[NFC] lifetime inference: check for prior defaults before error

Don't diagnose a lifetime error if a previous default already handled the
non-Escapable output in question.

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -1117,7 +1117,8 @@ class LifetimeDependenceChecker {
     bool nonEscapableSelf = isDiagnosedNonEscapable(dc->getSelfTypeInContext());
     if (nonEscapableSelf && accessor->getImplicitSelfDecl()->isInOut()) {
       // First, infer the dependency of the inout non-Escapable 'self'. This may
-      // result in two inferred dependencies for accessors.
+      // result in two inferred dependencies for accessors (one targetting
+      // selfIndex here, and one targetting resultIndex below).
       inferMutatingAccessor(accessor);
     }
     // Handle synthesized wrappers...
@@ -1255,6 +1256,9 @@ class LifetimeDependenceChecker {
     if (!resultDeps)
       return; // .sil implicit initializers may have been annotated.
 
+    if (!resultDeps->empty())
+      return; // same-type inferrence applied; don't issue diagnostics.
+
     unsigned paramIndex = 0;
     for (auto *param : *afd->getParameters()) {
       SWIFT_DEFER { paramIndex++; };
@@ -1293,6 +1297,9 @@ class LifetimeDependenceChecker {
     if (!resultDeps)
       return;
 
+    if (!resultDeps->empty())
+      return; // same-type inferrence applied; don't issue diagnostics.
+
     bool nonEscapableSelf = isDiagnosedNonEscapable(dc->getSelfTypeInContext());
     // Do not infer the result's dependence when the method is mutating and
     // 'self' is non-Escapable. Independently, a missing dependence on inout
@@ -1359,6 +1366,9 @@ class LifetimeDependenceChecker {
     if (!resultDeps)
       return;
 
+    if (!resultDeps->empty())
+      return; // same-type inferrence applied; don't issue diagnostics.
+
     // Strict inference only handles a single escapable parameter,
     // which is an unambiguous borrow dependence.
     if (afd->getParameters()->size() == 0) {