Sema: more robust diagnostic for circular inheritance

The code would generate different diagnosics depending on
the cycle having length 1 or longer. The length 1 case
was broken if the path had a prefix that wasn't part
of the cycle, eg if we have C : A, A : A and visit C
first.

Swift SVN r28317

lib/Sema/TypeCheckDecl.cpp

@@ -616,26 +616,27 @@ static void checkCircularity(TypeChecker &tc, T *decl,
 
   case CircularityCheck::Checking: {
     // We're already checking this protocol, which means we have a cycle.
-
-    // The type directly references itself.
-    if (path.size() == 1) {
-      tc.diagnose((*path.begin())->getLoc(),
+
+    // The beginning of the path might not be part of the cycle, so find
+    // where the cycle starts.
+    auto cycleStart = path.end() - 1;
+    while (*cycleStart != decl) {
+      assert(cycleStart != path.begin() && "Missing cycle start?");
+      --cycleStart;
+    }
+
+    // If the path length is 1 the type directly references itself.
+    if (path.end() - cycleStart == 1) {
+      tc.diagnose(path.back()->getLoc(),
                   circularDiag,
-                  (*path.begin())->getName().str());
-      
+                  path.back()->getName().str());
+
       decl->setInvalid();
       decl->overwriteType(ErrorType::get(tc.Context));
       breakInheritanceCycle(decl);
       break;
     }
 
-    // Find the beginning of the cycle within the full path.
-    auto cycleStart = path.end()-2;
-    while (*cycleStart != decl) {
-      assert(cycleStart != path.begin() && "Missing cycle start?");
-      --cycleStart;
-    }
-
     // Form the textual path illustrating the cycle.
     llvm::SmallString<128> pathStr;
     for (auto i = cycleStart, iEnd = path.end(); i != iEnd; ++i) {

test/decl/class/circular_inheritance.swift

@@ -6,3 +6,6 @@ class A : C { } // expected-note{{class 'A' declared here}}
 
 class TrivialCycle : TrivialCycle {} // expected-error{{circular class inheritance TrivialCycle}}
 protocol P : P {} // expected-error{{circular protocol inheritance P}}
+
+class Isomorphism : Automorphism { }
+class Automorphism : Automorphism { } // expected-error{{circular class inheritance Automorphism}}

test/decl/protocol/protocols.swift

@@ -81,6 +81,9 @@ protocol CircleMiddle : CircleStart { func circle_middle() } // expected-error{{
 protocol CircleStart : CircleEnd { func circle_start() } // expected-note{{protocol 'CircleStart' declared here}}
 protocol CircleEnd : CircleMiddle { func circle_end()} // expected-note{{protocol 'CircleEnd' declared here}}
 
+protocol CircleEntry : CircleTrivial { }
+protocol CircleTrivial : CircleTrivial { } // expected-error{{circular protocol inheritance CircleTrivial}}
+
 struct Circle {
   func circle_start() {}
   func circle_middle() {}

validation-test/compiler_crashers/0394-void.swift → validation-test/compiler_crashers_fixed/0394-void.swift

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 
 // Distributed under the terms of the MIT license
 // Test case submitted to project by https://github.com/practicalswift (practicalswift)

validation-test/compiler_crashers/0876-x.swift → validation-test/compiler_crashers_fixed/0876-x.swift

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 
 // Distributed under the terms of the MIT license
 // Test case submitted to project by https://github.com/practicalswift (practicalswift)

validation-test/compiler_crashers/0896-void.swift → validation-test/compiler_crashers_fixed/0896-void.swift

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 
 // Distributed under the terms of the MIT license
 // Test case submitted to project by https://github.com/practicalswift (practicalswift)

validation-test/compiler_crashers/1973-void.swift → validation-test/compiler_crashers_fixed/1973-void.swift

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 
 // Distributed under the terms of the MIT license
 // Test case submitted to project by https://github.com/practicalswift (practicalswift)

validation-test/compiler_crashers/2206-void.swift → validation-test/compiler_crashers_fixed/2206-void.swift

@@ -1,4 +1,4 @@
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 
 // Distributed under the terms of the MIT license
 // Test case submitted to project by https://github.com/practicalswift (practicalswift)