go/types: improve error messages for unexpected ListExprs

This CL is a mix of CL 312149 and CL 314409, adding the
Checker.singleIndex method to provide better error messages when an
unexpected ListExpr is encountered.

Change-Id: I45d6de9b4dfc299dc2d356ca14d05c9191de818d
Reviewed-on: https://go-review.googlesource.com/c/go/+/314869
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 37 insertions, 5 deletions

diff --git a/src/go/types/index.go b/src/go/types/index.go
index f51c3f6acf..2ba3475f89 100644
--- a/src/go/types/index.go
+++ b/src/go/types/index.go
@@ -77,8 +77,13 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		x.typ = typ.elem
 
 	case *Map:
+		index := check.singleIndex(e)
+		if index == nil {
+			x.mode = invalid
+			return
+		}
 		var key operand
-		check.expr(&key, e.Index)
+		check.expr(&key, index)
 		check.assignment(&key, typ.key, "map index")
 		// ok to continue even if indexing failed - map element type is known
 		x.mode = mapindex
@@ -132,8 +137,13 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 			// If there are maps, the index expression must be assignable
 			// to the map key type (as for simple map index expressions).
 			if nmaps > 0 {
+				index := check.singleIndex(e)
+				if index == nil {
+					x.mode = invalid
+					return
+				}
 				var key operand
-				check.expr(&key, e.Index)
+				check.expr(&key, index)
 				check.assignment(&key, tkey, "map index")
 				// ok to continue even if indexing failed - map element type is known
 
@@ -170,8 +180,8 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		return
 	}
 
-	if e.Index == nil {
-		check.invalidAST(e, "missing index for %s", x)
+	index := check.singleIndex(e)
+	if index == nil {
 		x.mode = invalid
 		return
 	}
@@ -183,7 +193,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		x.typ = Typ[Invalid]
 	}
 
-	check.index(e.Index, length)
+	check.index(index, length)
 	return false
 }
 
@@ -298,6 +308,28 @@ L:
 	}
 }
 
+// singleIndex returns the (single) index from the index expression e.
+// If the index is missing, or if there are multiple indices, an error
+// is reported and the result is nil.
+func (check *Checker) singleIndex(e *ast.IndexExpr) ast.Expr {
+	index := e.Index
+	if index == nil {
+		check.invalidAST(e, "missing index for %s", e)
+		return nil
+	}
+
+	indexes := typeparams.UnpackExpr(index)
+	if len(indexes) == 0 {
+		check.invalidAST(index, "index expression %v with 0 indices", index)
+		return nil
+	}
+	if len(indexes) > 1 {
+		// TODO(rFindley) should this get a distinct error code?
+		check.invalidOp(indexes[1], _InvalidIndex, "more than one index")
+	}
+	return indexes[0]
+}
+
 // index checks an index expression for validity.
 // If max >= 0, it is the upper bound for index.
 // If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.