[Clang] Fix evaluation context of lambdas appearing in discarded statements

Fixes 2 bugs reported in #146063

 - The body of a lambda appearing in a discarded statement was sometimes
   considered discarded itself
 - A lambda conversion operator that was not odr-used was sometimes not
   defined even if it was needed

Fixes #146063

Author: cor3ntin

clang/lib/Sema/SemaExpr.cpp

@@ -18252,6 +18252,11 @@ static bool isImplicitlyDefinableConstexprFunction(FunctionDecl *Func) {
 
   if (Func->isImplicitlyInstantiable() || !Func->isUserProvided())
     return true;
+
+  // Lambda conversion operators are never user provided
+  if (CXXConversionDecl *Conv = dyn_cast<CXXConversionDecl>(Func))
+    return isLambdaConversionOperator(Conv);
+
   auto *CCD = dyn_cast<CXXConstructorDecl>(Func);
   return CCD && CCD->getInheritedConstructor();
 }

clang/lib/Sema/TreeTransform.h

@@ -15723,13 +15723,9 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
 
   // FIXME: Sema's lambda-building mechanism expects us to push an expression
   // evaluation context even if we're not transforming the function body.
-  getSema().PushExpressionEvaluationContext(
-      E->getCallOperator()->isConsteval() ?
-      Sema::ExpressionEvaluationContext::ImmediateFunctionContext :
-      Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
-  getSema().currentEvaluationContext().InImmediateEscalatingFunctionContext =
-      getSema().getLangOpts().CPlusPlus20 &&
-      E->getCallOperator()->isImmediateEscalating();
+  getSema().PushExpressionEvaluationContextForFunction(
+      Sema::ExpressionEvaluationContext::PotentiallyEvaluated,
+      E->getCallOperator());
 
   Sema::CodeSynthesisContext C;
   C.Kind = clang::Sema::CodeSynthesisContext::LambdaExpressionSubstitution;

clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp

@@ -218,4 +218,26 @@ void regression() {
 
 }
 
+namespace GH146063 {
+template <typename>
+struct A {
+  static_assert([]() constexpr { return true; }());
+};
+
+void f1() {
+  if constexpr (false) {
+    A<int> a;
+  }
+}
+
+void f2() {
+  if constexpr (false) {
+    static_assert([]{});
+    // expected-warning@-1 {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  }
+}
+
+}
+
+
 #endif