[CS] Delay type resolution of variables for variables in named pattern

Author: ahoppen

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -239,6 +239,9 @@ CUSTOM_LOCATOR_PATH_ELT(SyntacticElement)
 /// The element of the pattern binding declaration.
 CUSTOM_LOCATOR_PATH_ELT(PatternBindingElement)
 
+/// The variable declared by a named pattern.
+SIMPLE_LOCATOR_PATH_ELT(NamedPatternDecl)
+
 #undef LOCATOR_PATH_ELT
 #undef CUSTOM_LOCATOR_PATH_ELT
 #undef SIMPLE_LOCATOR_PATH_ELT

lib/Sema/CSBindings.cpp

@@ -109,6 +109,16 @@ bool BindingSet::isDelayed() const {
     if (locator->directlyAt<NilLiteralExpr>())
       return true;
 
+    // When inferring the type of a variable in a pattern, delay its resolution
+    // so that we resolve type variables inside the expression as placeholders
+    // instead of marking the type of the variable itself as a placeholder. This
+    // allows us to produce more specific errors because the type variable in
+    // the expression that introduced the placeholder might be diagnosable using
+    // fixForHole.
+    if (locator->isLastElement<LocatorPathElt::NamedPatternDecl>()) {
+      return true;
+    }
+
     // It's possible that type of member couldn't be determined,
     // and if so it would be beneficial to bind member to a hole
     // early to propagate that information down to arguments,

lib/Sema/CSGen.cpp

@@ -2327,9 +2327,10 @@ namespace {
         }
 
         if (!varType) {
-          varType =
-              CS.createTypeVariable(CS.getConstraintLocator(locator),
-                                    TVO_CanBindToNoEscape | TVO_CanBindToHole);
+          varType = CS.createTypeVariable(
+              CS.getConstraintLocator(pattern,
+                                      LocatorPathElt::NamedPatternDecl()),
+              TVO_CanBindToNoEscape | TVO_CanBindToHole);
 
           // If this is either a `weak` declaration or capture e.g.
           // `weak var ...` or `[weak self]`. Let's wrap type variable

lib/Sema/ConstraintLocator.cpp

@@ -97,6 +97,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::PackType:
   case ConstraintLocator::PackElement:
   case ConstraintLocator::PatternBindingElement:
+  case ConstraintLocator::NamedPatternDecl:
     return 0;
 
   case ConstraintLocator::FunctionArgument:
@@ -441,6 +442,11 @@ void LocatorPathElt::dump(raw_ostream &out) const {
         << llvm::utostr(patternBindingElt.getIndex());
     break;
   }
+
+  case ConstraintLocator::NamedPatternDecl: {
+    out << "named pattern decl";
+    break;
+  }
   }
 }
 

lib/Sema/ConstraintSystem.cpp

@@ -5256,6 +5256,13 @@ void constraints::simplifyLocator(ASTNode &anchor,
       continue;
     }
 
+    case ConstraintLocator::NamedPatternDecl: {
+      auto pattern = cast<NamedPattern>(anchor.get<Pattern *>());
+      anchor = pattern->getDecl();
+      path = path.slice(1);
+      break;
+    }
+
     case ConstraintLocator::ImplicitConversion:
       break;