Fix Python requirements when resolving lock

poetry/puzzle/provider.py

@@ -275,10 +275,6 @@ class Provider:
         else:
             dependencies = package.requires
 
-        dependencies = [
-            dep for dep in dependencies if dep.name not in self.UNSAFE_PACKAGES
-        ]
-
         if not self._package.python_constraint.allows_any(package.python_constraint):
             return [
                 Incompatibility(
@@ -295,6 +291,14 @@ class Provider:
                 )
             ]
 
+        dependencies = [
+            dep
+            for dep in dependencies
+            if dep.name not in self.UNSAFE_PACKAGES
+            and self._package.python_constraint.allows_any(dep.python_constraint)
+            and self._package.platform_constraint.matches(dep.platform_constraint)
+        ]
+
         return [
             Incompatibility(
                 [Term(package.to_dependency(), True), Term(dep, False)],
@@ -316,7 +320,7 @@ class Provider:
             for r in package.requires
             if r.is_activated()
             and self._package.python_constraint.allows_any(r.python_constraint)
-            and self._package.platform_constraint.matches(package.platform_constraint)
+            and self._package.platform_constraint.matches(r.platform_constraint)
         ]
 
         # Searching for duplicate dependencies

tests/installation/fixtures/with-optional-dependencies.test

@@ -8,18 +8,6 @@ python-versions = "*"
 platform = "*"
 
 [[package]]
-name = "B"
-version = "1.1"
-description = ""
-category = "main"
-optional = false
-python-versions = "*"
-platform = "*"
-
-[package.requirements]
-python = "~2.4"
-
-[[package]]
 name = "C"
 version = "1.3"
 description = ""
@@ -56,6 +44,5 @@ content-hash = "123456789"
 
 [metadata.hashes]
 A = []
-B = []
 C = []
 D = []