Fix: Do not consider global Path in def parameter restriction as free variable

spec/compiler/semantic/def_overload_spec.cr

@@ -235,6 +235,27 @@ describe "Semantic: def overload" do
     ") { int32 }
   end
 
+  it "does not consider global paths as free variables (1)" do
+    assert_error <<-CR, "undefined constant ::Foo"
+      def foo(x : ::Foo) forall Foo
+      end
+
+      foo(1)
+      CR
+  end
+
+  it "does not consider global paths as free variables (2)" do
+    assert_error <<-CR, "no overload matches"
+      class Foo
+      end
+
+      def foo(x : ::Foo) forall Foo
+      end
+
+      foo(1)
+      CR
+  end
+
   it "prefers more specific overload than one with free variables" do
     assert_type("
       require \"prelude\"

src/compiler/crystal/semantic/abstract_def_checker.cr

@@ -360,10 +360,8 @@ class Crystal::AbstractDefChecker
     end
 
     def visit(node : Path)
-      if !node.global? && node.names.size == 1
+      if name = node.single_name?
         # Check if it matches any of the generic type vars
-        name = node.names.first
-
         type_var = @generic_type.type_vars[name]?
         if type_var.is_a?(Var)
           # Check that it's actually a type parameter on the base type

src/compiler/crystal/semantic/ast.cr

@@ -268,8 +268,8 @@ module Crystal
     def free_var?(node : Path)
       free_vars = @free_vars
       return false unless free_vars
-
-      !node.global? && node.names.size == 1 && free_vars.includes?(node.names.first)
+      name = node.single_name?
+      !name.nil? && free_vars.includes?(name)
     end
 
     def free_var?(any)

src/compiler/crystal/semantic/restrictions.cr

@@ -481,8 +481,8 @@ module Crystal
       # Match all concrete types first
       free_var_count = other.types.count do |other_type|
         other_type.is_a?(Path) &&
-          other_type.names.size == 1 &&
-          context.has_def_free_var?(other_type.names.first)
+          (first_name = other_type.single_name?) &&
+          context.has_def_free_var?(first_name)
       end
       if free_var_count > 1
         other.raise "can't specify more than one free var in union restriction"
@@ -495,21 +495,18 @@ module Crystal
     end
 
     def restrict(other : Path, context)
-      single_name = other.names.size == 1
-      if single_name
-        first_name = other.names.first
+      if first_name = other.single_name?
         if context.has_def_free_var?(first_name)
           return context.set_free_var(first_name, self)
         end
       end
 
-      if single_name
+      if first_name
         owner = context.instantiated_type
 
         # Special case: if we have an *uninstantiated* generic type like Foo(X)
         # and a restriction X, it matches, and we add X to the free vars.
         if owner.is_a?(GenericType)
-          first_name = other.names.first
           if owner.type_vars.includes?(first_name)
             context.set_free_var(first_name, self)
             return self
@@ -530,8 +527,7 @@ module Crystal
         return restrict ident_type, context
       end
 
-      if single_name
-        first_name = other.names.first
+      if first_name
         if context.defining_type.type_var?(first_name)
           return context.set_free_var(first_name, self)
         end
@@ -653,8 +649,8 @@ module Crystal
       # Match all concrete types first
       free_vars, other_types = other.types.partition do |other_type|
         other_type.is_a?(Path) &&
-          other_type.names.size == 1 &&
-          context.has_def_free_var?(other_type.names.first)
+          (first_name = other_type.single_name?) &&
+          context.has_def_free_var?(first_name)
       end
       if free_vars.size > 1
         other.raise "can't specify more than one free var in union restriction"
@@ -871,17 +867,15 @@ module Crystal
             return type_var
           end
         when Path
-          if other_type_var.names.size == 1
-            name = other_type_var.names.first
-
+          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(name)
+            existing = context.get_free_var(first_name)
             if existing && existing != type_var
               return nil
             end
 
-            context.set_free_var(name, type_var)
+            context.set_free_var(first_name, type_var)
             return type_var
           end
         else
@@ -1091,9 +1085,7 @@ module Crystal
     end
 
     def restrict(other : Path, context)
-      single_name = other.names.size == 1
-      if single_name
-        first_name = other.names.first
+      if first_name = other.single_name?
         if context.has_def_free_var?(first_name)
           return context.set_free_var(first_name, self)
         end
@@ -1105,9 +1097,7 @@ module Crystal
           return self
         end
       else
-        single_name = other.names.size == 1
-        if single_name
-          first_name = other.names.first
+        if first_name = other.single_name?
           if context.defining_type.type_var?(first_name)
             return context.set_free_var(first_name, self)
           else

src/compiler/crystal/syntax/ast.cr

@@ -1383,6 +1383,11 @@ module Crystal
       names.size == 1 && names.first == name
     end
 
+    # Returns this path's name if it has only one part and is not global
+    def single_name?
+      names.first if names.size == 1 && !global?
+    end
+
     def clone_without_location
       ident = Path.new(@names.clone, @global)
       ident