diff --git a/lib/scope.js b/lib/scope.js
index 630ee9b1a..ab822f3f3 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -116,6 +116,11 @@ const MASK_EXPORT_WANT_MANGLE = 1 << 1;
 
 let function_defs = null;
 let unmangleable_names = null;
+/**
+ * When defined, there is a function declaration somewhere that's inside of a block.
+ * See https://tc39.es/ecma262/multipage/additional-ecmascript-features-for-web-browsers.html#sec-block-level-function-declarations-web-legacy-compatibility-semantics
+*/
+let scopes_with_block_defuns = null;
 
 class SymbolDef {
     constructor(scope, orig, init) {
@@ -666,6 +671,15 @@ AST_Scope.DEFMETHOD("def_variable", function(symbol, init) {
 });
 
 function next_mangled(scope, options) {
+    let defun_scope;
+    if (
+        scopes_with_block_defuns
+        && (defun_scope = scope.get_defun_scope())
+        && scopes_with_block_defuns.has(defun_scope)
+    ) {
+        scope = defun_scope;
+    }
+
     var ext = scope.enclosed;
     var nth_identifier = options.nth_identifier;
     out: while (true) {
@@ -800,6 +814,13 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
             lname = save_nesting;
             return true;        // don't descend again in TreeWalker
         }
+        if (
+            node instanceof AST_Defun
+            && !(tw.parent() instanceof AST_Scope)
+        ) {
+            scopes_with_block_defuns = scopes_with_block_defuns || new Set();
+            scopes_with_block_defuns.add(node.parent_scope.get_defun_scope());
+        }
         if (node instanceof AST_Scope) {
             node.variables.forEach(collect);
             return;
@@ -848,6 +869,7 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options) {
 
     function_defs = null;
     unmangleable_names = null;
+    scopes_with_block_defuns = null;
 
     function collect(symbol) {
         if (symbol.export & MASK_EXPORT_DONT_MANGLE) {
diff --git a/test/compress/blocks.js b/test/compress/blocks.js
index a3247f93f..96927aa93 100644
--- a/test/compress/blocks.js
+++ b/test/compress/blocks.js
@@ -183,6 +183,61 @@ issue_1672_if_strict: {
     expect_stdout: true
 }
 
+issue_t1155_function_in_block: {
+    options = { unused: false }
+    mangle = {}
+    input: {
+        function f1() {
+            if ("aaaaaaa") {
+                function f2() {
+                    return 1;
+                }
+        
+                const var1 = "bbbbbbb";
+            }
+        }
+    }
+    expect: {
+        function f1() {
+            if ("aaaaaaa") {
+                function a() {
+                    return 1;
+                }
+        
+                const b = "bbbbbbb";
+            }
+        }
+    }
+}
+
+issue_t1155_function_in_other_block: {
+    options = { unused: false }
+    mangle = {}
+    input: {
+        function f1() {
+            if ("aaaaaaaaaannnnnnn") {
+                function f2() {
+                    return 1;
+                }
+            }
+            if ("aaaaaaaaaannnnnnn") {
+                const f2 = "aaaaaaaaaannnnnnn";
+            }
+        }
+    }
+    expect: {
+        function f1() {
+            if ("aaaaaaaaaannnnnnn")
+                function a() {
+                    return 1;
+                }
+            if ("aaaaaaaaaannnnnnn") {
+                const n = "aaaaaaaaaannnnnnn";
+            }
+        }
+    }
+}
+
 issue_2946_else_const: {
     input: {
         if (1) {