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

[Timm Baeder via cfe-commits]

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@@ -1776,15 +1777,28 @@ bool CheckBitCast(InterpState &S, CodePtr OpPC, const Type *TargetType,
   return true;
 }
 
-static void compileFunction(InterpState &S, const Function *Func) {
-  const FunctionDecl *Definition;
-  if (!Func->getDecl()->getBody(Definition))
-    return;
-  if (!Definition)
+static void compileFunction(InterpState &S, const Function *Func,
+                            CodePtr OpPC) {
+  const FunctionDecl *Fn = Func->getDecl();
+
+  // [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.
+  if (FunctionDefinitionCanBeLazilyInstantiated(Fn) && S.inConstantContext()) {
+    SemaProxy *SP = S.getSemaProxy();
+    if (!SP)
+      return;
+    SP->instantiateFunctionDefinition(S.Current->getLocation(OpPC),
+                                      const_cast<FunctionDecl *>(Fn));
+  }
+  Fn = Fn->getDefinition();
----------------
tbaederr wrote:

This code used `getDefinition()` before, but that wasn't enough: 2cc6b14b11010b8e537a5dfe627159ce96f5a690

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