[CSSimplify] PointerToCPointer: Allow element type inference for "argument" pointers

If the type of the "argument" pointer is not yet resolved the solver
should equate it to that of the "parameter" pointer as regular
conversions do.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -13768,6 +13768,19 @@ ConstraintSystem::simplifyPointerToCPointerRestriction(
     return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
   };
 
+  auto elementLoc = locator.withPathElement(LocatorPathElt::GenericArgument(0));
+
+  if (swiftPtr->isTypeVariableOrMember()) {
+    // Inference between the equivalent pointer kinds is
+    // handled by regular pointer conversions.
+    if (swiftPtrKind == cPtrKind)
+      return SolutionKind::Error;
+
+    addConstraint(ConstraintKind::BindToPointerType, swiftPtr, cPtr,
+                  elementLoc);
+    return markSupported();
+  }
+
   // If pointers have the same element type there is nothing to do.
   if (swiftPtr->isEqual(cPtr))
     return markSupported();
@@ -13783,15 +13796,10 @@ ConstraintSystem::simplifyPointerToCPointerRestriction(
     // Unsafe[Mutable]Pointer<T> -> Unsafe[Mutable]Pointer<[U]Int8>
     if (cPtr->isInt8() || cPtr->isUInt8()) {
       // <T> can default to the type of C pointer.
-      addConstraint(
-          ConstraintKind::Defaultable, swiftPtr, cPtr,
-          locator.withPathElement(LocatorPathElt::GenericArgument(0)));
+      addConstraint(ConstraintKind::Defaultable, swiftPtr, cPtr, elementLoc);
       return markSupported();
     }
 
-    auto elementLoc =
-        locator.withPathElement(LocatorPathElt::GenericArgument(0));
-
     // Unsafe[Mutable]Pointer<Int{8, 16, ...}> <->
     // Unsafe[Mutable]Pointer<UInt{8, 16, ...}>
     if (swiftPtr->isInt() || swiftPtr->isUInt()) {