Merge pull request #58432 from DougGregor/downgrade-sendable-function-conversions-5.7

Downgrade missing @sendable to a warning in Swift 5.x mode.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -2672,11 +2672,15 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   }
 
   /// Whether to downgrade to a concurrency warning.
-  auto isConcurrencyWarning = [&] {
-    if (contextRequiresStrictConcurrencyChecking(DC, GetClosureType{*this})
-        && !hasPreconcurrencyCallee(this, locator))
+  auto isConcurrencyWarning = [&](bool forSendable) {
+    // Except for Sendable warnings, don't downgrade to an error in strict
+    // contexts without a preconcurrency callee.
+    if (!forSendable &&
+        contextRequiresStrictConcurrencyChecking(DC, GetClosureType{*this}) &&
+        !hasPreconcurrencyCallee(this, locator))
       return false;
 
+    // We can only handle the downgrade for conversions.
     switch (kind) {
     case ConstraintKind::Conversion:
     case ConstraintKind::ArgumentConversion:
@@ -2696,7 +2700,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
 
       auto *fix = AddSendableAttribute::create(
           *this, func1, func2, getConstraintLocator(locator),
-          isConcurrencyWarning());
+          isConcurrencyWarning(true));
       if (recordFix(fix))
         return getTypeMatchFailure(locator);
     }
@@ -2731,7 +2735,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
 
       auto *fix = MarkGlobalActorFunction::create(
           *this, func1, func2, getConstraintLocator(locator),
-          isConcurrencyWarning());
+          isConcurrencyWarning(false));
 
       if (recordFix(fix))
         return getTypeMatchFailure(locator);

test/Concurrency/sendable_checking.swift

@@ -59,7 +59,7 @@ func testCV(
   acceptCV(ns4) // expected-warning{{type 'NS4' does not conform to the 'Sendable' protocol}}
   acceptCV(fn) // expected-warning{{type '() -> Void' does not conform to the 'Sendable' protocol}}
   // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
-  acceptSendableFn(fn) // expected-error{{passing non-sendable parameter 'fn' to function expecting a @Sendable closure}}
+  acceptSendableFn(fn) // expected-warning{{passing non-sendable parameter 'fn' to function expecting a @Sendable closure}}
 }
 
 // rdar://83942484 - spurious Sendable diagnostics