consistently uses the caller's specific to get the callee's pattern type id (#7036)

`DoVarPreWorkImpl` was provided an incorrect pattern type while trying
to match `var` parameters, which caused the toolchain to crash in
`Convert`. This commit changes `DoVarPreWorkImpl`'s API so it derives
the pattern type from the work item's pattern ID, rather than relying on
an external source.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/pattern_match.cpp

@@ -19,6 +19,7 @@
 #include "toolchain/check/type.h"
 #include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/sem_ir/expr_info.h"
+#include "toolchain/sem_ir/inst_kind.h"
 #include "toolchain/sem_ir/pattern.h"
 
 namespace Carbon::Check {
@@ -180,15 +181,21 @@ class MatchContext {
     return value_id;
   }
 
-  // Performs the core logic of matching a variable pattern whose type is
-  // `pattern_type_id`, but returns the scrutinee that its subpattern should be
-  // matched with, rather than pushing it onto the worklist. This is factored
-  // out so it can be reused when handling a `FormBindingPattern` or
-  // `FormParamPattern` with an initializing form.
+  // Performs the core logic of matching a variable pattern, returning the
+  // scrutinee that its subpattern should be matched with, rather than pushing
+  // it onto the worklist. This is factored out so it can be reused when
+  // handling a `FormBindingPattern` or `FormParamPattern` with an initializing
+  // form.
   auto DoVarPreWorkImpl(State state, SemIR::TypeId pattern_type_id,
                         SemIR::InstId scrutinee_id, WorkItem entry) const
       -> SemIR::InstId;
 
+  // Returns the scrutinee type from `pattern_id` when passed a `CallerState`,
+  // and `param_pattern_type_id` otherwise.
+  auto GetSpecificPatternTypeId(State state, SemIR::InstId pattern_id,
+                                SemIR::TypeId param_pattern_type_id)
+      -> SemIR::TypeId;
+
   // The stack of work to be processed.
   llvm::SmallVector<WorkItem> stack_;
 
@@ -444,10 +451,28 @@ static auto ParamKindFor(Context& context, SemIR::Inst param_pattern,
   }
 }
 
+auto MatchContext::GetSpecificPatternTypeId(State state,
+                                            SemIR::InstId pattern_id,
+                                            SemIR::TypeId param_pattern_type_id)
+    -> SemIR::TypeId {
+  CARBON_KIND_SWITCH(state) {
+    case CARBON_KIND(CallerState* caller): {
+      auto& sem_ir = context_.sem_ir();
+      return ExtractScrutineeType(
+          sem_ir, SemIR::GetTypeOfInstInSpecific(
+                      sem_ir, caller->callee_specific_id, pattern_id));
+    }
+    default:
+      return param_pattern_type_id;
+  }
+}
+
 auto MatchContext::DoPreWork(State state, SemIR::AnyParamPattern param_pattern,
                              SemIR::InstId scrutinee_id, WorkItem entry)
     -> void {
   AddAsPostWork(entry);
+  auto pattern_type_id =
+      GetSpecificPatternTypeId(state, entry.pattern_id, param_pattern.type_id);
 
   // If `param_pattern` has initializing form, match it as a `VarPattern`
   // before matching it as a parameter pattern.
@@ -463,7 +488,7 @@ auto MatchContext::DoPreWork(State state, SemIR::AnyParamPattern param_pattern,
     }
     case SemIR::VarParamPattern::Kind: {
       scrutinee_id =
-          DoVarPreWorkImpl(state, param_pattern.type_id, scrutinee_id, entry);
+          DoVarPreWorkImpl(state, pattern_type_id, scrutinee_id, entry);
       entry.allow_unmarked_ref = true;
       break;
     }
@@ -477,15 +502,10 @@ auto MatchContext::DoPreWork(State state, SemIR::AnyParamPattern param_pattern,
       if (scrutinee_id == SemIR::ErrorInst::InstId) {
         caller_state->call_args.push_back(SemIR::ErrorInst::InstId);
       } else {
-        auto scrutinee_type_id = ExtractScrutineeType(
-            context_.sem_ir(),
-            SemIR::GetTypeOfInstInSpecific(context_.sem_ir(),
-                                           caller_state->callee_specific_id,
-                                           entry.pattern_id));
         caller_state->call_args.push_back(
             Convert(context_, SemIR::LocId(scrutinee_id), scrutinee_id,
                     {.kind = ConversionKindFor(context_, param_pattern, entry),
-                     .type_id = scrutinee_type_id}));
+                     .type_id = pattern_type_id}));
       }
       // Do not traverse farther or schedule PostWork, because the caller side
       // of the pattern ends here.
@@ -603,8 +623,10 @@ auto MatchContext::DoPostWork(State state,
 auto MatchContext::DoPreWork(State state, SemIR::VarPattern var_pattern,
                              SemIR::InstId scrutinee_id, WorkItem entry)
     -> void {
+  auto pattern_type_id =
+      GetSpecificPatternTypeId(state, entry.pattern_id, var_pattern.type_id);
   auto new_scrutinee_id =
-      DoVarPreWorkImpl(state, var_pattern.type_id, scrutinee_id, entry);
+      DoVarPreWorkImpl(state, pattern_type_id, scrutinee_id, entry);
   if (need_subpattern_results()) {
     AddAsPostWork(entry);
   }
@@ -638,8 +660,7 @@ auto MatchContext::DoVarPreWorkImpl(State state, SemIR::TypeId pattern_type_id,
     case CARBON_KIND(CallerState* _): {
       storage_id = AddInst<SemIR::TemporaryStorage>(
           context_, SemIR::LocId(entry.pattern_id),
-          {.type_id =
-               ExtractScrutineeType(context_.sem_ir(), pattern_type_id)});
+          {.type_id = pattern_type_id});
       CARBON_CHECK(scrutinee_id.has_value());
       break;
     }