go/types: better cycle reporting for some cyclic composite literals

To evaluate the type of composite literals, the type checker called
Checker.typ which breaks cycles. As a result, certain cycles were
not reported with actual cycle reporting, but caught due to other
uninitialized fields (with less nice error message).

The change now calls Checker.typExpr at the relevant call site.

For #18643.

Change-Id: Iecb3f0e1afb4585b85553b6c581212f52ac3a1c4
Reviewed-on: https://go-review.googlesource.com/115456
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/builtins.go

@@ -174,7 +174,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			}
 		}
 
-		if mode == invalid {
+		if mode == invalid && typ != Typ[Invalid] {
 			check.invalidArg(x.pos(), "%s for %s", x, bin.name)
 			return
 		}

src/go/types/expr.go

@@ -1064,7 +1064,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 					break
 				}
 			}
-			typ = check.typ(e.Type)
+			typ = check.typExpr(e.Type, nil, nil)
 			base = typ
 
 		case hint != nil:

src/go/types/testdata/cycles.src

@@ -147,7 +147,7 @@ type (
 // test cases for issue 18643
 // (type cycle detection when non-type expressions are involved)
 type (
-	T14 [len(T14 /* ERROR cycle */ {})]int
+	T14 /* ERROR cycle */ [len(T14{})]int
 	T15 [][len(T15 /* ERROR cycle */ {})]int
 	T16 map[[len(T16 /* ERROR cycle */ {1:2})]int]int
 	T17 map[int][len(T17 /* ERROR cycle */ {1:2})]int

src/go/types/typexpr.go

@@ -161,6 +161,11 @@ func (check *Checker) typExpr(e ast.Expr, def *Named, path []*TypeName) (T Type)
 	return
 }
 
+// typ is like typExpr (with a nil argument for the def parameter),
+// but typ breaks type cycles. It should be called for components of
+// types that break cycles, such as pointer base types, slice or map
+// element types, etc. See the comment in typExpr for details.
+//
 func (check *Checker) typ(e ast.Expr) Type {
 	// typExpr is called with a nil path indicating an indirection:
 	// push indir sentinel on object path