Have Germany and Poland ever signed a peace treaty after WWII?

My understanding is that the former government of Poland, the second Polish Republic, never formally declared war on the National Socialist Republic of Germany, and it was a de facto war.

The People's Republic of Poland was not reconstituted until 1952. By that time the Third Reich was long gone and there was nothing to declare war on.

Note that the former government of Poland, in exile in London during the war, was never restored to power.

So, the bottom line here is that both the governments of Germany and Poland were eliminated during the war and by the time the countries had new governments (and borders) a state of peace existed.

You are making the mistake of thinking that "countries" declare war. That is not how it works in diplomacy. People in the form of their governments declare war on each other. The land has nothing to do with it. For example, if you read the declaration of war by the United States Congress it specifically is against the government of Germany, not Germany, the GOVERNMENT of Germany.

They did not, and it was a complex diplomatic issue until successfully addressed with a rather obscure instrument.

The problem was not peace itself but the border which according to the Potsdam Agreement was supposed to become final only upon the conclusion of the peace treaty. As the so called "2+4 process" on the unification of Germany in 1990 did not include Poland as a participant and was expected to produce the Final Settlement treaty (not the Peace Treaty), Poland got concerned least the legal loophole was not closed and her borders could be challenged. So the negotiating parties invited Polish representative for a short meeting in Paris held separately of the main 2+4 negotiations venue and agreed on a special protocol which stipulated that united Germany will see the borders as final, will speedily conclude a treaty with Poland, and will not allow any circumstances to affect this matter. It was also added that no peace treaty will be expected.

This protocol was drafted in French only and was formally delivered to Poland by the French Embassy in Warsaw on July 23, 1990. It has not been published until quite recently despite the fact that Poland considers it a key accomplishment of its diplomacy and a cornerstone of its rapprochement with Germany.

In other words, Germany (or rather both Germanies as it was going on before the ultimate unification) formally renounced the peace treaty requirement for the finality of border settlement but it was done with as little fanfare as possible.

A peace treaty with just Poland really makes little sense; it would have had to be a peace treaty with the Soviet Union (and the US, Britain and France) as well.

Until 1990, West Germany would not have considered a peace treaty because it would mean permanently ceding East Germany. The Germans preferred to have the formal status of occupied country.

After 1990, a peace treaty really made little sense; it was made obsolete by historic events. If anything, it could have re-ignited old memories that everybody involved would prefer to bury. Poland and (post-Soviet) Russia both self-identified as having transformed themselves, and Germany was all to happy to agree with that. So WW II pretty much ended by mutually forgetting about it, rather than a formal peace treaty.

No, there was no state of war between Germany and Poland. A state of war can end either with a peace treaty or with a surrender. In this case, there was a surrender of Germany.

Furthermore. The German state ceased to exist in mid-1945. If was completely demolished and as such, its foreign relations as well. After a while, two new states were instituted by the former Allies. With these new states, Poland could sign or not to sign a peace treaty, but they were never in a state of war.