Add -verify-all-substitution-maps frontend flag

include/swift/Basic/LangOptions.h

@@ -322,6 +322,9 @@ namespace swift {
     /// `@differentiable` declaration attribute, etc.
     bool EnableExperimentalDifferentiableProgramming = false;
 
+    /// Enable verification when every SubstitutionMap is constructed.
+    bool VerifyAllSubstitutionMaps = false;
+
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///
@@ -512,6 +515,7 @@ namespace swift {
     /// Enable constraint solver support for experimental
     ///        operator protocol designator feature.
     bool SolverEnableOperatorDesignatedTypes = false;
+    
   };
 } // end namespace swift
 

include/swift/Option/FrontendOptions.td

@@ -549,6 +549,9 @@ def sil_merge_partial_modules : Flag<["-"], "sil-merge-partial-modules">,
 def sil_verify_all : Flag<["-"], "sil-verify-all">,
   HelpText<"Verify SIL after each transform">;
   
+def verify_all_substitution_maps : Flag<["-"], "verify-all-substitution-maps">,
+  HelpText<"Verify all SubstitutionMaps on construction">;
+
 def sil_debug_serialization : Flag<["-"], "sil-debug-serialization">,
   HelpText<"Do not eliminate functions in Mandatory Inlining/SILCombine dead "
            "functions. (for debugging only)">;

lib/AST/SubstitutionMap.cpp

@@ -56,7 +56,12 @@ SubstitutionMap::SubstitutionMap(
                                 GenericSignature genericSig,
                                 ArrayRef<Type> replacementTypes,
                                 ArrayRef<ProtocolConformanceRef> conformances)
-  : storage(Storage::get(genericSig, replacementTypes, conformances)) { }
+  : storage(Storage::get(genericSig, replacementTypes, conformances)) {
+#ifndef NDEBUG
+  if (genericSig->getASTContext().LangOpts.VerifyAllSubstitutionMaps)
+    verify();
+#endif
+}
 
 ArrayRef<Type> SubstitutionMap::getReplacementTypesBuffer() const {
   return storage ? storage->getReplacementTypes() : ArrayRef<Type>();

lib/Frontend/CompilerInvocation.cpp

@@ -565,6 +565,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                    Target.isOSDarwin());
   Opts.EnableSILOpaqueValues |= Args.hasArg(OPT_enable_sil_opaque_values);
 
+  Opts.VerifyAllSubstitutionMaps |= Args.hasArg(OPT_verify_all_substitution_maps);
+
   Opts.UseDarwinPreStableABIBit =
     (Target.isMacOSX() && Target.isMacOSXVersionLT(10, 14, 4)) ||
     (Target.isiOS() && Target.isOSVersionLT(12, 2)) ||