Do not recurse infinitely when type checking constructor parameters.

Fully-qualified references to associated types in parameter lists of
constructors could result in infinite recursion and crash the compiler
when the typealias for the associated type is not defined.

Use the same approach used in normal function parameter lists of setting
IsBeingTypeChecked on the enclosing type to avoid going into an infinite
recursion here.

Resolves rdar://problem/27680407.

(cherry picked from commit 126b9fc)

Author: rudkx

lib/Sema/TypeCheckDecl.cpp

@@ -6425,17 +6425,22 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       }
     }
 
-    // Type check the constructor parameters.
-    if (CD->isInvalid() || semaFuncParamPatterns(CD)) {
-      CD->overwriteType(ErrorType::get(TC.Context));
-      CD->setInterfaceType(ErrorType::get(TC.Context));
-      CD->setInvalid();
-    } else {
-      configureConstructorType(CD, SelfTy,
-                               CD->getParameterList(1)->getType(TC.Context));
+    {
+      CD->setIsBeingTypeChecked(true);
+      SWIFT_DEFER { CD->setIsBeingTypeChecked(false); };
+
+      // Type check the constructor parameters.
+      if (CD->isInvalid() || semaFuncParamPatterns(CD)) {
+        CD->overwriteType(ErrorType::get(TC.Context));
+        CD->setInterfaceType(ErrorType::get(TC.Context));
+        CD->setInvalid();
+      } else {
+        configureConstructorType(CD, SelfTy,
+                                 CD->getParameterList(1)->getType(TC.Context));
 
-      if (!CD->isGenericContext())
-        TC.configureInterfaceType(CD);
+        if (!CD->isGenericContext())
+          TC.configureInterfaceType(CD);
+      }
     }
 
     validateAttributes(TC, CD);

test/Sema/circular_decl_checking.swift

@@ -66,3 +66,13 @@ struct SomeStruct<A> {
   typealias A = A // expected-error {{type alias 'A' circularly references itself}}
 }
 
+// <rdar://problem/27680407> Infinite recursion when using fully-qualified associatedtype name that has not been defined with typealias
+protocol rdar27680407Proto {
+  associatedtype T // expected-note {{protocol requires nested type 'T'; do you want to add it?}}
+
+  init(value: T)
+}
+
+struct rdar27680407Struct : rdar27680407Proto { // expected-error {{type 'rdar27680407Struct' does not conform to protocol 'rdar27680407Proto'}}
+  init(value: rdar27680407Struct.T) {}
+}