Merge pull request #15701 from CodaFi/humdinger

[DiagnosticsQoI] Bail on nil literal diagnostics if the source is Optional

Author: CodaFi

lib/Sema/CSDiag.cpp

@@ -3032,6 +3032,10 @@ bool FailureDiagnosis::diagnoseContextualConversionError(
   // If we're diagnostic an issue with 'nil', produce a specific diagnostic,
   // instead of uttering ExpressibleByNilLiteral.
   if (isa<NilLiteralExpr>(expr->getValueProvidingExpr())) {
+    // If the source type is some kind of optional, the contextual conversion
+    // to 'nil' didn't fail, something else did.
+    if (contextualType->getOptionalObjectType())
+      return false;
     diagnose(expr->getLoc(), nilDiag, contextualType);
     if (nilFollowup)
       nilFollowup();

test/decl/init/nil.swift

@@ -26,3 +26,5 @@ var _: Int = nil
 // expected-error@-1 {{nil cannot initialize specified type 'Int'}} 
 // expected-note@-2 {{add '?' to form the optional type 'Int?'}} {{11-11=?}}
 
+// 'nil' can initialize the specified type, if its generic parameters are bound
+var _: Array? = nil // expected-error {{generic parameter 'Element' could not be inferred}}