diff --git a/clang/include/clang/Basic/IdentifierTable.h b/clang/include/clang/Basic/IdentifierTable.h
--- a/clang/include/clang/Basic/IdentifierTable.h
+++ b/clang/include/clang/Basic/IdentifierTable.h
@@ -223,7 +223,7 @@
   }
   void setObjCKeywordID(tok::ObjCKeywordKind ID) { ObjCOrBuiltinID = ID; }
 
-  /// True if setNotBuiltin() was called.
+  /// True if revertBuiltin() was called.
   bool hasRevertedBuiltin() const {
     return ObjCOrBuiltinID == tok::NUM_OBJC_KEYWORDS;
   }
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3756,24 +3756,21 @@
   // If the previous declaration was an implicitly-generated builtin
   // declaration, then at the very least we should use a specialized note.
   unsigned BuiltinID;
-  if (Old->isImplicit() && (BuiltinID = Old->getBuiltinID())) {
+  bool RevertedBuiltin{Old->getIdentifier()->hasRevertedBuiltin()};
+  if (Old->isImplicit() &&
+      ((BuiltinID = Old->getBuiltinID()) || RevertedBuiltin)) {
     // If it's actually a library-defined builtin function like 'malloc'
     // or 'printf', just warn about the incompatible redeclaration.
-    if (Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID)) {
+    // Also warn if it's a redeclaration of a builtin redeclaration. We know if
+    // it is if the previous declaration is a reverted builtin.
+    if (RevertedBuiltin ||
+        Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID)) {
       Diag(New->getLocation(), diag::warn_redecl_library_builtin) << New;
       Diag(OldLocation, diag::note_previous_builtin_declaration)
         << Old << Old->getType();
 
-      // If this is a global redeclaration, just forget hereafter
-      // about the "builtin-ness" of the function.
-      //
-      // Doing this for local extern declarations is problematic.  If
-      // the builtin declaration remains visible, a second invalid
-      // local declaration will produce a hard error; if it doesn't
-      // remain visible, a single bogus local redeclaration (which is
-      // actually only a warning) could break all the downstream code.
-      if (!New->getLexicalDeclContext()->isFunctionOrMethod())
-        New->getIdentifier()->revertBuiltin();
+      // Forget hereafter about the "builtin-ness" of the function.
+      New->getIdentifier()->revertBuiltin();
 
       return false;
     }
diff --git a/clang/test/CodeGen/builtin-redeclaration.c b/clang/test/CodeGen/builtin-redeclaration.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/builtin-redeclaration.c
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -emit-llvm-only %s
+
+// PR45410
+// Ensure we mark local extern redeclarations with a different type as non-builtin.
+void non_builtin() {
+  extern float exp();
+  exp(); // Will crash due to wrong number of arguments if this calls the builtin.
+}
+
+// PR45410
+// We mark exp() builtin as const with -fno-math-errno (default).
+// We mustn't do that for extern redeclarations of builtins where the type differs.
+float attribute() {
+  extern float exp();
+  return exp(1);
+}