"Mr Trump had asked a foreign-policy expert "why can't we use" nuclear weapons, according to MSNBC host Joe Scarborough, who said he spoke to the expert.
"I'll be very careful here. Several months ago, a foreign policy expert, on the international level, went to advise Donald Trump, and three times he asked about the use of nuclear weapons," Scarborough said on his program Morning Joe.
"Three times he asked. At one point, 'If we have them, why can't we use them?'" Scarborough continued."

Not so much interested in what Mr Joe Scarborough thinks about this, the nuance and weighting Mr. Joe Scarborough tries to infer. I would be more interested in the full context of the conversation between Trump and the foreign-policy expert from the expert himself.

"The whole idea behind nuclear deterrence is that you don't use the damn things,"

Not strictly the full story that is it. Between two adversaries who both have nuclear weapons the deterrence factor comes in the form of firm belief that either side will have no qualms about using them, add to that equation "Nuke Triad" capability on both sides and the reality of Mutually Assured Destruction - then you have a deterrent.

"Towards the end of May, Mr Trump said that if the Islamic State group was to attack the US, he might return fire with a nuke"

Ah so no original thought there then:

"On 19 January 2006, Chirac said that France was prepared to launch a nuclear strike against any country that sponsors a terrorist attack against French interests. He said his country's nuclear arsenal had been reconfigured to include the ability to make a tactical strike in retaliation for terrorism."

France's nuclear strike force - Force de Frappe - Like that of the US Military id "nuke triad" capable. Before leaving office as Iran's nuclear weapons programme became undeniable and they reported that they now had weapons capable of hitting any target in Europe, Chirac had France's SSBNs weapons load reconfigured so that four of the missile tubes were loaded with tactical warheads instead of full multiple individually targeted warheads.

Early days of the Cold War, NATO it was made clear to the Soviets and Warsaw Pact forces in Europe that any use by them of Chemical or Biological weapons would result in immediate use by NATO of tactical nuclear weapons. Reagan reaffirmed this strategy, in both cases the U.S.S.R. considered the threat credible and no such attack was ever contemplated. The Russians passed this on to Saddam Hussein in 1990 which is why Saddam did not use the Chemical Weapons he undoubtedly did have against Coalition forces in 1991.