[CSSimplify] Specialization: Fix a crash when specialized declaration is not generic

Check whether there are any opened generic parameters associated
with a declaration and if not, produce a fix which would be later
diagnosed as either a warning (in Swift 5 mode) or an error (if it
was a concrete type or the compiler is in Swift 6 language mode).

Resolves: rdar://135610320

lib/Sema/CSSimplify.cpp

@@ -13868,6 +13868,23 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     return genericParams;
   };
 
+  auto fixInvalidSpecialization = [&](ValueDecl *decl) -> SolutionKind {
+    if (isa<AbstractFunctionDecl>(decl)) {
+      return recordFix(AllowFunctionSpecialization::create(
+                 *this, decl, getConstraintLocator(locator)))
+                 ? SolutionKind::Error
+                 : SolutionKind::Solved;
+    }
+
+    // Allow concrete macros to have specializations with just a warning.
+    return recordFix(AllowConcreteTypeSpecialization::create(
+               *this, type1, decl, getConstraintLocator(locator),
+               isa<MacroDecl>(decl) ? FixBehavior::DowngradeToWarning
+                                    : FixBehavior::Error))
+               ? SolutionKind::Error
+               : SolutionKind::Solved;
+  };
+
   ValueDecl *decl;
   SmallVector<OpenedType, 2> openedTypes;
   if (auto *bound = dyn_cast<TypeAliasType>(type1.getPointer())) {
@@ -13926,6 +13943,12 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     decl = overloadChoice.getDecl();
 
     auto openedOverloadTypes = getOpenedTypes(overloadLocator);
+    // Attempting to specialize a non-generic declaration.
+    if (openedOverloadTypes.empty()) {
+      // Note that this is unconditional because the fix is
+      // downgraded to a warning in swift language modes < 6.
+      return fixInvalidSpecialization(decl);
+    }
 
     auto genericParams = getGenericParams(decl);
     if (genericParams) {
@@ -13941,22 +13964,8 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
   }
 
   auto genericParams = getGenericParams(decl);
-  if (!decl->getAsGenericContext() || !genericParams) {
-    if (isa<AbstractFunctionDecl>(decl)) {
-      return recordFix(AllowFunctionSpecialization::create(
-                 *this, decl, getConstraintLocator(locator)))
-                 ? SolutionKind::Error
-                 : SolutionKind::Solved;
-    }
-
-    // Allow concrete macros to have specializations with just a warning.
-    return recordFix(AllowConcreteTypeSpecialization::create(
-               *this, type1, decl, getConstraintLocator(locator),
-               isa<MacroDecl>(decl) ? FixBehavior::DowngradeToWarning
-                                    : FixBehavior::Error))
-               ? SolutionKind::Error
-               : SolutionKind::Solved;
-  }
+  if (!decl->getAsGenericContext() || !genericParams)
+    return fixInvalidSpecialization(decl);
 
   // Map the generic parameters we have over to their opened types.
   bool hasParameterPack = false;

test/Constraints/specialization_with_shadowing.swift

@@ -0,0 +1,76 @@
+// RUN: %empty-directory(%t/src)
+// RUN: split-file %s %t/src
+
+/// Build the library A
+// RUN: %target-swift-frontend -emit-module %t/src/A.swift \
+// RUN:   -module-name A -swift-version 5 -enable-library-evolution \
+// RUN:   -emit-module-path %t/A.swiftmodule \
+// RUN:   -emit-module-interface-path %t/A.swiftinterface
+
+/// Build the library B
+// RUN: %target-swift-frontend -emit-module %t/src/B.swift \
+// RUN:   -module-name B -swift-version 5 -enable-library-evolution \
+// RUN:   -emit-module-path %t/B.swiftmodule \
+// RUN:   -emit-module-interface-path %t/B.swiftinterface
+
+// Build the client AB (language mode 5)
+// RUN: %target-swift-frontend -emit-module %t/src/ClientAB.swift \
+// RUN:   -module-name Client -I %t -swift-version 5 \
+// RUN:   -verify
+
+// Build the client AB (language mode 6)
+// RUN: %target-swift-frontend -emit-module %t/src/ClientAB.swift \
+// RUN:   -module-name Client -I %t -swift-version 6 \
+// RUN:   -verify
+
+// Build the client BA (language mode 5)
+// RUN: %target-swift-frontend -emit-module %t/src/ClientBA.swift \
+// RUN:   -module-name Client -I %t -swift-version 5 \
+// RUN:   -verify
+
+// Build the client BA (language mode 6)
+// RUN: %target-swift-frontend -emit-module %t/src/ClientBA.swift \
+// RUN:   -module-name Client -I %t -swift-version 6 \
+// RUN:   -verify
+
+//--- A.swift
+public protocol EventSource {
+}
+
+//--- B.swift
+public struct Event<T> {
+}
+
+public class EventSource<Parameter> {
+    public var event: Event<Parameter> {
+        fatalError()
+    }
+
+    public init() {}
+}
+
+//--- ClientAB.swift
+import B
+// Note: import order is important in this case because successful match might
+//       mean that the other overload is not going to be attempted and we want
+//       to attempt protocol EventSource always.
+import A
+
+func test() {
+    let test: B.Event<Void>
+    test = EventSource<Void>().event
+    print(test)
+}
+
+//--- ClientBA.swift
+import B
+// Note: import order is important in this case because successful match might
+//       mean that the other overload is not going to be attempted and we want
+//       to attempt protocol EventSource always.
+import A
+
+func test() {
+    let test: B.Event<Void>
+    test = EventSource<Void>().event
+    print(test)
+}