[dev.go2go] go/types: don't crash instantiating defined types that are not fully set up

Addresses crash #16 of #39634.

Updates #39634.

Change-Id: I33063d6e2176586f097d2d1391f490f1775a2202
Reviewed-on: https://go-review.googlesource.com/c/go/+/241122
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/fixedbugs/issue39634.go2

@@ -27,6 +27,10 @@ type foo8(type A) interface { type A }
 func bar8(type A foo8(A))(a A) {}
 func main8() {}
 
+// crash 16
+type Foo16(type T) r16 /* ERROR not a type */
+func r16(type T)() Foo16((Foo16(T)))
+
 // crash 17
 type Y17 interface{ c() }
 type Z17 interface {

src/go/types/subst.go

@@ -337,8 +337,6 @@ func (subst *subster) typ(typ Type) Type {
 			}
 		}
 
-		assert(t.underlying != nil)
-
 		if t.tparams == nil {
 			dump(">>> %s is not parameterized", t)
 			return t // type is not parameterized
@@ -399,7 +397,7 @@ func (subst *subster) typ(typ Type) Type {
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
-		named.underlying = subst.typ(t.underlying)
+		named.underlying = subst.typOrNil(t.underlying)
 		named.orig = named.underlying // for cycle detection (Checker.validType)
 
 		return named