[CSFix] Fix a null pointer dereference in getStructuralTypeContext

First problem - the logic used constraint system, which shouldn't
be required, second - it expects the type to be always present in
`ContextualTypeInfo` but that's not the case for some patterns.

Resolves: rdar://131819800

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -1797,19 +1797,33 @@ class Solution {
   /// "resolved" concrete type.
   Type getResolvedType(ASTNode node) const;
 
-  Type getContextualType(ASTNode anchor) const {
+  std::optional<ContextualTypeInfo>
+  getContextualTypeInfo(ASTNode anchor) const {
     for (const auto &entry : contextualTypes) {
-      if (entry.first == anchor) {
-        // The contextual information record could contain the purpose
-        // without a type i.e. when the context is an optional-some or
-        // an invalid pattern binding.
-        if (auto contextualTy = entry.second.getType())
+      if (entry.first == anchor)
+        return entry.second;
+    }
+    return std::nullopt;
+  }
+
+  Type getContextualType(ASTNode anchor) const {
+    if (auto info = getContextualTypeInfo(anchor)) {
+      // The contextual information record could contain the purpose
+      // without a type i.e. when the context is an optional-some or
+      // an invalid pattern binding.
+      if (auto contextualTy = info->getType())
           return simplifyType(contextualTy);
-      }
     }
     return Type();
   }
 
+  ContextualTypePurpose getContextualTypePurpose(ASTNode anchor) const {
+    if (auto info = getContextualTypeInfo(anchor)) {
+      return info->purpose;
+    }
+    return CTP_Unused;
+  }
+
   /// Retrieve the generic environment for the opened element of a given pack
   /// expansion, or \c nullptr if no environment was recorded.
   GenericEnvironment *

lib/Sema/CSFix.cpp

@@ -592,12 +592,16 @@ getStructuralTypeContext(const Solution &solution, ConstraintLocator *locator) {
     assert(locator->isLastElement<LocatorPathElt::ContextualType>() ||
            locator->isLastElement<LocatorPathElt::FunctionArgument>());
 
-    auto &cs = solution.getConstraintSystem();
     auto anchor = locator->getAnchor();
-    auto contextualType = cs.getContextualType(anchor, /*forConstraint=*/false);
-    auto exprType = cs.getType(anchor);
-    return std::make_tuple(contextualTypeElt->getPurpose(), exprType,
-                           contextualType);
+    auto contextualInfo = solution.getContextualTypeInfo(anchor);
+    // For some patterns the type could be empty and the entry is
+    // there to indicate the purpose only.
+    if (!contextualInfo || !contextualInfo->getType())
+      return std::nullopt;
+
+    auto exprType = solution.getType(anchor);
+    return std::make_tuple(contextualInfo->purpose, exprType,
+                           contextualInfo->getType());
   } else if (auto argApplyInfo = solution.getFunctionArgApplyInfo(locator)) {
     Type fromType = argApplyInfo->getArgType();
     Type toType = argApplyInfo->getParamType();
@@ -607,9 +611,8 @@ getStructuralTypeContext(const Solution &solution, ConstraintLocator *locator) {
       auto fromFnType = fromType->getAs<FunctionType>();
       auto toFnType = toType->getAs<FunctionType>();
       if (fromFnType && toFnType) {
-        auto &cs = solution.getConstraintSystem();
         return std::make_tuple(
-            cs.getContextualTypePurpose(locator->getAnchor()),
+            solution.getContextualTypePurpose(locator->getAnchor()),
             fromFnType->getResult(), toFnType->getResult());
       }
     }

test/Constraints/patterns.swift

@@ -805,3 +805,18 @@ func testMatchingNonErrorConformingTypeInClosure(_ x: any Error) {
     }
   }
 }
+
+// rdar://131819800 - crash in `transformWithPosition` while trying to emit diagnostics for `AllowFunctionTypeMismatch` fix
+do {
+  enum E {
+  case test(kind: Int, defaultsToEmpty: Bool = false)
+  }
+
+  func test(e: E) {
+    if case .test(kind: _, // expected-error {{tuple pattern has the wrong length for tuple type '(Int, Bool)'}}
+                  name: let name?,
+                  defaultsToEmpty: _,
+                  deprecateName: let deprecatedName?) = e {
+    }
+  }
+}