cmd/compile/internal/types2: fix indexing of generic types

Correctly track if the index expression is addressable.
Rewrote code slightly.

Fixes #49275.

Change-Id: Ic54edd0213a091173ff5403ab0e3e1f1fca0e361
Reviewed-on: https://go-review.googlesource.com/c/go/+/360603
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -101,77 +101,80 @@ func (check *Checker) indexExpr(x *operand, e *syntax.IndexExpr) (isFuncInst boo
 
 	case *TypeParam:
 		// TODO(gri) report detailed failure cause for better error messages
-		var tkey, telem Type // tkey != nil if we have maps
+		var key, elem Type // key != nil: we must have all maps
+		mode := variable   // non-maps result mode
+		// TODO(gri) factor out closure and use it for non-typeparam cases as well
 		if typ.underIs(func(u Type) bool {
-			var key, elem Type
-			alen := int64(-1) // valid if >= 0
+			l := int64(-1) // valid if >= 0
+			var k, e Type  // k is only set for maps
 			switch t := u.(type) {
 			case *Basic:
-				if !isString(t) {
-					return false
+				if isString(t) {
+					e = universeByte
+					mode = value
 				}
-				elem = universeByte
 			case *Array:
-				elem = t.elem
-				alen = t.len
-			case *Pointer:
-				a, _ := under(t.base).(*Array)
-				if a == nil {
-					return false
+				l = t.len
+				e = t.elem
+				if x.mode != variable {
+					mode = value
+				}
+			case *Pointer:
+				if t := asArray(t.base); t != nil {
+					l = t.len
+					e = t.elem
 				}
-				elem = a.elem
-				alen = a.len
 			case *Slice:
-				elem = t.elem
+				e = t.elem
 			case *Map:
-				key = t.key
-				elem = t.elem
-			default:
+				k = t.key
+				e = t.elem
+			}
+			if e == nil {
 				return false
 			}
-			assert(elem != nil)
-			if telem == nil {
+			if elem == nil {
 				// first type
-				tkey, telem = key, elem
-				length = alen
-			} else {
-				// all map keys must be identical (incl. all nil)
-				if !Identical(key, tkey) {
-					return false
-				}
-				// all element types must be identical
-				if !Identical(elem, telem) {
-					return false
-				}
-				tkey, telem = key, elem
-				// track the minimal length for arrays
-				if alen >= 0 && alen < length {
-					length = alen
-				}
+				length = l
+				key, elem = k, e
+				return true
+			}
+			// all map keys must be identical (incl. all nil)
+			// (that is, we cannot mix maps with other types)
+			if !Identical(key, k) {
+				return false
+			}
+			// all element types must be identical
+			if !Identical(elem, e) {
+				return false
+			}
+			// track the minimal length for arrays, if any
+			if l >= 0 && l < length {
+				length = l
 			}
 			return true
 		}) {
 			// For maps, the index expression must be assignable to the map key type.
-			if tkey != nil {
+			if key != nil {
 				index := check.singleIndex(e)
 				if index == nil {
 					x.mode = invalid
 					return false
 				}
-				var key operand
-				check.expr(&key, index)
-				check.assignment(&key, tkey, "map index")
+				var k operand
+				check.expr(&k, index)
+				check.assignment(&k, key, "map index")
 				// ok to continue even if indexing failed - map element type is known
 				x.mode = mapindex
-				x.typ = telem
+				x.typ = elem
 				x.expr = e
 				return false
 			}
 
 			// no maps
 			valid = true
-			x.mode = variable
-			x.typ = telem
+			x.mode = mode
+			x.typ = elem
 		}
 	}
 

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

@@ -114,6 +114,14 @@ func _[T interface{ [10]int }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERR
 func _[T interface{ [10]byte | string }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERROR out of bounds */ ] }
 func _[T interface{ [10]int | *[20]int | []int }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERROR out of bounds */ ] }
 
+// indexing with strings and non-variable arrays (assignment not permitted)
+func _[T string](x T) { _ = x[0]; x /* ERROR cannot assign */ [0] = 0 }
+func _[T []byte | string](x T) { x /* ERROR cannot assign */ [0] = 0 }
+func _[T [10]byte]() { f := func() (x T) { return }; f /* ERROR cannot assign */ ()[0] = 0 }
+func _[T [10]byte]() { f := func() (x *T) { return }; f /* ERROR cannot index */ ()[0] = 0 }
+func _[T [10]byte]() { f := func() (x *T) { return }; (*f())[0] = 0 }
+func _[T *[10]byte]() { f := func() (x T) { return }; f()[0] = 0 }
+
 // slicing
 
 func _[T interface{ ~[10]E }, E any] (x T, i, j, k int) { var _ []E = x[i:j] }