[clang] [Clang] Instantiate functions from constant evaluation. (PR #205557)

[Corentin Jabot via cfe-commits]

================
@@ -6997,6 +6999,30 @@ static bool handleTrivialCopy(EvalInfo &Info, const ParmVarDecl *Param,
       CopyObjectRepresentation);
 }
 
+bool FunctionDefinitionCanBeLazilyInstantiated(const FunctionDecl *FD) {
+  if (FD->isDefined() || !FD->isImplicitlyInstantiable() || !FD->isConstexpr())
+    return false;
+
+  FunctionDecl *Pattern = FD->getTemplateInstantiationPattern();
+  return Pattern && Pattern->isDefined();
+}
+
+static void TryInstantiateFunctionBeforeCall(const FunctionDecl *FD,
+                                             EvalInfo &Info,
+                                             SourceLocation Loc) {
+
+  // [C++26] [temp.inst] p5
+  // [...] the function template specialization is implicitly instantiated
+  // when the specialization is referenced in a context that requires a function
+  // definition to exist or if the existence of the definition affects the
+  // semantics of the program.
+
+  SemaProxy *SP = Info.getSemaProxy();
+  if (SP && FunctionDefinitionCanBeLazilyInstantiated(FD) &&
+      Info.InConstantContext)
+    SP->instantiateFunctionDefinition(Loc, const_cast<FunctionDecl *>(FD));
+}
+
----------------
cor3ntin wrote:

So, in your approach, do we still need `Info.InConstantContext` or is that true by construction and it should be an assert?

https://github.com/llvm/llvm-project/pull/205557