AST: Improve robustness with invalid nesting of extensions in generic context

Extensions cannot capture generic parameters from outer contexts.
Their generic parameter list does not point to the outer
parameter list, the signature does not contain outer parameters
and they do not appear in the extension's generic environment.

Furthermore, the depth/index of the extension's parameters may
clash with outer parameters, which would break all kinds of
invariants.

This patch changes these DeclContext methods to return false or
nullptr even if an extension is contained in a generic context:

- isGenericContext()
- getGenericParamsOfContext()
- getGenericSignatureOfContext()
- getGenericEnvironmentOfContext()

Author: slavapestov

lib/AST/DeclContext.cpp

@@ -206,9 +206,8 @@ GenericParamList *DeclContext::getGenericParamsOfContext() const {
       continue;
 
     case DeclContextKind::ExtensionDecl:
-      if (auto GP = cast<ExtensionDecl>(dc)->getGenericParams())
-        return GP;
-      continue;
+      // Extensions do not capture outer generic parameters.
+      return cast<ExtensionDecl>(dc)->getGenericParams();
     }
     llvm_unreachable("bad DeclContextKind");
   }
@@ -247,9 +246,8 @@ GenericSignature *DeclContext::getGenericSignatureOfContext() const {
 
     case DeclContextKind::ExtensionDecl: {
       auto ED = cast<ExtensionDecl>(dc);
-      if (auto genericSig = ED->getGenericSignature())
-        return genericSig;
-      continue;
+      // Extensions do not capture outer generic parameters.
+      return ED->getGenericSignature();
     }
     }
     llvm_unreachable("bad DeclContextKind");
@@ -288,9 +286,8 @@ GenericEnvironment *DeclContext::getGenericEnvironmentOfContext() const {
 
     case DeclContextKind::ExtensionDecl: {
       auto ED = cast<ExtensionDecl>(dc);
-      if (auto genericCtx = ED->getGenericEnvironment())
-        return genericCtx;
-      continue;
+      // Extensions do not capture outer generic parameters.
+      return ED->getGenericEnvironment();
     }
     }
     llvm_unreachable("bad DeclContextKind");
@@ -468,17 +465,20 @@ bool DeclContext::isGenericContext() const {
     case DeclContextKind::AbstractFunctionDecl:
       if (cast<AbstractFunctionDecl>(dc)->getGenericParams())
         return true;
+      // Check parent context.
       continue;
 
     case DeclContextKind::GenericTypeDecl:
       if (cast<GenericTypeDecl>(dc)->getGenericParams())
         return true;
+      // Check parent context.
       continue;
 
     case DeclContextKind::ExtensionDecl:
       if (cast<ExtensionDecl>(dc)->getGenericParams())
         return true;
-      continue;
+      // Extensions do not capture outer generic parameters.
+      return false;
     }
     llvm_unreachable("bad decl context kind");
   }

validation-test/compiler_crashers/28212-swift-typechecker-resolvetypeincontext.swift renamed to validation-test/compiler_crashers_fixed/28212-swift-typechecker-resolvetypeincontext.swift

@@ -5,7 +5,7 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -typecheck
+// RUN: not %target-swift-frontend %s -typecheck
 // REQUIRES: asserts
 struct B{let f= <a
 extension{

validation-test/compiler_crashers/28332-swift-archetypebuilder-getgenericsignature.swift renamed to validation-test/compiler_crashers_fixed/28332-swift-archetypebuilder-getgenericsignature.swift

@@ -5,7 +5,7 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -typecheck
+// RUN: not %target-swift-frontend %s -typecheck
 // REQUIRES: asserts
 class A{var b=a
 struct Q<h{

validation-test/compiler_crashers/28445-gp-getouterparameters-proto-getdeclcontext-getgenericparamsofcontext-failed.swift renamed to validation-test/compiler_crashers_fixed/28445-gp-getouterparameters-proto-getdeclcontext-getgenericparamsofcontext-failed.swift

@@ -5,7 +5,7 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 // REQUIRES: asserts
 class A{let f= <c
 protocol A{