go/types, types2: better error message for bad assignment

If the LHS of an assignment is neither addressable nor a map expression
(and not the blank identifier), explicitly say so for a better error
message.

For #3117.

Change-Id: I4bffc35574fe390a0567e89182b23585eb5a90de
Reviewed-on: https://go-review.googlesource.com/c/go/+/492875
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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

@@ -222,7 +222,7 @@ func (check *Checker) lhsVar(lhs syntax.Expr) Type {
 				return Typ[Invalid]
 			}
 		}
-		check.errorf(&x, UnassignableOperand, "cannot assign to %s", &x)
+		check.errorf(&x, UnassignableOperand, "cannot assign to %s (neither addressable nor a map index expression)", x.expr)
 		return Typ[Invalid]
 	}
 

src/go/types/assignments.go

@@ -221,7 +221,7 @@ func (check *Checker) lhsVar(lhs ast.Expr) Type {
 				return Typ[Invalid]
 			}
 		}
-		check.errorf(&x, UnassignableOperand, "cannot assign to %s", &x)
+		check.errorf(&x, UnassignableOperand, "cannot assign to %s (neither addressable nor a map index expression)", x.expr)
 		return Typ[Invalid]
 	}
 

src/internal/types/testdata/fixedbugs/issue3117.go

@@ -0,0 +1,13 @@
+// Copyright 2023 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 p
+
+type S struct {
+	a [1]int
+}
+
+func _(m map[int]S, key int) {
+	m /* ERROR "cannot assign to m[key].a[0] (neither addressable nor a map index expression)" */ [key].a[0] = 0
+}