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diff --git a/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions
diff --git a/‎src/compiler/scanner.ts
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+107-20Lines changed: 107 additions & 20 deletions b/‎src/compiler/scanner.ts
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+107-20Lines changed: 107 additions & 20 deletions
diff --git a/‎src/compiler/types.ts
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+2-1Lines changed: 2 additions & 1 deletion b/‎src/compiler/types.ts
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+2-1Lines changed: 2 additions & 1 deletion
diff --git a/‎src/compiler/utilities.ts
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+1-1Lines changed: 1 addition & 1 deletion b/‎src/compiler/utilities.ts
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+1-1Lines changed: 1 addition & 1 deletion
diff --git a/‎tests/baselines/reference/parser.numericSeparators.binary.js
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+12Lines changed: 12 additions & 0 deletions b/‎tests/baselines/reference/parser.numericSeparators.binary.js
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+12Lines changed: 12 additions & 0 deletions
diff --git a/‎tests/baselines/reference/parser.numericSeparators.binary.symbols
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+7Lines changed: 7 additions & 0 deletions b/‎tests/baselines/reference/parser.numericSeparators.binary.symbols
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+7Lines changed: 7 additions & 0 deletions
diff --git a/‎tests/baselines/reference/parser.numericSeparators.binary.types
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+13Lines changed: 13 additions & 0 deletions b/‎tests/baselines/reference/parser.numericSeparators.binary.types
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+13Lines changed: 13 additions & 0 deletions
diff --git a/‎tests/baselines/reference/parser.numericSeparators.binaryNegative.errors.txt
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+50Lines changed: 50 additions & 0 deletions b/‎tests/baselines/reference/parser.numericSeparators.binaryNegative.errors.txt
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diff --git a/‎tests/baselines/reference/parser.numericSeparators.binaryNegative.js
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+34Lines changed: 34 additions & 0 deletions b/‎tests/baselines/reference/parser.numericSeparators.binaryNegative.js
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+34Lines changed: 34 additions & 0 deletions
@@ -3420,6 +3420,10 @@
         "category": "Message",
         "code": 6187
     },
+    "Numeric separators are not allowed here.": {
+        "category": "Error",
+        "code": 6188
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
 
@@ -561,9 +561,9 @@ namespace ts {
         return false;
     }
 
-    function scanConflictMarkerTrivia(text: string, pos: number, error?: ErrorCallback) {
+    function scanConflictMarkerTrivia(text: string, pos: number, error?: (diag: DiagnosticMessage, pos?: number, len?: number) => void) {
         if (error) {
-            error(Diagnostics.Merge_conflict_marker_encountered, mergeConflictMarkerLength);
+            error(Diagnostics.Merge_conflict_marker_encountered, pos, mergeConflictMarkerLength);
         }
 
         const ch = text.charCodeAt(pos);
@@ -852,34 +852,86 @@ namespace ts {
             scanRange,
         };
 
-        function error(message: DiagnosticMessage, length?: number): void {
+        function error(message: DiagnosticMessage): void;
+        function error(message: DiagnosticMessage, errPos: number, length: number): void;
+        function error(message: DiagnosticMessage, errPos: number = pos, length?: number): void {
             if (onError) {
+                const oldPos = pos;
+                pos = errPos;
                 onError(message, length || 0);
+                pos = oldPos;
             }
         }
 
+        function scanNumberFragment(): string {
+            let start = pos;
+            let allowSeparator = false;
+            let result = "";
+            while (true) {
+                const ch = text.charCodeAt(pos);
+                if (ch === CharacterCodes._) {
+                    tokenFlags |= TokenFlags.ContainsSeparator;
+                    if (allowSeparator) {
+                        allowSeparator = false;
+                        result += text.substring(start, pos);
+                    }
+                    else {
+                        error(Diagnostics.Numeric_separators_are_not_allowed_here, pos, 1);
+                    }
+                    pos++;
+                    start = pos;
+                    continue;
+                }
+                if (isDigit(ch)) {
+                    allowSeparator = true;
+                    pos++;
+                    continue;
+                }
+                break;
+            }
+            if (text.charCodeAt(pos - 1) === CharacterCodes._) {
+                error(Diagnostics.Numeric_separators_are_not_allowed_here, pos - 1, 1);
+            }
+            return result + text.substring(start, pos);
+        }
+
         function scanNumber(): string {
             const start = pos;
-            while (isDigit(text.charCodeAt(pos))) pos++;
+            const mainFragment = scanNumberFragment();
+            let decimalFragment: string;
+            let scientificFragment: string;
             if (text.charCodeAt(pos) === CharacterCodes.dot) {
                 pos++;
-                while (isDigit(text.charCodeAt(pos))) pos++;
+                decimalFragment = scanNumberFragment();
             }
             let end = pos;
             if (text.charCodeAt(pos) === CharacterCodes.E || text.charCodeAt(pos) === CharacterCodes.e) {
                 pos++;
                 tokenFlags |= TokenFlags.Scientific;
                 if (text.charCodeAt(pos) === CharacterCodes.plus || text.charCodeAt(pos) === CharacterCodes.minus) pos++;
-                if (isDigit(text.charCodeAt(pos))) {
-                    pos++;
-                    while (isDigit(text.charCodeAt(pos))) pos++;
-                    end = pos;
+                const preNumericPart = pos;
+                const finalFragment = scanNumberFragment();
+                if (!finalFragment) {
+                    error(Diagnostics.Digit_expected);
                 }
                 else {
-                    error(Diagnostics.Digit_expected);
+                    scientificFragment = text.substring(end, preNumericPart) + finalFragment;
+                    end = pos;
+                }
+            }
+            if (tokenFlags & TokenFlags.ContainsSeparator) {
+                let result = mainFragment;
+                if (decimalFragment) {
+                    result += "." + decimalFragment;
                 }
+                if (scientificFragment) {
+                    result += scientificFragment;
+                }
+                return "" + +result;
+            }
+            else {
+                return "" + +(text.substring(start, end)); // No need to use all the fragments; no _ removal needed
             }
-            return "" + +(text.substring(start, end));
         }
 
         function scanOctalDigits(): number {
@@ -894,23 +946,36 @@ namespace ts {
          * Scans the given number of hexadecimal digits in the text,
          * returning -1 if the given number is unavailable.
          */
-        function scanExactNumberOfHexDigits(count: number): number {
-            return scanHexDigits(/*minCount*/ count, /*scanAsManyAsPossible*/ false);
+        function scanExactNumberOfHexDigits(count: number, canHaveSeparators: boolean): number {
+            return scanHexDigits(/*minCount*/ count, /*scanAsManyAsPossible*/ false, canHaveSeparators);
         }
 
         /**
          * Scans as many hexadecimal digits as are available in the text,
          * returning -1 if the given number of digits was unavailable.
          */
-        function scanMinimumNumberOfHexDigits(count: number): number {
-            return scanHexDigits(/*minCount*/ count, /*scanAsManyAsPossible*/ true);
+        function scanMinimumNumberOfHexDigits(count: number, canHaveSeparators: boolean): number {
+            return scanHexDigits(/*minCount*/ count, /*scanAsManyAsPossible*/ true, canHaveSeparators);
         }
 
-        function scanHexDigits(minCount: number, scanAsManyAsPossible: boolean): number {
+        function scanHexDigits(minCount: number, scanAsManyAsPossible: boolean, canHaveSeparators: boolean): number {
             let digits = 0;
             let value = 0;
+            let allowSeparator = false;
             while (digits < minCount || scanAsManyAsPossible) {
                 const ch = text.charCodeAt(pos);
+                if (canHaveSeparators && ch === CharacterCodes._) {
+                    tokenFlags |= TokenFlags.ContainsSeparator;
+                    if (allowSeparator) {
+                        allowSeparator = false;
+                    }
+                    else {
+                        error(Diagnostics.Numeric_separators_are_not_allowed_here, pos, 1);
+                    }
+                    pos++;
+                    continue;
+                }
+                allowSeparator = canHaveSeparators;
                 if (ch >= CharacterCodes._0 && ch <= CharacterCodes._9) {
                     value = value * 16 + ch - CharacterCodes._0;
                 }
@@ -929,6 +994,9 @@ namespace ts {
             if (digits < minCount) {
                 value = -1;
             }
+            if (text.charCodeAt(pos - 1) === CharacterCodes._) {
+                error(Diagnostics.Numeric_separators_are_not_allowed_here, pos - 1, 1);
+            }
             return value;
         }
 
@@ -1097,7 +1165,7 @@ namespace ts {
         }
 
         function scanHexadecimalEscape(numDigits: number): string {
-            const escapedValue = scanExactNumberOfHexDigits(numDigits);
+            const escapedValue = scanExactNumberOfHexDigits(numDigits, /*canHaveSeparators*/ false);
 
             if (escapedValue >= 0) {
                 return String.fromCharCode(escapedValue);
@@ -1109,7 +1177,7 @@ namespace ts {
         }
 
         function scanExtendedUnicodeEscape(): string {
-            const escapedValue = scanMinimumNumberOfHexDigits(1);
+            const escapedValue = scanMinimumNumberOfHexDigits(1, /*canHaveSeparators*/ false);
             let isInvalidExtendedEscape = false;
 
             // Validate the value of the digit
@@ -1162,7 +1230,7 @@ namespace ts {
             if (pos + 5 < end && text.charCodeAt(pos + 1) === CharacterCodes.u) {
                 const start = pos;
                 pos += 2;
-                const value = scanExactNumberOfHexDigits(4);
+                const value = scanExactNumberOfHexDigits(4, /*canHaveSeparators*/ false);
                 pos = start;
                 return value;
             }
@@ -1218,8 +1286,22 @@ namespace ts {
             // For counting number of digits; Valid binaryIntegerLiteral must have at least one binary digit following B or b.
             // Similarly valid octalIntegerLiteral must have at least one octal digit following o or O.
             let numberOfDigits = 0;
+            let separatorAllowed = false;
             while (true) {
                 const ch = text.charCodeAt(pos);
+                // Numeric seperators are allowed anywhere within a numeric literal, except not at the beginning, or following another separator
+                if (ch === CharacterCodes._) {
+                    tokenFlags |= TokenFlags.ContainsSeparator;
+                    if (separatorAllowed) {
+                        separatorAllowed = false;
+                    }
+                    else {
+                        error(Diagnostics.Numeric_separators_are_not_allowed_here, pos, 1);
+                    }
+                    pos++;
+                    continue;
+                }
+                separatorAllowed = true;
                 const valueOfCh = ch - CharacterCodes._0;
                 if (!isDigit(ch) || valueOfCh >= base) {
                     break;
@@ -1232,6 +1314,11 @@ namespace ts {
             if (numberOfDigits === 0) {
                 return -1;
             }
+            if (text.charCodeAt(pos - 1) === CharacterCodes._) {
+                // Literal ends with underscore - not allowed
+                error(Diagnostics.Numeric_separators_are_not_allowed_here, pos - 1, 1);
+                return value;
+            }
             return value;
         }
 
@@ -1435,7 +1522,7 @@ namespace ts {
                     case CharacterCodes._0:
                         if (pos + 2 < end && (text.charCodeAt(pos + 1) === CharacterCodes.X || text.charCodeAt(pos + 1) === CharacterCodes.x)) {
                             pos += 2;
-                            let value = scanMinimumNumberOfHexDigits(1);
+                            let value = scanMinimumNumberOfHexDigits(1, /*canHaveSeparators*/ true);
                             if (value < 0) {
                                 error(Diagnostics.Hexadecimal_digit_expected);
                                 value = 0;
 
@@ -1490,8 +1490,9 @@ namespace ts {
         HexSpecifier = 1 << 6,      // e.g. `0x00000000`
         BinarySpecifier = 1 << 7,   // e.g. `0b0110010000000000`
         OctalSpecifier = 1 << 8,    // e.g. `0o777`
+        ContainsSeparator = 1 << 9, // e.g. `0b1100_0101`
         BinaryOrOctalSpecifier = BinarySpecifier | OctalSpecifier,
-        NumericLiteralFlags = Scientific | Octal | HexSpecifier | BinarySpecifier | OctalSpecifier
+        NumericLiteralFlags = Scientific | Octal | HexSpecifier | BinarySpecifier | OctalSpecifier | ContainsSeparator
     }
 
     export interface NumericLiteral extends LiteralExpression {
 
@@ -346,7 +346,7 @@ namespace ts {
     export function getLiteralText(node: LiteralLikeNode, sourceFile: SourceFile) {
         // If we don't need to downlevel and we can reach the original source text using
         // the node's parent reference, then simply get the text as it was originally written.
-        if (!nodeIsSynthesized(node) && node.parent) {
+        if (!nodeIsSynthesized(node) && node.parent && !(isNumericLiteral(node) && node.numericLiteralFlags & TokenFlags.ContainsSeparator)) {
             return getSourceTextOfNodeFromSourceFile(sourceFile, node);
         }
 
 
@@ -0,0 +1,12 @@
+//// [parser.numericSeparators.binary.ts]
+0b00_11;
+0B0_1;
+0b1100_0011;
+0B0_11_0101;
+
+
+//// [parser.numericSeparators.binary.js]
+3;
+1;
+195;
+53;
@@ -0,0 +1,7 @@
+=== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/parser.numericSeparators.binary.ts ===
+0b00_11;
+No type information for this code.0B0_1;
+No type information for this code.0b1100_0011;
+No type information for this code.0B0_11_0101;
+No type information for this code.
+No type information for this code.
@@ -0,0 +1,13 @@
+=== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/parser.numericSeparators.binary.ts ===
+0b00_11;
+>0b00_11 : 3
+
+0B0_1;
+>0B0_1 : 1
+
+0b1100_0011;
+>0b1100_0011 : 195
+
+0B0_11_0101;
+>0B0_11_0101 : 53
+
@@ -0,0 +1,50 @@
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/1.ts(1,5): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/2.ts(1,3): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/3.ts(1,2): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/3.ts(1,3): error TS1005: ';' expected.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/3.ts(1,3): error TS2304: Cannot find name 'B0101'.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/4.ts(1,6): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/5.ts(1,13): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/6.ts(1,3): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/6.ts(1,4): error TS6188: Numeric separators are not allowed here.
+tests/cases/conformance/parser/ecmascriptnext/numericSeparators/6.ts(1,5): error TS6188: Numeric separators are not allowed here.
+
+
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/1.ts (1 errors) ====
+    0b00_
+        ~
+!!! error TS6188: Numeric separators are not allowed here.
+    
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/2.ts (1 errors) ====
+    0b_110
+      ~
+!!! error TS6188: Numeric separators are not allowed here.
+    
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/3.ts (3 errors) ====
+    0_B0101
+     ~
+!!! error TS6188: Numeric separators are not allowed here.
+      ~~~~~
+!!! error TS1005: ';' expected.
+      ~~~~~
+!!! error TS2304: Cannot find name 'B0101'.
+    
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/4.ts (1 errors) ====
+    0b01__11
+         ~
+!!! error TS6188: Numeric separators are not allowed here.
+    
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/5.ts (1 errors) ====
+    0B0110_0110__
+                ~
+!!! error TS6188: Numeric separators are not allowed here.
+    
+==== tests/cases/conformance/parser/ecmascriptnext/numericSeparators/6.ts (3 errors) ====
+    0b___0111010_0101_1
+      ~
+!!! error TS6188: Numeric separators are not allowed here.
+       ~
+!!! error TS6188: Numeric separators are not allowed here.
+        ~
+!!! error TS6188: Numeric separators are not allowed here.
+    
@@ -0,0 +1,34 @@
+//// [tests/cases/conformance/parser/ecmascriptnext/numericSeparators/parser.numericSeparators.binaryNegative.ts] ////
+
+//// [1.ts]
+0b00_
+
+//// [2.ts]
+0b_110
+
+//// [3.ts]
+0_B0101
+
+//// [4.ts]
+0b01__11
+
+//// [5.ts]
+0B0110_0110__
+
+//// [6.ts]
+0b___0111010_0101_1
+
+
+//// [1.js]
+0;
+//// [2.js]
+6;
+//// [3.js]
+0;
+B0101;
+//// [4.js]
+7;
+//// [5.js]
+102;
+//// [6.js]
+1867;
Original file line number	Diff line number	Diff line change
`@@ -346,7 +346,7 @@ namespace ts {`
`346`	`346`	`export function getLiteralText(node: LiteralLikeNode, sourceFile: SourceFile) {`
`347`	`347`	`// If we don't need to downlevel and we can reach the original source text using`
`348`	`348`	`// the node's parent reference, then simply get the text as it was originally written.`
`349`		`- if (!nodeIsSynthesized(node) && node.parent) {`
	`349`	`+ if (!nodeIsSynthesized(node) && node.parent && !(isNumericLiteral(node) && node.numericLiteralFlags & TokenFlags.ContainsSeparator)) {`
`350`	`350`	`return getSourceTextOfNodeFromSourceFile(sourceFile, node);`
`351`	`351`	`}`
`352`	`352`