Fix restriction of numeral generic argument against non-free variable…

… `Path` (#12784)

spec/compiler/semantic/restrictions_spec.cr

@@ -617,8 +617,7 @@ describe "Restrictions" do
           CRYSTAL
       end
 
-      # TODO: enable in #12784
-      pending "inserts constant before free variable with same name" do
+      it "inserts constant before free variable with same name" do
         assert_type(<<-CRYSTAL) { tuple_of([char, bool]) }
           class Foo(T); end
 
@@ -636,7 +635,7 @@ describe "Restrictions" do
           CRYSTAL
       end
 
-      pending "keeps constant before free variable with same name" do
+      it "keeps constant before free variable with same name" do
         assert_type(<<-CRYSTAL) { tuple_of([char, bool]) }
           class Foo(T); end
 
@@ -1140,6 +1139,31 @@ describe "Restrictions" do
       "expected argument #2 to 'foo' to be StaticArray(UInt8, 10), not StaticArray(UInt8, 11)"
   end
 
+  it "does not treat single path as free variable when given number (1) (#11859)" do
+    assert_error <<-CR, "expected argument #1 to 'Foo(1)#foo' to be Foo(1), not Foo(2)"
+      class Foo(T)
+        def foo(x : Foo(T))
+        end
+      end
+
+      Foo(1).new.foo(Foo(2).new)
+      CR
+  end
+
+  it "does not treat single path as free variable when given number (2) (#11859)" do
+    assert_error <<-CR, "expected argument #1 to 'foo' to be Foo(1), not Foo(2)"
+      X = 1
+
+      class Foo(T)
+      end
+
+      def foo(x : Foo(X))
+      end
+
+      foo(Foo(2).new)
+      CR
+  end
+
   it "restricts aliased typedef type (#9474)" do
     assert_type(%(
       lib A

src/compiler/crystal/semantic/restrictions.cr

@@ -705,7 +705,20 @@ module Crystal
     end
 
     def restriction_of?(other : Generic, owner, self_free_vars = nil, other_free_vars = nil)
-      return true if self == other
+      # The two `Foo(X)`s below are not equal because only one of them is bound
+      # and the other one is unbound, so we compare the free variables too:
+      # (`X` is an alias or a numeric constant)
+      #
+      # ```
+      # def foo(x : Foo(X)) forall X
+      # end
+      #
+      # def foo(x : Foo(X))
+      # end
+      # ```
+      #
+      # See also the todo in `Path#restriction_of?(Path)`
+      return true if self == other && self_free_vars == other_free_vars
       return false unless name == other.name && type_vars.size == other.type_vars.size
 
       # Special case: NamedTuple against NamedTuple
@@ -1242,15 +1255,17 @@ module Crystal
           end
         when Path
           if first_name = other_type_var.single_name?
-            # If the free variable is already set to another
-            # number, there's no match
-            existing = context.get_free_var(first_name)
-            if existing && existing != type_var
-              return nil
-            end
+            if context.has_def_free_var?(first_name)
+              # If the free variable is already set to another
+              # number, there's no match
+              existing = context.get_free_var(first_name)
+              if existing && existing != type_var
+                return nil
+              end
 
-            context.set_free_var(first_name, type_var)
-            return type_var
+              context.set_free_var(first_name, type_var)
+              return type_var
+            end
           end
         else
           # Restriction is not possible (maybe return nil here?)