[dev.go2go] go/types: generic types in selector expressions must be instantiated

Addresses crash 24 of #39634.

Updates #39634.

Change-Id: If44e834834a07719e7f727d12cc24ac4af0997e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/245739
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/call.go

@@ -521,6 +521,11 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 		goto Error
 	}
 
+	if x.mode == typexpr && isGeneric(x.typ) {
+		check.errorf(e.Pos(), "cannot use generic type %s without instantiation", x.typ)
+		goto Error
+	}
+
 	obj, index, indirect = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
 		switch {

src/go/types/fixedbugs/issue39634.go2

@@ -64,6 +64,10 @@ func F20(type t Z20)() { F20(t /* ERROR invalid composite literal type */ {}) }
 type Z21 /* ERROR illegal cycle */ interface{ Z21 }
 func F21(type *T Z21)() { ( /* ERROR not used */ F21(Z21)) }
 
+// crash 24
+type T24(type P) P
+func (r T24(P)) m() { T24 /* ERROR without instantiation */ .m() }
+
 // crash 27
 func e27(type T)() interface{ (x27 /* ERROR not a type */ ) }
 func x27() { e27() /* ERROR cannot infer T */ }