@@ -12,6 +12,30 @@
     <AssemblyOriginatorKeyFile>..\..\AngouriMath\key.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
 
+  <!--
+  Set by `Sources/Utils/benchmark_key_commits.sh`, and by nothing else. The inter-version run
+  overlays this project onto an old checkout of the kernel, and the cases that reach internals
+  cannot compile against a kernel that had not made them internal-visible yet: `MatchedRules` and
+  `Patterns` were internal until v2.3.0, so the whole project failed to build for v2.2.0 and
+  earlier and those rows read "did not build" — while `CommonFunctionsInterVersion`, the only case
+  that run executes, uses nothing but the public API and would have compiled.
+
+  The flag is per-purpose, not per-commit: the script passes it for *every* row including master,
+  so what is compiled is the same in each column and the case that runs is the same file. That is
+  the property the comparison rests on, and conditioning per commit would have broken it.
+  https://github.com/asc-community/AngouriMath/issues/529
+  -->
+  <ItemGroup Condition="'$(InterVersionOnly)' == 'true'">
+    <Compile Remove="MatchingEngine.cs" />
+    <Compile Remove="TransformationLayer.cs" />
+    <Compile Remove="RAMUsageTest.cs" />
+    <Compile Remove="BenchLinqCompilation.cs" />
+    <Compile Remove="CacheCompiledFunc.cs" />
+  </ItemGroup>
+  <PropertyGroup Condition="'$(InterVersionOnly)' == 'true'">
+    <DefineConstants>$(DefineConstants);INTERVERSION_ONLY</DefineConstants>
+  </PropertyGroup>
+
   <ItemGroup>
     <None Include="..\..\.editorconfig" Link=".editorconfig" />
     <PackageReference Include="BenchmarkDotNet" Version="0.13.0" />
 
@@ -79,12 +79,17 @@ public static int Main(string[] args)
                 .Select(arg =>
                     arg switch
                     {
-                        "RAMUsageTest" => GetReportByBenchmark(typeof(RAMUsageTest), "Gen 0", "Gen 1", "Gen 2", "Allocated"),
-                        // Allocated: the class has carried [MemoryDiagnoser] all along, so the
-                        // figure was being collected on every run and then dropped here.
-                        "CommonFunctionsInterVersion" => GetReportByBenchmark(typeof(CommonFunctionsInterVersion), "Mean", "Error", "StdDev", "Allocated"),
+// Every case but the inter-version one is compiled out when the project is built for the
+// key-commit run, because a case that reaches API an older kernel did not have stops the whole
+// project compiling and takes the row with it. Only CommonFunctionsInterVersion is executed by
+// that run. https://github.com/asc-community/AngouriMath/issues/529
                         // Allocated as well as Mean: the regressions this one exists to catch
                         // show up in allocation and are invisible in the timings.
+                        "CommonFunctionsInterVersion" => GetReportByBenchmark(typeof(CommonFunctionsInterVersion), "Mean", "Error", "StdDev", "Allocated"),
+#if !INTERVERSION_ONLY
+                        // Allocated: the class has carried [MemoryDiagnoser] all along, so the
+                        // figure was being collected on every run and then dropped here.
+                        "RAMUsageTest" => GetReportByBenchmark(typeof(RAMUsageTest), "Gen 0", "Gen 1", "Gen 2", "Allocated"),
                         "TransformationLayer" => GetReportByBenchmark(typeof(TransformationLayer), "Mean", "Error", "StdDev", "Allocated"),
                         // Ratio as well: this one exists to compare two forms of the same rule,
                         // and the absolute nanoseconds matter far less than the factor between
@@ -97,6 +102,7 @@ public static int Main(string[] args)
                         "BenchLinqCompilation" => GetReportByBenchmark(typeof(BenchLinqCompilation), "Mean", "Error", "StdDev", "Allocated"),
                         "CacheCompiledFunc" => GetReportByBenchmark(typeof(CacheCompiledFunc), "Mean", "Error", "StdDev", "Allocated"),
                         "NumbersBenchmark" => GetReportByBenchmark(typeof(NumbersBenchmark), "Mean", "Error", "StdDev"),
+#endif
                         _ => throw new($"Unexpected benchmark {arg}")
                     }).ToArray(); // active action
             Console.WriteLine();
 
@@ -19,12 +19,21 @@
 # is reported as not building rather than skipped, since "this cannot be measured any more" is
 # itself worth seeing.
 #
-# Measured, so the bound is known rather than discovered. At the time of writing v2.3.0 and later
-# build; v2.2.0 and earlier do not, because the benchmark's MatchingEngine case reaches
-# `MatchedRules` and `Patterns`, which were internal until v2.3.0. Reaching further back means
-# compiling only the inter-version case, which means conditioning the benchmark project on which
-# commit it is pointed at -- and the whole reason these columns compare is that the benchmark does
-# not vary. Left as it is; the table says which rows it could not measure.
+# How far back this reaches, and why it is further than it was. v2.3.0 and later used to be the
+# bound: v2.2.0 and earlier did not build, because the benchmark's MatchingEngine case reaches
+# `MatchedRules` and `Patterns`, which were internal until v2.3.0, and TransformationLayer names
+# `Transformation.Rationalization`, which did not exist at v2.1.0.
+#
+# Neither of those cases is executed by this run. Only CommonFunctionsInterVersion is, and it uses
+# nothing but the public API -- so the rows were being lost to files that were compiled and never
+# called. `-p:InterVersionOnly=true` compiles the case that runs and leaves the rest out, and the
+# script passes it for every row.
+#
+# The earlier note here said reaching further back would mean conditioning the project on which
+# commit it is pointed at, and that this would break the comparison because the benchmark must not
+# vary. The first half was right and the second does not follow: the flag is per-purpose, not
+# per-commit, so every column compiles the same file and runs the same file. Measured: v2.1.0 and
+# v2.2.0 build and run under it.
 
 v2.1.0
 v2.2.0
 
@@ -18,6 +18,13 @@
 # calls the public API as it is now, so an old enough commit genuinely cannot be measured this
 # way, and that is worth seeing rather than silently omitting.
 #
+# What stopped older commits building was not that case, though. It was the *other* cases in the
+# same project -- MatchingEngine reaches internals that were internal until v2.3.0, and
+# TransformationLayer names Transformation.Rationalization, which did not exist at v2.1.0 -- so
+# the project failed to compile and took the row with it while the case that runs would have
+# compiled fine. `-p:InterVersionOnly=true` leaves those out. It is passed for every row, so the
+# columns still compare: the same file is compiled and the same file runs in each of them.
+#
 # Run from anywhere.
 
 set -uo pipefail
@@ -60,7 +67,12 @@ for commit in $commits; do
     cp -r "$repository/$benchmark" "$tree/Sources/Tests/"
     rm -rf "$tree/$benchmark/bin" "$tree/$benchmark/obj" "$tree/$benchmark/benchmark_results.csv"
 
-    if ! (cd "$tree/$benchmark" && dotnet build -c Release > "$results/$commit.build.log" 2>&1); then
+    # InterVersionOnly compiles the one case this run executes and leaves out the rest. A case
+    # that reaches API an older kernel did not have stops the whole project compiling and takes
+    # the row with it: MatchedRules and Patterns were internal until v2.3.0, and
+    # Transformation.Rationalization did not exist until after v2.1.0. Passed for every row
+    # including master, so what is compiled does not vary between columns.
+    if ! (cd "$tree/$benchmark" && dotnet build -c Release -p:InterVersionOnly=true > "$results/$commit.build.log" 2>&1); then
         echo "  did not build — see $results/$commit.build.log"
         echo "did not build" > "$results/$commit.status"
         continue