add tests for operators that are not supposed to be suffixable

test/parse.jl

@@ -1233,6 +1233,9 @@ end === (3, String)
 @test parse("3 +̂′ 4") == Expr(:call, :+̂′, 3, 4)
 @test parse("3 +⁽¹⁾ 4") == Expr(:call, :+⁽¹⁾, 3, 4)
 @test parse("3 +₍₀₎ 4") == Expr(:call, :+₍₀₎, 3, 4)
+for bad in ('=', '$', ':', "||", "&&", "->", "<:")
+    @test_throws ParseError parse("3 $(bad)⁽¹⁾ 4")
+end
 @test Base.operator_precedence(:+̂) == Base.operator_precedence(:+)
 
 # issue #19351