[dev.go2go] go/types: remove incorrect assert and replace with explanatory comment

Addresses crash 26 of #39634.
Also, added test case for crash 25 which was fixed with
https://golang.org/cl/245739.

Updates #39634.

Change-Id: I19793eb79fce63052ba055cde0b1dafdd9476895
Reviewed-on: https://go-review.googlesource.com/c/go/+/245740
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/fixedbugs/issue39634.go2

@@ -68,6 +68,15 @@ func F21(type *T Z21)() { ( /* ERROR not used */ F21(Z21)) }
 type T24(type P) P
 func (r T24(P)) m() { T24 /* ERROR without instantiation */ .m() }
 
+// crash 25
+type T25(type A) int
+func (t T25(A)) m1() {}
+var x T25 /* ERROR without instantiation */ .m1
+
+// crash 26
+type T26 = interface{ F26(type Z)() }
+func F26(type Z)() T26 { return F26() /* ERROR cannot infer Z */ }
+
 // crash 27
 func e27(type T)() interface{ (x27 /* ERROR not a type */ ) }
 func x27() { e27() /* ERROR cannot infer T */ }

src/go/types/infer.go

@@ -200,7 +200,9 @@ func isParameterized(typ Type, seen map[Type]bool) (res bool) {
 		return isParameterizedList(t.types, seen)
 
 	case *Signature:
-		assert(t.tparams == nil) // TODO(gri) is this correct?
+		// t.tparams may not be nil if we are looking at a signature
+		// of a function type (or an interface method) that is part of
+		// the type we're testing. We don't care about these parameters.
 		// TODO(gri) Rethink check below.
 		//assert(t.recv == nil || !isParameterized(t.recv.typ))
 		return isParameterized(t.params, seen) || isParameterized(t.results, seen)