[Sema] only check conformance of synth. extensions

Author: kavon

lib/Sema/TypeChecker.cpp

@@ -293,8 +293,18 @@ TypeCheckSourceFileRequest::evaluate(Evaluator &eval, SourceFile *SF) const {
     // Type-check macro-generated or implicitly-synthesized extensions.
     if (auto *synthesizedSF = SF->getSynthesizedFile()) {
       for (auto *decl : synthesizedSF->getTopLevelDecls()) {
-        assert(isa<ExtensionDecl>(decl));
-        TypeChecker::typeCheckDecl(decl);
+        auto extension = cast<ExtensionDecl>(decl);
+
+        // Limit typechecking of synthesized _impicit_ extensions to conformance
+        // checking. This is done because a conditional conformance to Copyable
+        // is synthesized as an extension, based on the markings of `~Copyable`
+        // in a value type. This call to `checkConformancesInContext` will
+        // the actual check to verify that the conditional extension is correct,
+        // as it may be an invalid conformance.
+        if (extension->isImplicit())
+          TypeChecker::checkConformancesInContext(extension);
+        else
+          TypeChecker::typeCheckDecl(extension);
       }
     }
     SF->typeCheckDelayedFunctions();