Merge pull request #42036 from xedin/dont-infer-from-magic-defaults

[TypeChecker] SE-0347: Avoid type inference from caller-site defaults

lib/AST/Decl.cpp

@@ -7067,6 +7067,10 @@ Expr *ParamDecl::getTypeCheckedDefaultExpr() const {
 Type ParamDecl::getTypeOfDefaultExpr() const {
   auto &ctx = getASTContext();
 
+  // If this is a caller-side default, the type is determined based on
+  // a particular call site.
+  assert(!hasCallerSideDefaultExpr());
+
   if (Type type = evaluateOrDefault(
           ctx.evaluator,
           DefaultArgumentTypeRequest{const_cast<ParamDecl *>(this)}, nullptr)) {

lib/Sema/CSSimplify.cpp

@@ -1782,6 +1782,11 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
         if (!PD->getInterfaceType()->hasTypeParameter())
           continue;
 
+        // The type of the default value is going to be determined
+        // based on a type deduced for the parameter at this call site.
+        if (PD->hasCallerSideDefaultExpr())
+          continue;
+
         auto defaultExprType = PD->getTypeOfDefaultExpr();
 
         // A caller side default.

lib/Sema/TypeCheckConstraints.cpp

@@ -468,6 +468,11 @@ Type TypeChecker::typeCheckParameterDefault(Expr *&defaultValue,
     if (!ctx.TypeCheckerOpts.EnableTypeInferenceFromDefaultArguments)
       return Type();
 
+    // Caller-side defaults are always type-checked based on the concrete
+    // type of the argument deduced at a particular call site.
+    if (isa<MagicIdentifierLiteralExpr>(defaultValue))
+      return Type();
+
     // Parameter type doesn't have any generic parameters mentioned
     // in it, so there is nothing to infer.
     if (!paramInterfaceTy->hasTypeParameter())

test/Constraints/type_inference_from_default_exprs.swift

@@ -167,3 +167,11 @@ func main() {
   takesRectangle(.init())
   // expected-error@-1 {{cannot convert default value of type 'Rectangle' to expected argument type 'Circle' for parameter #0}}
 }
+
+func test_magic_defaults() {
+  func with_magic(_: Int = #function) {} // expected-error {{default argument value of type 'String' cannot be converted to type 'Int'}}
+  func generic_with_magic<T>(_: T = #line) -> T {} // expected-error {{default argument value of type 'Int' cannot be converted to type 'T'}}
+
+  let _ = with_magic()
+  let _: String = generic_with_magic()
+}