diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index df4c13c9ad97aa..550454abdd1ef2 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3911,9 +3911,20 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     assert(!MayBeEmittedEagerly(Global));
     addDeferredDeclToEmit(GD);
   } else {
-    // Otherwise, remember that we saw a deferred decl with this name.  The
-    // first use of the mangled name will cause it to move into
-    // DeferredDeclsToEmit.
+    // Otherwise, remember that we saw a deferred decl with this name.
+    auto DDI = DeferredDecls.find(MangledName);
+    const auto *FD = dyn_cast<FunctionDecl>(Global);
+    if (FD && DDI != DeferredDecls.end()) {
+      bool IsDefinitionAvailableExternally =
+          getContext().GetGVALinkageForFunction(FD) == GVA_AvailableExternally;
+      if (!IsDefinitionAvailableExternally && !FD->isMultiVersion() &&
+          !FD->hasAttr<OMPDeclareTargetDeclAttr>())
+        getDiags().Report(Global->getLocation(),
+                          diag::err_duplicate_mangled_name)
+            << MangledName;
+    }
+    // The first use of the mangled name will cause it to move
+    // into DeferredDeclsToEmit.
     DeferredDecls[MangledName] = GD;
   }
 }
diff --git a/clang/test/CodeGenCXX/duplicate-mangled-name.cpp b/clang/test/CodeGenCXX/duplicate-mangled-name.cpp
index 04e6fee506ebd7..cfa70623de53cd 100644
--- a/clang/test/CodeGenCXX/duplicate-mangled-name.cpp
+++ b/clang/test/CodeGenCXX/duplicate-mangled-name.cpp
@@ -2,6 +2,7 @@
 // RUN: %clang_cc1 -triple %itanium_abi_triple-only %s -verify -DTEST2 -emit-llvm -o - | FileCheck %s
 // RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only %s -verify -DTEST3
 // RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only %s -verify -DTEST4
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm-only %s -verify -DTEST5
 
 #ifdef TEST1
 
@@ -70,6 +71,13 @@ float foo() {
   return _ZN2nm3abcE + nm::abc;
 }
 
+#elif TEST5
+
+inline void f(int) asm("foo");
+inline void f(int) {}
+inline void f() asm("foo");
+inline void f(){} // expected-error {{definition with same mangled name 'foo' as another definition}}
+
 #else
 
 #error Unknown test
diff --git a/clang/test/CodeGenCXX/lambda-instantiation-mangling-conflicts.cpp b/clang/test/CodeGenCXX/lambda-instantiation-mangling-conflicts.cpp
new file mode 100644
index 00000000000000..3a5adc5cb7c7d0
--- /dev/null
+++ b/clang/test/CodeGenCXX/lambda-instantiation-mangling-conflicts.cpp
@@ -0,0 +1,30 @@
+// REQUIRES: aarch64-registered-target
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -triple aarch64 -target-feature +sme -target-feature +sme2 -emit-llvm -o - %t/a.cpp -verify
+// RUN: %clang_cc1 -triple aarch64 -target-feature +sme -target-feature +sme2 -emit-llvm -o - %t/b.cpp -verify
+
+;--- a.cpp
+
+int normal_fn(int);
+int vector_pcs_fn(int) __attribute__((aarch64_vector_pcs));
+
+// expected-error@+3 {{definition with same mangled name '_ZZ23function_pcs_attributesvENK3$_0clIFiiEEEDaRT_' as another definition}}
+__attribute__((aarch64_vector_pcs))
+int function_pcs_attributes() {
+  auto a = [](auto &fn) { return fn(42); };
+  return a(normal_fn) + a(vector_pcs_fn);
+}
+
+;--- b.cpp
+
+int normal_fn(int);
+int streaming_fn(int) __arm_streaming;
+
+// expected-error@+2 {{definition with same mangled name '_ZZ32function_params_normal_streamingvENK3$_0clIFiiEEEDaRT_' as another definition}}
+int function_params_normal_streaming() {
+  auto a = [](auto &fn) { return fn(42); };
+  return a(normal_fn) + a(streaming_fn);
+}
diff --git a/clang/test/CodeGenCXX/mangle-lambdas-gh88906.cpp b/clang/test/CodeGenCXX/mangle-lambdas-gh88906.cpp
index e7592cec5da776..6130756af0e618 100644
--- a/clang/test/CodeGenCXX/mangle-lambdas-gh88906.cpp
+++ b/clang/test/CodeGenCXX/mangle-lambdas-gh88906.cpp
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-linux-gnu %s -emit-llvm -mconstructor-aliases -o - | FileCheck %s
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -fclang-abi-compat=18 %s -emit-llvm -mconstructor-aliases  -o - | FileCheck --check-prefix=CLANG18 %s
+// RUN: not %clang_cc1 -triple x86_64-linux-gnu -fclang-abi-compat=18 %s -emit-llvm -mconstructor-aliases 2>&1 | FileCheck --check-prefix=CLANG18 %s
 // RUN: %clang_cc1 -triple i386-pc-win32 %s -emit-llvm -mconstructor-aliases -o - | FileCheck --check-prefix=MSABI %s
 
 
@@ -29,12 +29,7 @@ void GH88906(){
 // CHECK-LABEL: define internal void @_ZN4funcC2IN7GH889064Test1bMUlvE_EEET_
 // CHECK-LABEL: define internal void @_ZN4funcC2IN7GH889064Test1cMUlvE_EEET_
 
-// CLANG18-LABEL: define internal void @_ZZ7GH88906vEN4TestC2Ev
-// CLANG18: call void @_ZN4funcC2IZ7GH88906vEN4TestUlvE_EZ7GH88906vENS1_UlvE0_EEET_T0_
-// CLANG18: call void @_ZN4funcC2IZ7GH88906vEN4TestUlvE_EEET_
-// CLANG18: call void @_ZN4funcC2IZ7GH88906vEN4TestUlvE_EEET_
-
-
+// CLANG18: error: definition with same mangled name '_ZN4funcC1IZ7GH88906vEN4TestUlvE_EEET_' as another definition
 
 // MSABI-LABEL: define internal x86_thiscallcc noundef ptr @"??0Test@?1??GH88906@@YAXXZ@QAE@XZ"
 // MSABI: call x86_thiscallcc noundef ptr @"??$?0V<lambda_1>@a@Test@?1??GH88906@@YAXXZ@V<lambda_2>@12?1??3@YAXXZ@@func@@QAE@V<lambda_1>@a@Test@?1??GH88906@@YAXXZ@V<lambda_2>@23?1??4@YAXXZ@@Z"