Merge pull request #72915 from tshortli/lazy-conformance-silgen-take-2-6.0

[6.0] SILGen: Ignore placeholders and missing methods during conformance emission

Author: tshortli

lib/SILGen/SILGenType.cpp

@@ -554,11 +554,7 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     PrettyStackTraceConformance trace("generating SIL witness table",
                                       Conformance);
 
-    // Check whether the conformance is valid first.
     Conformance->resolveValueWitnesses();
-    if (Conformance->isInvalid())
-      return nullptr;
-
     auto *proto = Conformance->getProtocol();
     visitProtocolDecl(proto);
 
@@ -619,13 +615,12 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     return Conformance->getWitness(decl);
   }
 
-  void addPlaceholder(MissingMemberDecl *placeholder) {
-    llvm_unreachable("generating a witness table with placeholders in it");
-  }
-
-  void addMissingMethod(SILDeclRef requirement) {
-    llvm_unreachable("generating a witness table with placeholders in it");
-  }
+  // Treat placeholders and missing methods as no-ops. These may be encountered
+  // during lazy typechecking when SILGen triggers witness resolution and
+  // discovers and invalid conformance. The diagnostics emitted during witness
+  // resolution should cause compilation to fail.
+  void addPlaceholder(MissingMemberDecl *placeholder) {}
+  void addMissingMethod(SILDeclRef requirement) {}
 
   void addMethodImplementation(SILDeclRef requirementRef,
                                SILDeclRef witnessRef,