[6.2] Disable surprising lifetime inference of implicit initializers

include/swift/AST/DiagnosticsSema.def

@@ -8294,6 +8294,9 @@ ERROR(lifetime_dependence_cannot_infer_kind, none,
 ERROR(lifetime_dependence_cannot_infer_scope_ownership, none,
       "cannot borrow the lifetime of '%0', which has consuming ownership on %1",
       (StringRef, StringRef))
+ERROR(lifetime_dependence_cannot_infer_implicit_init, none,
+      "cannot infer implicit initialization lifetime. Add an initializer with "
+      "'@_lifetime(...)' for each parameter the result depends on", ())
 
 //------------------------------------------------------------------------------
 // MARK: Lifetime Dependence Experimental Inference

lib/AST/LifetimeDependence.cpp

@@ -1046,9 +1046,13 @@ class LifetimeDependenceChecker {
     return LifetimeDependenceKind::Scope;
   }
 
-  // Infer implicit initialization. The dependence kind can be inferred, similar
-  // to an implicit setter, because the implementation is simply an assignment
-  // to stored property.
+  // Infer implicit initialization. A non-Escapable initializer parameter can
+  // always be inferred, similar to an implicit setter, because the
+  // implementation is simply an assignment to stored property. Escapable
+  // parameters are ambiguous: they may either be borrowed or
+  // non-dependent. non-Escapable types often have incidental integer fields
+  // that are unrelated to lifetime. Avoid inferring any dependency on Escapable
+  // parameters unless it is the (unambiguously borrowed) sole parameter.
   void inferImplicitInit() {
     auto *afd = cast<AbstractFunctionDecl>(decl);
     if (afd->getParameters()->size() == 0) {
@@ -1068,15 +1072,28 @@ class LifetimeDependenceChecker {
       if (paramTypeInContext->hasError()) {
         return;
       }
+      if (!paramTypeInContext->isEscapable()) {
+        // An implicitly initialized non-Escapable value always copies its
+        // dependency.
+        targetDeps = std::move(targetDeps).add(paramIndex,
+                                               LifetimeDependenceKind::Inherit);
+        continue;
+      }
+      if (afd->getParameters()->size() > 1 && !useLazyInference()) {
+        diagnose(param->getLoc(),
+                 diag::lifetime_dependence_cannot_infer_implicit_init);
+        return;
+      }
+      // A single Escapable parameter must be borrowed.
       auto kind = inferLifetimeDependenceKind(paramTypeInContext,
                                               param->getValueOwnership());
       if (!kind) {
         diagnose(returnLoc,
                  diag::lifetime_dependence_cannot_infer_scope_ownership,
                  param->getParameterName().str(), diagnosticQualifier());
-        return;
       }
-      targetDeps = std::move(targetDeps).add(paramIndex, *kind);
+      targetDeps = std::move(targetDeps).add(paramIndex,
+                                             LifetimeDependenceKind::Scope);
     }
     pushDeps(std::move(targetDeps));
   }

test/Sema/lifetime_attr.swift

@@ -75,7 +75,7 @@ func immortalConflict(_ immortal: Int) -> NE { // expected-error{{conflict betwe
 }
 
 do {
-  struct Test: ~Escapable {
+  struct Test: ~Escapable { // expected-error{{cannot infer implicit initialization lifetime. Add an initializer with '@_lifetime(...)' for each parameter the result depends on}}
     var v1: Int
     var v2: NE
   }

test/Sema/lifetime_depend_infer.swift

@@ -577,3 +577,39 @@ struct NonEscapableMutableSelf: ~Escapable {
   @_lifetime(&self)
   mutating func mutatingMethodOneParamBorrow(_: NE) {}
 }
+
+// =============================================================================
+// Initializers
+// =============================================================================
+
+struct NE_Int: ~Escapable {
+  let i: Int
+}
+
+struct NE_C: ~Escapable { // expected-error{{cannot borrow the lifetime of 'c', which has consuming ownership on an implicit initializer}}
+  let c: C
+}
+
+struct NE_C_Int: ~Escapable { // expected-error{{cannot infer implicit initialization lifetime. Add an initializer with '@_lifetime(...)' for each parameter the result depends on}}
+  let c: C
+  let i: Int
+}
+
+struct NE_Int_Int: ~Escapable { // expected-error{{cannot infer implicit initialization lifetime. Add an initializer with '@_lifetime(...)' for each parameter the result depends on}}
+  let i: Int
+  let j: Int
+}
+
+struct NE_NE: ~Escapable {
+  let ne: NE
+}
+
+struct NE_NE_Int: ~Escapable { // expected-error{{cannot infer implicit initialization lifetime. Add an initializer with '@_lifetime(...)' for each parameter the result depends on}}
+  let ne: NE
+  let i: Int
+}
+
+struct NE_NE_C: ~Escapable { // expected-error{{cannot infer implicit initialization lifetime. Add an initializer with '@_lifetime(...)' for each parameter the result depends on}}
+  let ne: NE
+  let c: C
+}