Error when declaring a constant within another constant declaration

spec/compiler/parser/parser_spec.cr

@@ -1919,6 +1919,16 @@ module Crystal
     assert_syntax_error "{1, ->do\n|x| x\end }", "unexpected token: \"|\", proc literals specify their parameters like this: ->(x : Type) { ... }"
     assert_syntax_error "{1, ->{ |_| x } }", "unexpected token: \"|\", proc literals specify their parameters like this: ->(param : Type) { ... }"
 
+    # #2874
+    assert_syntax_error "A = B = 1", "dynamic constant assignment"
+    assert_syntax_error "A = (B = 1)", "dynamic constant assignment"
+    assert_syntax_error "A = foo(B = 1)", "dynamic constant assignment"
+    assert_syntax_error "A = foo { B = 1 }", "dynamic constant assignment"
+    assert_syntax_error "A = begin; B = 1; end", "dynamic constant assignment"
+    assert_syntax_error "A = begin; 1; rescue; B = 1; end", "dynamic constant assignment"
+    assert_syntax_error "A = begin; 1; rescue; 1; else; B = 1; end", "dynamic constant assignment"
+    assert_syntax_error "A = begin; 1; ensure; B = 1; end", "dynamic constant assignment"
+
     assert_syntax_error "1 while 3", "trailing `while` is not supported"
     assert_syntax_error "1 until 3", "trailing `until` is not supported"
     assert_syntax_error "x++", "postfix increment is not supported, use `exp += 1`"

src/compiler/crystal/syntax/parser.cr

@@ -36,6 +36,7 @@ module Crystal
       @def_nest = 0
       @fun_nest = 0
       @type_nest = 0
+      @is_constant_assignment = false
 
       # Keeps track of current call args starting locations,
       # so if we parse a type declaration exactly at those points we
@@ -370,10 +371,12 @@ module Crystal
           else
             break unless can_be_assigned?(atomic)
 
-            if atomic.is_a?(Path) && (inside_def? || inside_fun?)
+            if atomic.is_a?(Path) && (inside_def? || inside_fun? || @is_constant_assignment)
               raise "dynamic constant assignment. Constants can only be declared at the top level or inside other types."
             end
 
+            @is_constant_assignment = true if atomic.is_a?(Path)
+
             if atomic.is_a?(Var) && atomic.name == "self"
               raise "can't change the value of self", location
             end
@@ -422,6 +425,8 @@ module Crystal
               end
             end
 
+            @is_constant_assignment = false if atomic.is_a?(Path)
+
             push_var atomic
 
             atomic = Assign.new(atomic, atomic_value).at(location)