lang: funcs: Don't allow interpolation in printf format string

We'd like to pre-compute the interpolation if we can, so that we can run
this code properly... For now, we can't so it's a compile time error...
Hopefully we can remove this restriction in the future. The problem is
the string must be a constant, or it would be possible to switch it from
"%d %s" to "%s %d %d" or anything that changes the type signature.

lang/funcs/core/fmt/printf_func.go

@@ -85,6 +85,13 @@ func (obj *PrintfFunc) Polymorphisms(partialType *types.Type, partialValues []ty
 		}
 	}
 
+	// FIXME: we'd like to pre-compute the interpolation if we can, so that
+	// we can run this code properly... for now, we can't, so it's a compile
+	// time error...
+	if partialValues[0] == nil {
+		return nil, fmt.Errorf("could not determine type from format string")
+	}
+
 	format := partialValues[0].Str() // must not panic
 	typList, err := parseFormatToTypeList(format)
 	if err != nil {