Merge pull request #41436 from xedin/allow-specialization-from-default-expr

[TypeChecker] Allow inference from default expressions in certain scenarios (under a flag)

lib/Sema/CSDiagnostics.cpp

@@ -257,6 +257,11 @@ ValueDecl *RequirementFailure::getDeclRef() const {
       return cast<ValueDecl>(getDC()->getAsDecl());
     }
 
+    if (contextualPurpose == CTP_DefaultParameter ||
+        contextualPurpose == CTP_AutoclosureDefaultParameter) {
+      return cast<ValueDecl>(getDC()->getParent()->getAsDecl());
+    }
+
     return getAffectedDeclFromType(contextualTy);
   }
 
@@ -8087,3 +8092,27 @@ bool SwiftToCPointerConversionInInvalidContext::diagnoseAsError() {
                  paramType, callee->getDescriptiveKind(), callee->getName());
   return true;
 }
+
+bool DefaultExprTypeMismatch::diagnoseAsError() {
+  auto *locator = getLocator();
+
+  unsigned paramIdx =
+      locator->castLastElementTo<LocatorPathElt::ApplyArgToParam>()
+          .getParamIdx();
+
+  emitDiagnostic(diag::cannot_convert_default_value_type_to_argument_type,
+                 getFromType(), getToType(), paramIdx);
+
+  auto overload = getCalleeOverloadChoiceIfAvailable(locator);
+  assert(overload);
+
+  auto *PD = getParameterList(overload->choice.getDecl())->get(paramIdx);
+
+  auto note = emitDiagnosticAt(PD->getLoc(), diag::default_value_declared_here);
+
+  if (auto *defaultExpr = PD->getTypeCheckedDefaultExpr()) {
+    note.highlight(defaultExpr->getSourceRange());
+  }
+
+  return true;
+}