[dev.go2go] go/types: make sure arrays, slices always have an element type

Addresses crash #19 of #39634.

Updates #39634.

Change-Id: I42208511a3fc27432891243dcf4799e80432c2c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/240903
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/expr.go

@@ -1423,6 +1423,13 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 			goto Error
 		}
 
+		// In pathological (invalid) cases (e.g.: type T1 [][[]T1{}[0][0]]T0)
+		// the element type may be accessed before it's set. Make sure we have
+		// a valid type.
+		if x.typ == nil {
+			x.typ = Typ[Invalid]
+		}
+
 		check.index(e.Index, length)
 		// ok to continue
 

src/go/types/fixedbugs/issue39634.go2

@@ -5,13 +5,13 @@
 package p
 
 // crash 1
-type nt(type )interface{g /* ERROR undeclared name */ }
-type ph(type e nt,g(d /* ERROR undeclared name */ ))s /* ERROR undeclared name */
-func(*ph(e,e /* ERROR redeclared */ ))h(d /* ERROR undeclared name */ )
+type nt1(type )interface{g /* ERROR undeclared name */ }
+type ph1(type e nt1,g(d /* ERROR undeclared name */ ))s /* ERROR undeclared name */
+func(*ph1(e,e /* ERROR redeclared */ ))h(d /* ERROR undeclared name */ )
 
 // crash 2
-type Numeric2 interface{t /* ERROR not a type */ }
-func t(type T Numeric2)(s[]T){0 /* ERROR not a type */ (){s /* ERROR cannot index */ [0][0]}}
+type Numeric2 interface{t2 /* ERROR not a type */ }
+func t2(type T Numeric2)(s[]T){0 /* ERROR not a type */ (){s /* ERROR cannot index */ [0][0]}}
 
 // crash 4
 type Numeric4 interface{t4 /* ERROR not a type */ }
@@ -35,6 +35,9 @@ type Z17 interface {
 }
 func F17(type T Z17)(T)
 
+// crash 19
+type Z19 [][[]Z19{}[0][0]]c19 /* ERROR undeclared */
+
 // crash 20
 type Z20 /* ERROR illegal cycle */ interface{ Z20 }
 func F20(type t Z20)() { F20(t /* ERROR invalid composite literal type */ {}) }