indexes() {
 	var ok mybool
 	_, ok = m["bar"]
 	_ = ok
+	_ = m[0 /* ERROR "cannot convert 0" */ ] + "foo" // ERROR "cannot convert"
 
 	var t string
 	_ = t[- /* ERROR "negative" */ 1]

src/cmd/compile/internal/types2/testdata/typeparams.go2

@@ -94,11 +94,19 @@ func _[T any] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
 func _[T interface{ type int }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
 func _[T interface{ type string }] (x T, i int) { _ = x[i] }
 func _[T interface{ type []int }] (x T, i int) { _ = x[i] }
-func _[T interface{ type [10]int, *[20]int, map[string]int }] (x T, i int) { _ = x[i] }
+func _[T interface{ type [10]int, *[20]int, map[int]int }] (x T, i int) { _ = x[i] }
 func _[T interface{ type string, []byte }] (x T, i int) { _ = x[i] }
 func _[T interface{ type []int, [1]rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
 func _[T interface{ type string, []rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
 
+// indexing with various combinations of map types in type lists (see issue #42616)
+func _[T interface{ type []E, map[int]E }, E any](x T, i int) { _ = x[i] }
+func _[T interface{ type []E }, E any](x T, i int) { _ = &x[i] }
+func _[T interface{ type map[int]E }, E any](x T, i int) { _, _ = x[i] } // comma-ok permitted
+func _[T interface{ type []E, map[int]E }, E any](x T, i int) { _ = &x /* ERROR cannot take address */ [i] }
+func _[T interface{ type []E, map[int]E, map[uint]E }, E any](x T, i int) { _ = x /* ERROR cannot index */ [i] } // different map element types
+func _[T interface{ type []E, map[string]E }, E any](x T, i int) { _ = x[i /* ERROR cannot use i */ ] }
+
 // slicing
 // TODO(gri) implement this