SILGen: Fix incorrect assertion

Since 'decl' was already a StructDecl, 'decl->getDeclContext()' was
giving us a non-generic module scope context.

Fixes <https://bugs.swift.org/browse/SR-10075>.

Author: slavapestov

lib/SILGen/SILGenConstructor.cpp

@@ -152,9 +152,7 @@ static void emitImplicitValueConstructor(SILGenFunction &SGF,
       if (!field->isStatic() && field->isLet() &&
           field->getParentInitializer()) {
 #ifndef NDEBUG
-        auto fieldTy = decl->getDeclContext()->mapTypeIntoContext(
-            field->getInterfaceType());
-        assert(fieldTy->isEqual(field->getParentInitializer()->getType())
+        assert(field->getType()->isEqual(field->getParentInitializer()->getType())
                && "Checked by sema");
 #endif
 

test/SILGen/default_constructor.swift

@@ -121,3 +121,15 @@ struct I {
   // CHECK: assign [[RESULT]] to [[X_ADDR]] : $*Int
   init<T>(_: T) {}
 }
+
+// https://bugs.swift.org/browse/SR-10075
+func defaultValue<T>() -> T {
+    fatalError()
+}
+
+struct S<T> {
+  let value1: T = defaultValue()
+  let value2: Int
+
+  // CHECK-LABEL: sil hidden [ossa] @$s19default_constructor1SV6value2ACyxGSi_tcfC : $@convention(method) <T> (Int, @thin S<T>.Type) -> @out S<T>
+}